Meiosis APBio

Meiosis

Meiosis is the process of gamete formation

This type of cell division reduces the number of sets of chromosomes.

Each sperm and egg are haploid (n).  

That means the zygote will be 2n = diploid

Humans have a total of 46 chromosomes

2 sets of 23 chromosomes.

One parent gives one copy of 23 chromosomes.

One parent gives another copy of the same 23 chromosomes.

The diploid number is 46 and the haploid number is 23.

n = 23

2n = 46

Haploid vs. Diploid

When a cell has two complete sets of chromosomes, they are diploid.  
They have two DIFFERENT versions of each chromosome – one version/set from each parent.

When a cell only has one complete set of chromosomes, they are haploid.

They have one copy of each chromosome 

When two haploid cells combine at fertilization, they form a diploid zygote. 

Homologous chromosomes are the same chromosomes with just different alleles.  For example, Chromosome #1  has certain genes on it (here genes A and B).  One parent passed along the A&B alleles and one parent passed along the a&b alleles.  It’s different versions of the same chromosome. 

Sister chromatids are exact copies of the same chromosome.  Here, both chromosomes are A&B because they are just copies of each other formed during DNA replication.

Nonhomologous chromosomes are two different chromosomes entirely.  Here, one chromosome has A&B and one has C&D.  They do not have the same genes.  For example, this would be like Chromosome #1 vs. Chromosome #2.

Meiosis is the process of taking a diploid, somatic cell and forming a haploid gametic cell.

Somatic cells = body cells and are diploid

Gametic cells = sex cells and are haploid

Meiosis begins the same way as mitosis.

G₁ = cell gets the signal that gametes need to be formed

S = DNA replicates

G₂ = cell prepares to divide

MEIOSIS I

• Meiosis I has several key events

  • Independent Assortment

  • Crossing Over

• The product is 2 haploid cells. Two copies of one set of information.

MEIOSIS II

• The cell immediately moves into the next division. 

• 4 haploid cells are produced.  Each cell has one copy of the one set of information.

During Meiosis I, the homologous chromosomes line up next to each other  (horizontally).

Nonhomologous chromosomes line up vertically.

These sets of nonhomologous chromosomes line up independently of each other.  This is why Meiosis I has two different options.

This is called Independent Assortment

When the homologous chromosomes line up next to each other, nonsister chromatids swap information.

This crossing over produces chromosomes with a different combination of alleles than either parent. 

Two main sources of variation are

1. Crossing Over

2. Independent Assortment

When the chromosomes line up for the first division, Mitosis lines up 1x1 whereas Meiosis pairs homologous chromosomes together.  This allows the 2 cells at the end of mitosis to be identical and the two cells at the end of Meiosis to be different and haploid.