GEN 4.2 Sexual Reproduction (Meiosis)- Pre AP Biology Study Notes - New Syllabus.
GEN 4.2 Sexual Reproduction (Meiosis)- Pre AP Biology Study Notes
GEN 4.2 Sexual Reproduction (Meiosis)- Pre AP Biology Study Notes – New Syllabus.
LEARNING OBJECTIVE
GEN 4.2(a) Explain why reduction division must occur to produce gametes.
GEN 4.2(b) Explain how meiotic cellular division followed by fertilization leads to genetic diversity within a population.
GEN 4.2(c) Create and/or use models to explain how chromosome number is halved during meiosis.
Key Concepts:
GEN 4.2.1 Some unicellular and most eukaryotic organisms reproduce sexually, requiring a process called meiosis that results in genetic variation in the population.
a. Meiotic division requires two distinct nuclear divisions in order to reduce one diploid (2N) cell into four haploid (N) cells.
During the first division in meiosis, homologous chromosomes pair together in a tetrad and crossing-over occurs, which increases genetic variation.
At the end of the first division (meiosis I), homologous chromosomes are separated and two daughter cells are formed.
At the end of the second meiotic division (meiosis II), the two cells are separated into four genetically diverse haploid cells, which in animals differentiate into gametes.
b. Sexual reproduction occurs via fertilization, when sperm and egg gametes fuse and form a zygote, restoring the diploid number of chromosomes.
Why Reduction Division Must Occur to Produce Gametes
🌿 Introduction
In sexual reproduction, organisms produce special reproductive cells called gametes.
Gametes include:
- Sperm
- Egg
These cells are different from body (somatic) cells because they contain half the number of chromosomes.
The process that reduces chromosome number by half is called: Reduction division, which occurs during meiosis.
🧠 Step 1 – Understanding Chromosome Number
Most body cells are:
- Diploid (2N)
- Contain two sets of chromosomes
- One set inherited from the mother
- One set inherited from the father
Example:
If a species has 6 chromosomes in body cells: 2N = 6
Each parent contributes 3 chromosomes.
This diploid number must remain constant for the species.
🧬 Step 2 – What Happens During Fertilization?
Fertilization occurs when:
Sperm + Egg → Zygote
Each gamete contributes chromosomes to the offspring.
If each gamete were diploid (2N), then:
2N + 2N = 4N
The chromosome number would double in every generation.
Over generations, it would become:
2N → 4N → 8N → 16N
This would cause:
- Chromosome imbalance
- Genetic instability
- Nonviable offspring
So, gametes must contain only half the chromosome number.
🧪 Step 3 – Purpose of Reduction Division
Reduction division ensures that:
- Diploid (2N) parent cells produce haploid (N) gametes
- Each gamete contains only one set of chromosomes
Then during fertilization:
N + N = 2N
This restores the correct diploid number in the zygote.
Therefore:
Reduction division prevents chromosome number from increasing every generation.
🧬 Step 4 – How Reduction Occurs in Meiosis
Meiosis includes two divisions:
- Meiosis I
- Meiosis II
The actual reduction happens in Meiosis I.
During Meiosis I:
- Homologous chromosome pairs separate
- Each new cell receives only one chromosome from each pair
This reduces chromosome number from:
2N → N
After Meiosis II:
- Sister chromatids separate
- Four haploid cells are formed
Each gamete contains one set of chromosomes.
🧠 Step 5 – Why This Is Essential for Sexual Reproduction
Sexual reproduction requires:
- Production of haploid gametes
- Fusion of two gametes
- Restoration of diploid chromosome number
Without reduction division:
- Chromosome number would double
- Species chromosome number would not remain stable
- Genetic balance would be disrupted
So, reduction division maintains:
- Chromosome stability across generations
- Correct gene dosage
- Proper organism development
📊 Simple Numerical Example
Let’s say a species has:
2N = 8
Without reduction:
Gametes would have 8 chromosomes.
Fertilization would produce: 16 chromosomes.
But with reduction division:
Meiosis produces gametes with:
N = 4
Then fertilization restores:
4 + 4 = 8
Chromosome number remains constant.
📦 Quick Recap
Body cells = Diploid (2N)
Gametes = Haploid (N)
Reduction division occurs in Meiosis I
Prevents chromosome doubling
N + N = 2N after fertilization
Maintains chromosome stability
How Meiosis Followed by Fertilization Leads to Genetic Diversity
🌿 Introduction
Sexual reproduction produces genetically diverse offspring.
This diversity does not happen randomly – it is generated through:
- Meiosis
- Fertilization
Meiosis creates genetically different gametes.
Fertilization randomly combines those gametes.
Together, these processes increase genetic variation within a population.
🧠 Step 1 – What Is Genetic Diversity?
Genetic diversity refers to:
- Variation in DNA sequences among individuals
- Different combinations of alleles in a population
- Differences in traits among individuals
Higher genetic diversity means:
- Greater variation
- Greater ability for a population to survive environmental change
Now let’s see how meiosis creates that variation.
🧬 Step 2 – Genetic Variation During Meiosis
Meiosis consists of two divisions:
- Meiosis I
- Meiosis II
The major sources of variation occur during Meiosis I.
A) Homologous Chromosome Pairing
During Prophase I:
- Homologous chromosomes pair together
- Each pair forms a structure called a tetrad
- Each tetrad contains four chromatids
This pairing sets the stage for variation.
B) Crossing Over
While homologous chromosomes are paired:
- They exchange segments of DNA
- This exchange is called crossing-over
Crossing-over:
- Swaps genetic material between homologous chromosomes
- Produces new combinations of alleles
- Creates recombinant chromosomes
This increases genetic diversity because:
The chromatids after crossing-over are not identical to the original parental chromosomes.
C) Separation of Homologous Chromosomes
During Meiosis I:
- Homologous chromosomes separate
- Each daughter cell receives one chromosome from each pair
Because of random orientation during separation:
Each gamete receives a different combination of maternal and paternal chromosomes.
So even without crossing-over, chromosome combinations vary.
D) Meiosis II
During Meiosis II:
- Sister chromatids separate
- Four haploid cells are formed
Each of the four gametes is genetically unique due to earlier recombination and separation.
🧠 Step 3 – How Fertilization Adds More Diversity
After meiosis:
- Each parent produces many genetically different gametes
Fertilization occurs when:
One sperm fuse with one egg
A diploid zygote form
Fertilization is random:
- Any sperm can fertilize any egg
- Each sperm and egg are genetically unique
So the resulting zygote receives:
- A random set of chromosomes from mother
- A random set from father
This creates even more genetic variation.
🧬 Step 4 – Combined Effect on Population
Meiosis increases diversity by:
- Crossing-over
- Random separation of homologous chromosomes
Fertilization increases diversity by:
- Random combination of two unique gametes
Together, they create enormous genetic variation within a population.
No two offspring (except identical twins) are genetically identical.
🧠 Step 5 – Why This Diversity Matters
Genetic diversity is important because:
- It allows adaptation to environmental changes
- It increases survival chances
- It reduces the risk that one disease wipes out entire population
Populations with higher genetic diversity are more likely to survive changing conditions.
📊 Summary Table
| Process | How It Increases Diversity |
|---|---|
| Crossing-over | Creates new allele combinations |
| Separation of homologous chromosomes | Random distribution of chromosomes |
| Formation of gametes | Four unique haploid cells |
| Fertilization | Random fusion of gametes |
📦 Quick Recap
Meiosis creates variation
Crossing-over → new allele combinations
Random chromosome separation
Four genetically unique gametes
Fertilization randomly combines gametes
Result → high genetic diversity in population
How Chromosome Number Is Halved During Meiosis
🌿 Introduction
Body cells are diploid (2N), meaning they contain:
- Two sets of chromosomes
- One set from each parent
Gametes must be haploid (N), meaning they contain:
- Only one set of chromosomes
To reduce chromosome number from 2N to N, a special type of cell division called meiosis occurs.
A model of meiosis must clearly show how this reduction happens.
🧠 Step 1 – Start with a Diploid Cell (2N)
Imagine a cell with:
- Two homologous chromosome pairs
Example model:
Chromosome Pair 1:
Red chromosome (from mother)
Blue chromosome (from father)
Chromosome Pair 2:
Red chromosome
Blue chromosome
This cell is diploid (2N) because it has two of each chromosome type.
🧬 Step 2 – DNA Replication (Still 2N)
Before meiosis begins:
- Each chromosome replicates
- Each chromosome now consists of two sister chromatids
Important:
Even though DNA is duplicated, chromosome number is still 2N.
Why?
Because sister chromatids are still attached and count as one chromosome.
So, at this stage:
Chromosome number has NOT changed.
🧬 Step 3 – Meiosis I (Reduction Division)
This is the most important step for halving chromosome number.
During Meiosis I:
- Homologous chromosomes pair together
- Crossing-over may occur
- Homologous chromosomes separate
Key point: Homologous chromosomes separate — not sister chromatids.
Each new cell receives:
- Only one chromosome from each homologous pair
So, if the original cell had 2 copies of each chromosome:
After Meiosis I:
Each daughter cell has only 1 copy of each chromosome.
Chromosome number changes from:
2N → N
This is called reduction division.
🧬 Step 4 – Meiosis II (No Further Reduction)
During Meiosis II:
- Sister chromatids separate
- Similar to mitosis
Important:
Chromosome number does NOT change in Meiosis II.
It remains:
N → N
At the end:
- Four haploid cells are produced
Each has:
- One set of chromosomes
- Half the original number
🧠 Step 5 – Why Chromosome Number Is Halved
Chromosome number is halved because:
- Homologous chromosomes separate in Meiosis I
- Each cell receives only one chromosome from each pair
Since diploid cells have two of each chromosome:
Removing one of each pair results in haploid cells.
This ensures:
Gametes have half the chromosome number.
🧬 Step 6 – Model Summary
Let’s use numbers.
Suppose:
Diploid cell (2N) = 6 chromosomes
After DNA replication:
Still 6 chromosomes (each duplicated)
After Meiosis I:
Each daughter cell has 3 chromosomes (N)
After Meiosis II:
Four cells each have 3 chromosomes (N)
Chromosome number successfully reduced.
📊 Model Table of Chromosome Number
| Stage | Chromosome Number |
|---|---|
| Parent cell | 2N |
| After replication | 2N |
| After Meiosis I | N |
| After Meiosis II | N |
📦 Quick Recap
Start with diploid (2N) cell
DNA replicates (still 2N)
Meiosis I separates homologous chromosomes
Chromosome number reduced to N
Meiosis II separates sister chromatids
Four haploid cells produced