Meiosis 11.4

Homologous chromosomes: pairs of chromosomes with genes for the same traits, but they can have different information about those traits

Homologous Chromosomes

Humans Humans have 46 chromosomes or 23 pairs

of homologous chromosomes. Somatic (body) cells contain 46

chromosomes. 46 is the diploid number for humans. - Diploid means that a cell has both

homologs for each chromosome Somatic cells reproduce by mitosis

½ the chromosomes in a cell come from Mom, ½ the chromosomes in a cell come from Dad

The Homolog’s

23 pairs of chromosomes

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Reproductive Cells Gametes(eggs and sperm) are called

haploid because they contain one set of chromosomes.

Eggs also called Ovum and sperm contain 23 chromosomes.

@Eggs and sperm are produced by meiosis- a process that reduces chromosomes by half@

What is Meiosis? A type of cell division where ONE diploid cell

(46 chromosomes) become FOUR haploid cells(23 chromosomes each).

The cells produced by meiosis are gametes (sex cells)

46

23 2323 23

Who does it? Plants Fungi Animals (including you)

◦ Males start meiosis at puberty.◦ Females start meiosis in utero.

Where does it occur? In your gonads

◦ Females- the ovary◦ Males-the testicles

Why does it occur? Genetic Variation

◦ Mixes genes from Mom and Dad to create a unique baby

@Reduces the chromosome number by half@

Ensures that each cell produced has one copy from each homologous pair of chromosomes

23 in sperm + 23 in egg unite (fertilization) leads to a 46 zygote(baby)

How does it occur? In two stages called Meiosis I and Meiosis II. Each stage contains 4 distinct phases prophase

metaphase anaphase

teleophasetelophase is followed by the division

of cytoplasm known as cytokinesis.

Homologous chromosomes join to form a tetrad

-chromosomes in the tetrad cross over and exchange genes

Prophase 1

@Crossing over produces new genetic combinations@ - It is another way to increase genetic diversity

Crossing Over

During Metaphase 1, the tetrads (groups of 4 homologous

chromosomes) line up down the center of the cell

- they assort independently -Chromosomes from each parent line up

independently of each other

Metaphase 1

Lesson Overview Meiosis

Anaphase I

During anaphase I, spindle fibers pull each homologous chromosome pair toward opposite ends of the cell.

 .

Lesson Overview Meiosis

Telophase I and CytokinesisDuring telophase I, a nuclear membrane forms around each cluster of chromosomes.

 Cytokinesis follows telophase I, forming two new cells.

Lesson Overview Meiosis

Meiosis II

@The two cells produced by meiosis I now enter a second meiotic division. @

Unlike the first division, neither cell goes through a round of chromosome replication before entering meiosis II.

Lesson Overview Meiosis

Prophase II

As the cells enter prophase II, their chromosomes—each consisting of two chromatids—become visible.

 The chromosomes do not pair to form tetrads, because the homologous pairs were already separated during meiosis I.

Lesson Overview Meiosis

Metaphase II

During metaphase of meiosis II, chromosomes line up in the middle of each cell.

Lesson Overview Meiosis

Anaphase II

As the cell enters anaphase, the paired chromatids separate.

Lesson Overview Meiosis

Telophase II, and Cytokinesis

These four daughter cells now contain the haploid number (N)—just two chromosomes each.

@Independent assortment increases genetic diversity@ - There are 23 pairs of homologous chromosomes

in humans - independent assortment ensures that there are 223 possible combinations

223= 8,388,608 combinations

- When two gametes join to form the zygote in fertilization the possible number of combinations is

223 X 223 = 64 trillion

Independent Assortment

Independent Assortment

Crossing over

What happens in prophase I that produces new genetic combinations?

23 chromosomes

How many chromosomes do human sperm/eggs have?

Mitosis creates 2 daughter cells with same amount of chromosomes

Meiosis creates 4 daughter cells with half amount of chromosomes

Meiosis goes through 2 cycles Mitosis occurs in somatic cells, Meiosis in

sex cells

How is mitosis different from meiosis?