Question
Describe the process of crossing over.
Explain the reason for linked genes not following the pattern of inheritance discovered by Mendel.
▶️Answer/Explanation
Markscheme
a. occurs during prophase I/during meiosis
b. homologous chromosomes form bivalents/pair up
c. breakage and rejoining of chromatids
d. exchange «of DNA/alleles» between non-sister chromatids/homologous chromosomes
[Max 2 Marks]
a. «linked genes are» on the same chromosome
b. Mendel’s genes were on different chromosomes
c. linked genes are inherited together
OR
no independent assortment
d. «linked genes» only separated by crossing over
OR
fewer recombinants than with unlinked genes
Reject sex-linkage
Question
Cells go through a repeating cycle of events in growth regions such as plant root tips and animal embryos. Outline this cell cycle.
Draw a labelled diagram of the formation of a chiasma by crossing over.
Explain the control of gene expression in eukaryotes.
▶️Answer/Explanation
Markscheme
a. mitosis is the division of a nucleus to produce two genetically identical daughter nuclei
b. consists of four phases: prophase, metaphase, anaphase, telophase
c. cytokinesis occurs after mitosis
d. interphase is the metabolically active phase between cell divisions OWTTE
e. the interphase consists of the S phase, G1 and G2
f. DNA replicates in the S phase
g. cell growth
OR
preparation for mitosis
OR
duplication of organelles in G1 and G2
a. «crossing over/chiasmata shown between» homologous chromosomes
b. centromere drawn and labelled
c. single strand break «SSB»/DNA cut between homologous chromosomes
d. non-sister chromatids labelled
OR
sister chromatids labelled
e. chiasma between homologous chromosomes labelled «shown forming after SSB»
Homologous chromosomes must be labelled and correctly drawn.
It is likely that more than one diagram will need to be included to demonstrate the stages.
a. mRNA conveys genetic information from DNA to the ribosomes «where it guides polypeptide production»
b. gene expression requires the production of specific mRNA «through transcription»
c. most genes are turned off/not being transcribed at any one time/regulated
OR
some genes are only expressed at certain times
d. some genes are only expressed in certain cells/tissues
OR
«cell» differentiation involves changes in gene expression
e. transcription factors/proteins can increase/decrease transcription
f. hormones/chemical environment of cell can affect gene expression
g. example of cell environment
eg: auxin/insulin/cytoplasmic gradient in embryo
h. transcription factors/proteins may prevent or enhance the binding of RNA polymerase
i. nucleosomes limit access of transcription factors to DNA/regulate gene expression/transcription
OR
activate or silence genes
j. DNA methylation/acetylation appears to control gene expression «as epigenetic factor»
OR
methylated genes are silenced
k. «some» DNA methylation patterns are inherited
l. introns may contain positive or negative gene regulators
OR
gene expression can be regulated by post-transcriptional modification/splicing/mRNA processing
Question
Outline the processes that occur during the first division of meiosis.
Prior to cell division, chromosomes replicate. Explain the process of DNA replication in prokaryotes.
Outline outcomes of the human genome project.
▶️Answer/Explanation
Markscheme
Remember, up to TWO “quality of construction” marks per essay.
a. (consists of) prophase, metaphase, anaphase and telophase;
b. chromosome number halved/reduced/(diploid) to haploid;
c. homologous chromosomes pair up/form a bivalent/synapsis in prophase;
d. crossing over between non-sister chromatids/chromatids of different homologues;
e. nuclear envelope breaks down (at end of prophase/start of metaphase);
f. tetrads/bivalents/homologous pairs move to/align on equator/cell centre/on metaphase plate in metaphase; (accept homologous chromosomes without pairs if pairing has already been described)
g. attachment of spindle fibres/microtubules to centromeres/kinetochores;
h. (homologous) chromosomes separate/pulled to opposite poles in anaphase;
i. nuclear envelopes reform/do not reform (because of meiosis II) in telophase;
Accept the above points in a series of annotated diagrams. Reject answers with single chromatids forming pairs in metaphase or separating or moving to opposite poles in anaphase.
Remember, up to TWO “quality of construction” marks per essay.
a. DNA replication is semi-conservative;
b. each (molecule formed) has one new strand and one from parent molecule;
c. helicase uncoils DNA;
d. helicase separates the two strands by breaking hydrogen bonds between bases; (reject unzips as an alternative to uncoils but accept as alternative to separates if breakage of hydrogen bonds is included)
e. RNA primase adds primer / primase adds (short) length of RNA;
f. DNA polymerase III binds to/starts at (RNA) primer;
g. DNA polymerase (III) adds nucleotides/bases in a 5’ → 3’ direction;
h. bases according to complementary base pairing / A–T and C–G;
i. (leading strand) built up continuously (towards the replication fork);
j. (lagging strand) built up in pieces/short lengths/Okazaki fragments;
k. DNA polymerase I removes RNA/primers and replaces them with DNA;
l. ligase seals gaps between nucleotides/fragments/makes sugar-phosphate bonds;
m. nucleoside triphosphates provide the energy to add nucleotides;
Accept the above points in annotated diagrams.
Remember, up to TWO “quality of construction” marks per essay.
a. complete human DNA/chromosomes sequenced;
b. identification of all human genes / find position/map (all) human genes;
c. find/discover protein structures/functions;
d. find evidence for evolutionary relationships/human origins/ancestors;
e. find mutations/base substitutions/single nucleotide polymorphisms;
f. find genes causing/increasing chance of/develop test for/screen for diseases;
g. develop new drugs (based on base sequences) / new gene therapies;
h. tailor medication to individual genetic variation / pharmacogenomics;
i. promote international co-operation/global endeavours;
Question
The probability of extinction of a species increases if the population is small with low genetic variation.
State two processes that cause population size to decrease.
Explain how meiosis promotes variation.
▶️Answer/Explanation
Markscheme
a. mortality / fatal disease / predation / competition / other cause of death;
b. emigration;
a. (in prophase I) crossing over/chiasmata formation (between homologous chromosomes);
b. random alignment of homologues/bivalents in metaphase I / independent assortment of homologues / chromosomes;
c. second division of meiosis separates alleles further;
d. combinations of alleles in gametes is unlimited/2n;
Question
The diagram below shows a small portion of the tissue in a transverse section of a testis.
Outline the process of in vitro fertilization (IVF).
Identify the cell labelled X.
Outline the function of this cell.
Explain how meiosis results in genetic variation in gametes.
▶️Answer/Explanation
Markscheme
mother receives hormone treatment/FSH to stimulate egg development;
eggs and sperm collected/harvested / eggs taken from ovary;
egg fertilized outside the body/in a dish/in a lab;
develops into embryo;
embryo(s) implanted (artificially) in mother’s body/uterus;
Do not accept egg/fertilized egg/zygote implanted.
Sertoli cell / nurse cell
nourishes maturing sperm(atozoa) / protects sperm from lymphocytes
crossing over in prophase 1/between chromatids;
random orientation of bivalents/homologous pairs in metaphase 1;
random orientation of chromatids/chromosomes in metaphase 2;