IB DP Biology HL D3.1 Reproduction Flashcards

[q] D3.1.1—Differences between sexual and asexual reproduction

[a] Include these relative advantages: asexual reproduction to produce genetically identical offspring by individuals that are adapted to an existing environment, sexual reproduction to produce offspring with new gene combinations and thus variation needed for adaptation to a changed environment.

[q] D3.1.2—Role of meiosis and fusion of gametes in the sexual life cycle

[a] Students should appreciate that meiosis breaks up parental combinations of alleles, and fusion of gametes produces new combinations.

Fusion of gametes is also known as fertilization.

[q] D3.1.3—Differences between male and female sexes in sexual reproduction

[a] Include the prime difference that the male gamete travels to the female gamete, so it is smaller, with less food reserves than the egg.

From this follow differences in the numbers of gametes and the reproductive strategies of males and females.

[q] D3.1.4—Anatomy of the human male and female reproductive systems

[a] Students should be able to draw diagrams of the male-typical and female-typical systems and annotate them with names of structures and functions.

[q] D3.1.5—Changes during the ovarian and uterine cycles and their hormonal regulation

[a] Include the roles of oestradiol, progesterone, luteinizing hormone (LH), follicle-stimulating hormone (FSH) and both positive and negative feedback.

The ovarian and uterine cycles together constitute the menstrual cycle.

[q] D3.1.6—Fertilization in humans

[a] Include the fusion of a sperm’s cell membrane with an egg cell membrane, entry to the egg of the sperm nucleus but destruction of the tail and mitochondria.

Also include dissolution of nuclear membranes of sperm and egg nuclei and participation of all the condensed chromosomes in a joint mitosis to produce two diploid nuclei.

[q] D3.1.7—Use of hormones in in vitro fertilization (IVF) treatment

[a] The normal secretion of hormones is suspended, and artificial doses of hormones induce superovulation.

[q] D3.1.8—Sexual reproduction in flowering plants

[a] Include production of gametes inside ovules and pollen grains, pollination, pollen development and fertilization to produce an embryo.

Students should understand that reproduction in flowering plants is sexual, even if a plant species is hermaphroditic.

[q] D3.1.9—Features of an insect-pollinated flower

[a] Students should draw diagrams annotated with names of structures and their functions.

[q] D3.1.10—Methods of promoting cross-pollination

[a] Include different maturation times for pollen and stigma, separate male and female flowers or male and female plants.

Also include the role of animals or wind in transferring pollen between plants.

[q] D3.1.11—Self-incompatibility mechanisms to increase genetic variation within a species

[a] Students should understand that self-pollination leads to inbreeding, which decreases genetic diversity and vigour.

They should also understand that genetic mechanisms in many plant species ensure male and female gametes fusing during fertilization are from different plants.

[q] D3.1.12—Dispersal and germination of seeds

[a] Distinguish seed dispersal from pollination. Include the growth and development of the embryo and the mobilization of food reserves.

[q] D3.1.13—Control of the developmental changes of puberty by gonadotropin-releasing hormone and steroid sex hormones

[a] Limit to the increased release of gonadotropin-releasing hormone (GnRH) by the hypothalamus in childhood triggering the onset of increased luteinizing hormone (LH) and follicle-stimulating hormone (FSH) release.

Ultimately the increased sex hormone production leads to the changes associated with puberty.

[q] D3.1.14—Spermatogenesis and oogenesis in humans

[a] Include mitosis, cell growth, two divisions of meiosis and differentiation. Students should understand how gametogenesis, in typical male and female bodies, results in different numbers of sperm and eggs, and different amounts of cytoplasm.

[q] D3.1.15—Mechanisms to prevent polyspermy

[a] The acrosome reaction allows a sperm to penetrate the zona pellucida and the cortical reaction prevents other sperm from passing through.

[q] D3.1.16—Development of a blastocyst and implantation in the endometrium

[a] Students are not required to know the names of other stages in embryo development.

[q] D3.1.17—Pregnancy testing by detection of human chorionic gonadotropin secretion

[a] Include the production of human chorionic gonadotropin (hCG) in the embryo or developing placenta and the use of monoclonal antibodies that bind to hCG.

[q] D3.1.18—Role of the placenta in foetal development inside the uterus

[a] Students are not required to know details of placental structure apart from the large surface area of the placental villi.

Students should understand which exchange processes occur in the placenta and that it allows the foetus to be retained in the uterus to a later stage of development than in mammals that do not develop a placenta.

[q] D3.1.19—Hormonal control of pregnancy and childbirth

[a] Emphasize that the continuity of pregnancy is maintained by progesterone secretion initially from the corpus luteum and then from the placenta, whereas the changes during childbirth are triggered by a decrease in progesterone levels, allowing increases in oxytocin secretion due to positive feedback.

[q] D3.1.20—Hormone replacement therapy and the risk of coronary heart disease

[a] NOS: In early epidemiological studies, it was argued that women undergoing hormone replacement therapy (HRT) had reduced incidence of coronary heart disease (CHD) and this was deemed to be a cause- and-effect relationship.

Later randomized controlled trials showed that use of HRT led to a small increase in the risk of CHD.

The correlation between HRT and decreased incidence of CHD is not actually a cause- and-effect relationship.

HRT patients have a higher socioeconomic status, and this status has a causal relationship with lower risk of CHD.

[q] How are gametes produced in plants?

[a] In flowering plants, male and female gametes are produced in the anther and ovule 

Male gametes are contained within pollen grains, which are released from the anthers

– The anther contains pollen sacs

– Each pollen sac contains a diploid mothercell which undergoes meiosis to form four haploid pollen grains (the gametes)

Mitosis occurs to produce more haploid male gametes

Female gametes are made in the ovules

– A single diploid cell within the ovule undergoes meiosis to produce four haploid egg cells

– Only one of these cells survives which undergoes mitosis to produce female gametes

[q] What is pollination? What are the different kinds of pollination?

[a] Pollination is the process of transferring pollen from the anther of one flower to the stigma of another – All pollination methods are forms of sexual reproduction because the gametes are produced by meiosis so there is fusion of gametes to form a diploid nucleus

Some flowering plants are hermaphroditic which means they contain both male and female parts – Self pollination can occur in some of these species when pollen is transferred between different flowers on the same plant, or even from anther to stigma within the same flower

Cross pollination is the transfer of pollen from one plant to another

[q] What is the process of fertilisation in plants?

[a] 1. After pollination has occurred, a pollen tube grows from the pollen grain down the style to the ovary of the plant

2. The male nuclei travel down the pollen tube to the female ovule

– Two male nuclei travel down the pollen tube to the ovule; one will fuse with an ovule nucleus to form the zygote while the other will go on the form the plant embryo’s food store

3. Fertilisation occurs when the haploid male and female nuclei fuse and a diploid zygote is formedAfter fertilisation, the ovule becomes a seed and the ovary develops into the fruit

[q] What is the anatomy of flower?

[a] The development of flowers occurs in the reproductive stage of the plant life cycle

Flowers contain all the necessary organs and tissues required for sexual reproduction by pollination

Key structures of the flower include

– The anther – where the male gamete, pollen, is found

– The stigma – part of the female reproductive organ which receives the pollen

– The ovary – where the female gametes are located

[q] What are the structures and their functions of a flower?

[a]

[q] Why do plants pollinate?

[a] Flowers are the reproductive organ of the plant

– They usually contain both male and female reproductive parts

Plants produce pollen which contains a nucleus inside that is the male gamete

– Unlike the male gamete in humans (sperm), pollen is not capable of locomotion (moving from one place to another)

– This means plants have to have mechanisms in place to transfer pollen from the anther to the stigma

This process is known as pollination and there are two main mechanisms by which it occurs: transferred by insects (or other animals like birds) or transferred by wind

[q] What are insect-pollinating plants?

[a] For the flowers of many plant species, the pollinating agents are insects (e.g. bees) – Insects often visit these flowers to collect nectar (a sugary substance produced by insect-pollinated flowers and the base of their petals, which provides the insects with energy)

1. As an insect enters the flowers in search of nectar, it often brushes against the anthers, which deposit sticky pollen onto the insect’s body

2. When the insect visits another flower, it may brush against the stigma of this second flower and in the process, may deposit some of the pollen from the first flower, resulting in pollination
The structures of an insect-pollinated flower ensure that the flower is well-adapted for pollination by insects

[q] How are insect-pollinating plants adapted?

[a]

[q] What are wind-pollinating plants?

[a] For wind-pollinated flowers, the process of pollination is more random than it is for insect-pollinated flowers

1. When ripe, the anthers open and shed their pollen into the open air

2. The pollen is then either blown by the wind or carried by air currents until it (by chance) lands on the stigma of a plant of the same species, resulting in pollination

The structures of a wind-pollinated flower ensure that the flower is well-adapted for pollination by the wind

[q] How are wind-pollinating plants adapted?

[a]

[q] Why do plants prefer cross pollination?

[a] Cross-pollination occurs when the pollen from one plant is transferred to the stigma of another plant of the same species

– This is the way most plants carry out pollination as it improves genetic variation

Cross-pollination relies completely on the presence of pollinators and this can be a problem if those pollinators are missing (e.g. the reduction in bee numbers is of great importance to humans as bees pollinate a large number of food crops) – this doesn’t apply to wind-pollinated plants

[q] What are some other ways plants promote cross-pollination?

[a] In addition to the mechanisms described above for insect and wind pollinated plants, plants also have a variety of other methods to ensure successful cross-pollination

Different maturation times for the pollen and ovules of the same flower. This prevents self-pollination from occurring

Self-incompatibility mechanisms are used in some species that ensure if pollen lands on the stigma from the same plant the plant produces chemicals that ensure a pollen tube does not grow

Plants can produce flowers that only have either male or female parts or the whole plant is either male or female

Wind-pollinated plants are less likely to self-pollinate due to the wind carrying the pollen far from the parent plant

[q] What is self pollination and why is it not preferred?

[a] The pollen from a flower can land on its own stigma or on the stigma of another flower on the same plant – this is known as self-pollination

– Self-pollination reduces genetic variety of the offspring as all the gametes come from the same parent (and are therefore genetically identical). this can lead to inbreeding

– Lack of variation in the offspring is a disadvantage if environmental conditions change, as it is less likely that any offspring will have adaptations that suit the new conditions well

[q] What are some examples of self-incompatibility mechanisms?

[a] Genetic mechanisms in many plant species ensure male and female gametes fusing during fertilisation are from different plants

– Each plant has a set of genes that controls the growth of a pollen tube so that when pollen lands on the stigma of a flower of the same plant protein interactions occur that prevent the growth of a pollen tube

– This is an example of a self-incompatibility mechanism

The mechanism may include

– A pollen grain fails to germinate into a pollen tube

– A pollen grain germinates but does not enter the style

– The pollen nuclei enters the ovule but it degenerates before fertilisation can occur

– Fertilisation occurs but the embryo degenerates before growth is established

[q] What is seed dispersal? How is it different to pollination?

[a] Seed dispersal is then required in order to distribute the seeds away from the parent plant and reduce competition between the offspring and the parent plant

Methods of seed dispersal include

– Wind or water – Parachute or wing shaped lightweight seeds will travel on the wind or float in water

– Animals – Fleshy fruit is eaten by animals and seeds distributed through egestion Sticky or hooked seeds catch on to the fur or feathers of passing animals

– Explosions – Some pods explode propelling the seeds away from the parent plant

Seed dispersal can often be confused with pollination

– Pollination is the transfer of pollen from anther to stigma, while seed dispersal is the distribution of mature seeds. Both processes can involve wind, water, or animals.

[q] What is germination?

[a] Once a seed has formed within the ovary of a flower they undergo a period of dormancy

When conditions become favourable the seed may germinate

– Germination is the start of growth in the seed

[q] What are the requirements for germination?

[a] Three factors are required for successful germination:

Water – allows the seed to swell up, which causes the seed coat (testa) to burst, allowing the growing embryo plant to exit the seed. Water also allows the enzymes in the embryo to start working so that growth can occur (increases metabolic activity)

Oxygen – required for respiration, so that energy can be released for germination

Warmth – germination improves as temperature rises (up to a certain point) as the reactions which take place are controlled by enzymes, which cannot function effectively when temperatures are too low

[q] What is the process of germination?

[a] 1. A seed contains a plant embryo and food reserves for its growth

2. The food reserves contain endosperm tissue which are transferred to the embryo through early leaf structures called cotyledons

3. Seeds needs to replenish water lost during dormancy and does so through a process called imbibition which activates the biochemistry of the embryo

4. The rate of respiration and protein synthesis increases and the embryo can prepare to emerge through the seed coat

5. A structure called the radicle is the first to emerge and forms the initial root structure which responds to gravity and grows downward into the soil

6. The first structure to appear above ground is called the hypocotyl, this is a curved portion of the plant shoot found below the cotyledons and grows upwards

7. As the shoot grows the first leaves begin to appear from the cotyledon and photosynthesis can begin

8. The root structure is also established and full plant growth can occur

[q] What is Sexual Reproduction?

[a] Sexual reproduction involves two parents and is the fusion of the nuclei of two gamete, to form a zygote, and the production of offspring that are genetically different from each other

[q] What is a gamete?

[a] A gamete is a sex cell 

– Gametes differ from normal cells as they contain half the number of chromosomes found in other body cells – we say they have a haploid nucleus

– This is because they only contain one copy of each chromosome, rather than the two copies found in other body cells

In human beings, a normal body cell contains 46 chromosomes but each gamete contains 23 chromosomesWhen the male and female gametes fuse, they become a zygote (fertilised egg cell)

– This contains the full 46 chromosomes, half of which came from the father and half from the mother – we say the zygote has a diploid nucleus

[q] What are the advantages and disadvantages to sexual reproduction?

[a]

[q] What is asexual reproduction?

[a]  Asexual reproduction does not involve gametes or fertilisation

– Only one parent is required so there is no fusion of gametes and no mixing of genetic information

– As a result, the offspring are genetically identical to the parent and to each other (they are clones)

Many plants reproduce via asexual reproduction

Bacteria produce exact genetic copies of themselves in a type of asexual reproduction called binary fission

[q] What are the advantages and disadvantages to asexual reproduction?

[a]

[q] What are the differences between sexual vs asexual reproduction?

[a] The key differences between sexual and asexual reproduction include:

– The number of parent organisms

– How offspring are produced (the type of cell division required)

– The level of genetic similarity between offspring

– The possible sources of genetic variation in offspring

– The number of offspring produced

– The time taken to produce offspring

[q] What is meiosis in sexual reproduction? How does it ensure genetic variation?

[a] Meiosis is a form of nuclear division that results in the production of haploid cells from diploid cells

– It produces gametes in plants and animals that are used in sexual reproduction

– It takes place in two successive divisions: meiosis I and meiosis II

During meiosis, specific mechanisms occur to lead to genetic variation within the resulting gametes, this breaks up parental combinations of alleles derived from the mother and father chromosomes

Crossing over – the process by which non-sister chromatids exchange alleles during meiosis I

Independent assortment – the production of different combinations of alleles in daughter cells due to the random alignment of homologous pairs of chromosomes during meiosis I

Random fertilisation – there are millions of combinations of sperm and egg cells and the fusion of these sperm and egg cell

The fusion of gametes during fertilisation produces new combinations of alleles leading to genetic variation

[q] Where does Meiosis occur and what does it produce?

[a] Meiosis occurs:

– In the testes of male animals and the ovaries of female animals

– In the anthers and ovaries of flowering plants

Meiosis leads to the production of the following haploid gametes:

– Spermatozoa, or sperm cells, in male animals, ova (singular ovum) in female animals

– Male plant gametes are carried in pollen grains and female plants gametes are held in the ovules within the plant ovary

[q] Compare the differences between female and male gametes

[a] The differences between male and female gametes, not just in humans, means that there are differences in the strategies developed for reproductive success

– Human females release only one egg cell (per menstrual cycle) whereas a male will release many thousands of sperm cells per ejaculation, this is because the majority of which will not reach the egg cell (only one sperm cell can fertilise an egg cell)

[q] Draw and label the female reproductive system

[a]

[q] What is the function of each part of the female reproductive system?

[a]

[q] Draw and label the male reproductive system

[a]

[q] What is the function of each part of the male reproductive system?

[a]

[q] What is the Menstrual Cycle?

[a] The menstrual cycle is the series of changes that take place in the female body leading up to and following the release of an egg from the ovaries

– It starts in early adolescence in girls and is controlled by hormones

– The average menstrual cycle is 28 days long

[q] Summarise the events of the menstrual cycle

[a] The uterus lining, or endometrium, thickens from day 7 through to day 28 of the cycle in preparation for receiving a fertilised egg

The release of an egg, or ovulation, occurs about halfway through the cycle on day 14, and the egg then travels down the oviduct to the uterus

– Eggs develop inside fluid-filled sacs known as egg follicles inside the ovary

– The follicle releases the egg at ovulation and becomes an empty follicle known as a corpus luteum

Failure to fertilise the egg leads to menstruation, commonly known as a period

– Menstruation involves the loss of menstrual blood via the vagina

– This is caused by the breakdown of the endometrium

Menstruation takes place roughly between days 1-7 of the cycle 

– The number of days during which menstruation occurs can vary

After menstruation finishes, the endometrium starts to thicken again in preparation for the possible implantation of a fertilised egg in the next cycle

[q] What hormones control the menstrual cycle and where are they produced?

[a] Four hormones control the events that occur during the menstrual cycle:

Hormones produced by the pituitary gland in the brain

– Follicle-stimulating hormone (FSH)

– Luteinising hormone (LH)

Hormones produced in the ovaries

– Oestrogen (also known as oestradiol); produced by the egg follicle, and by the corpus luteum after ovulation

– Progesterone; produced by the corpus luteum

[q] Summarise process of fertilisation

[a]

[q] D3.1.1—Differences between sexual and asexual reproduction 

Include these relative advantages: asexual reproduction to produce genetically identical offspring by individuals that are adapted to an existing environment, sexual reproduction to produce offspring with new gene combinations and thus variation needed for adaptation to a changed environment.

[a] 

[q] D3.1.2—Role of meiosis and fusion of gametes in the sexual life cycle 

Students should appreciate that meiosis breaks up parental combinations of alleles, and fusion of gametes produces new combinations.

Fusion of gametes is also known as fertilization

[a] Meiosis halves the number of chromosomes in order to break up parental combinations of alleles and create more variation.

Fertilization is the fusion of gametes in order to double the number of chromosomes again back to diploid.

[q] D3.1.3—Differences between male and female sexes in sexual reproduction 

Include the prime difference that the male gamete travels to the female gamete, so it is smaller, with less food reserves than the egg.

From this follow differences in the numbers of gametes and the reproductive strategies of males and females.

[a]

[q] D3.1.4—Anatomy of the human male and female reproductive systems 

Students should be able to draw diagrams of the male-typical and female-typical systems and annotate them with names of structures and functions.

[a]

[q] D3.1.5—Changes during the ovarian and uterine cycles and their hormonal regulation 

Include the roles of oestradiol, progesterone, luteinizing hormone (LH), follicle-stimulating hormone (FSH) and both positive and negative feedback.

The ovarian and uterine cycles together constitute the menstrual cycle

[a]

[q] D3.1.5—Changes during the ovarian and uterine cycles and their hormonal regulation 

Include the roles of oestradiol, progesterone, luteinizing hormone (LH), follicle-stimulating hormone (FSH) and both positive and negative feedback.

The ovarian and uterine cycles together constitute the menstrual cycle

[a]

[q] D3.1.6—Fertilization in humans 

Include the fusion of a sperm’s cell membrane with an egg cell membrane, entry to the egg of the sperm nucleus but destruction of the tail and mitochondria.

Also include dissolution of nuclear membranes of sperm and egg nuclei and participation of all the condensed chromosomes in a joint mitosis to produce two diploid nuclei.

[a] Fertilization occurs when the sperm and oocyte fuse their plasma membranes and combine their nuclei to form one diploid nucleus.

Now called a zygote, the cell completes its second meiotic division and then divides via mitosis through the participation of all the condensed chromosomes to produce two diploid nuclei.

Since the rest of the sperm’s body does not enter the oocyte’s cytoplasm, the remainder of the organelles are destroyed.

While the offspring inherits chromosomes from both parents, mitochondrial DNA is only inherited from the mother.

[q] D3.1.7—Use of hormones in in vitro fertilization (IVF) treatment 

The normal secretion of hormones is suspended, and artificial doses of hormones induce superovulation.

[a] • Step 1 | suspending normal secretion of hormones: the patient is given birth control pills or estrogen in order to allow the healthcare provider to control the timing of the menstrual cycle.
• Step 2 | ovarian stimulation: hormonal medications (FSH and LH) are used to induce the development of multiple oocytes.
• Step 3 | egg retrieval: via a minor surgery, the oocytes are removed from the patient’s body.
• Step 4 | insemination and fertilization: concentrated sperm are mixed together with the eggs (insemination) and then stored in a controlled chamber until fertilization.
• Step 5 | embryo culture and transfer: the zygote is left to grow for a few days until it becomes a blastocyst and then is transferred to the patient’s body and implanted into the uterus.

[q] D3.1.9—Features of an insect-pollinated flower 

Students should draw diagrams annotated with names of structures and their functions. 

[a]

[q] D3.1.8—Sexual reproduction in flowering plants 

Include production of gametes inside ovules and pollen grains, pollination, pollen development and fertilization to produce an embryo.

Students should understand that reproduction in flowering plants is sexual, even if a plant species is hermaphroditic. 

Production of male gametes:

[a] Flowering plants (angiosperms) reproduce sexually as they can undergo meiosis, even if they are hermaphroditic (possessing both male and female gametes).

Production of male gametes:
• Within the anther’s microsporangia (pollen sacs), microspore mother cells each divide by meiosis to produce four microspores, all four of which form a pollen grain
• Each pollen grain will develop into two cells, a pollen tube cell and a generative cell (which is contained within the larger pollen tube cell)
• The pollen tube cell develops the pollen tube upon germination which the generative cell travels through to enter the ovary
• During transit in the pollen tube, the generative cell divides to form two male gametes (sperm cells)
• Pollen grains are released from the anther upon maturity when the pollen sacs burst

[q] D3.1.8—Sexual reproduction in flowering plants 

Include production of gametes inside ovules and pollen grains, pollination, pollen development and fertilization to produce an embryo.

Students should understand that reproduction in flowering plants is sexual, even if a plant species is hermaphroditic. 

Production of female gametes: 

[a] Flowering plants (angiosperms) reproduce sexually as they can undergo meiosis, even if they are hermaphroditic (possessing both male and female gametes).

Production of female gametes: 

• A single diploid cell in an area of tissue in the ovules divides by meiosis to form 4 haploid cells, of which only 1 survives
• The surviving cell then undergoes mitosis to produce 8 nucleate cells, now forming the embryo sac
• Only one cell develops into the egg; the rest assist in fertilization and embryo development then degenerate

[q] D3.1.10—Methods of promoting cross-pollination 

Include different maturation times for pollen and stigma, separate male and female flowers or male and female plants.

Also include the role of animals or wind in transferring pollen between plants.

[a] Cross-pollination is the transfer of pollen from the anther of one flower to the stigma of another flower on a different plant- thus promoting genetic diversity, adaptability, and hybrid vigor. This is achieved through:
• Transferring pollen by wind, insects, and animals
• Different maturation times for pollen and stigma
• Separate male and female parts within the same plant or on a different plant

[q] D3.1.11—Self-incompatibility mechanisms to increase genetic variation within a species 

Students should understand that self-pollination leads to inbreeding, which decreases genetic diversity and vigour.

They should also understand that genetic mechanisms in many plant species ensure male and female gametes fusing during fertilization are from different plants.

[a] To prevent self-pollination, plants have evolved self-incompatibility mechanisms to increase genetic variation within a species and prevent inbreeding.

This is genetically controlled by the S (sterility) locus, which encodes for enzymes that detect and degrades self-pollen to prevent fertilization.

[q] D3.1.12—Dispersal and germination of seeds 

Distinguish seed dispersal from pollination. Include the growth and development of the embryo and the mobilization of food reserves.

[a] Pollination is the transfer of pollen from the anther to the stigma, whereas seed dispersal is scattering the plant seeds far away from the parent plant to reduce competition between them.
After fertilization, the embryo begins to develop inside the seed, and only when there is no more enough room for growth is the seed ready for dispersal.

Embryonic growth is suspended until seed germination, and the developing seedling will rely on the food reserves in its cotyledons until leaves grow for photosynthesis.

[q] D3.1.13—Control of the developmental changes of puberty by gonadotropin-releasing hormone and steroid sex hormones

Limit to the increased release of gonadotropin-releasing hormone (GnRH) by the hypothalamus in childhood triggering the onset of increased luteinizing hormone (LH) and follicle-stimulating hormone (FSH) release.

Ultimately the increased sex hormone production leads to the changes associated with puberty

[a] The hypothalamus produces GnRH, a peptide hormone that signals to the anterior pituitary gland to produce the gonadotropins FSH and LH in both males and females.

GnRH is secreted during early childbirth but stops, and resumes during teenage years to mark the beginning of puberty (the process of transitioning to sexual maturity).
At around ages 8 or 9, the hypothalamus becomes less sensitive to estrogen and testosterone concentrations.

Since these hormones are in negative feedback with LH and FSH, this causes the gradual rise of LH and FSH levels.

The gonads, however, become more sensitive to FSH and LH, leading to their slow but imminent development throughout teenage years (along with secondary sexual traits).

[q] D3.1.14—Spermatogenesis and oogenesis in humans 

Include mitosis, cell growth, two divisions of meiosis and differentiation.

Students should understand how gametogenesis, in typical male and female bodies, results in different numbers of sperm and eggs, and different amounts of cytoplasm.

[a]

Oogenesis:
• Oogonia (ovarian stem cells) form during fetal development and divide by mitosis to form primary oocytes before birth
• Primary oocytes begin meiosis I but are arrested at prophase I before birth and until puberty
• LH stimulates a few primary oocytes to complete meiosis I and they are again arrested at metaphase II, forming secondary oocytes (a polar body is produced upon completion of meiosis I, which is a byproduct cell smaller than the main oocyte and eventually disintegrates)
• If fertilization occurs, the fusion of the two nuclei is followed by the completion of meiosis II and the formation of the zygote

[q] D3.1.14—Spermatogenesis and oogenesis in humans 

Include mitosis, cell growth, two divisions of meiosis and differentiation.

Students should understand how gametogenesis, in typical male and female bodies, results in different numbers of sperm and eggs, and different amounts of cytoplasm.

[a] Spermatogenesis:
• Spermatogonia (germ cells lining the basement membrane) divide by mitosis to produce primary spermatocytes
• Primary spermatocytes undergo meiosis I to produce secondary spermatocytes
• Secondary spermatocytes undergo meiosis II to produce spermatids
• Sertoli and Leydig cells both help control this process
• Spermatozoa are then transported to the epididymis for the next step of sperm maturation

[q] D3.1.15—Mechanisms to prevent polyspermy 

The acrosome reaction allows a sperm to penetrate the zona pellucida and the cortical reaction prevents other sperm from passing through.

[a] • Acrosome reaction: after burrowing through the corona radiata and binding to receptors on the zona pellucida, the acrosome releases its stored digestive enzymes in order to clear away the zona pellucida.

This enables the sperm to contact the plasma membrane of the oocyte, fusing with it and releasing its nucleus into its cytoplasm.
• Cortical reaction: cortical granules situated below the oocyte’s plasma membrane fuse with the membrane and release proteins that destroy sperm receptors and cause the release of any other attached sperm.

They also secrete polysaccharides that form an impenetrable barrier around the zygote, called the fertilization membrane.

[q] D3.1.16—Development of a blastocyst and implantation in the endometrium 

Students are not required to know the names of other stages in embryo development. 

[a] The blastocyst is developed in the uterus where its cells begin to secrete and organize themselves around the blastocoel, a fluid-filled cavity.

The trophoblasts are the cells forming the outer shell of the blastocyst, and when they come in contact with the uterine wall they adhere to and embed themselves in it, beginning implantation.

If the endometrium is not yet fully developed, the blastocyst will detach and find a better place.

Successful implantation could be accompanied by minor bleeding, but if the blastocyst fails to implant it is shed during menses with the endometrium.

[q] D3.1.17—Pregnancy testing by detection of human chorionic gonadotropin secretion 

Include the production of human chorionic gonadotropin (hCG) in the embryo or developing placenta and the use of monoclonal antibodies that bind to hCG.

[a] hCG functions:
• hCG is a peptide hormone released by the embryo during early weeks of pregnancy (first 4-18 weeks)
• hCG maintains the corpus luteum until the placenta is developed and able to function
• Once developed, the placenta is able to produce the required amount of progesterone and estrogen, so the embryo eventually reduces hCG production
• Reduction in levels of hCG/halting production causes corpus luteum degeneration
• If hCG levels are insufficient during early pregnancy, the endometrial lining may be too weak for blastocyst implantation or sustainment of developing embryo, possibly resulting in a miscarriage

[q] D3.1.17—Pregnancy testing by detection of human chorionic gonadotropin secretion 

Include the production of human chorionic gonadotropin (hCG) in the embryo or developing placenta and the use of monoclonal antibodies that bind to hCG.

[a] Pregnancy tests:
• Monoclonal antibodies are antibodies produced by hybridoma cells (a B cell and a cancer cell fused together)
• A monoclonal antibody that can bind to hCG is used in pregnancy tests, changing color if hCG attaches to it
• Present in the test is a control band with hCG previously attached to the antibody
• The remaining antibodies do not have hCG bound to them
• hCG is excreted in urine and so if a woman is pregnant, the empty antibodies will bind to hCG and change color, thus displaying two colored bands and indicating a positive result

[q] D3.1.18—Role of the placenta in foetal development inside the uterus 

Students are not required to know details of placental structure apart from the large surface area of the placental villi.

Students should understand which exchange processes occur in the placenta and that it allows the foetus to be retained in the uterus to a later stage of development than in mammals that do not develop a placenta.

[a]

The placenta completes development by 14-16 weeks and carries out the following functions:
• Provides nutrition through the umbilical vein
• Removes excreted material from the fetus through the umbilical artery
• Carries out respiration for the fetus by allowing exchange of gases through chorionic villi
• Secretes several hormones to stimulate fetal development
• Prevents mixing of maternal and fetal blood to avoid inducing an immune response

[q] D3.1.19—Hormonal control of pregnancy and childbirth D3.1.19—Hormonal control of pregnancy and childbirth 

Emphasize that the continuity of pregnancy is maintained by progesterone secretion initially from the corpus luteum and then from the placenta, whereas the changes during childbirth are triggered by a decrease in progesterone levels, allowing increases in oxytocin secretion due to positive feedback.

[a] During the first 8-12 weeks of pregnancy, the corpus luteum is responsible for the majority of progesterone secretion, and then the placenta takes charge of this function.
Towards the end of pregnancy, progesterone levels begin to drop but estrogen continues increasing, causing the endometrium to become more sensitive to contractions.

Uterine contractions stimulate the release of oxytocin from the pituitary, which in turn triggers more powerful contractions (positive feedback loop), ultimately resulting in parturition (childbirth).

[q] NOS: In early epidemiological studies, it was argued that women undergoing hormone replacement therapy (HRT) had reduced incidence of coronary heart disease (CHD) and this was deemed to be a cause-and-effect relationship.

Later randomized controlled trials showed that use of HRT led to a small increase in the risk of CHD.

The correlation between HRT and decreased incidence of CHD is not actually a cause-and-effect relationship.

HRT patients have a higher socioeconomic status, and this status has a causal relationship with lower risk of CHD.

[a] Correlation does not equal causation. 

[x] Exit text