Chromosomes and Chromosome Number

Human body cells have 46 chromosomes

Each parent contributes 23 chromosomes

Homologous chromosomes -one of two paired chromosomes, one from each parent

Homologous Chromosomes Same length Same centromere position Carry genes that control the same

inherited traits One came from Dad, & one came from Mom!

Why Meiosis?

An organism produces gametes to maintain the same number of chromosomes from generation to generation, and also introduce and maintain variation within the population.

We will discuss later, what happens when there is an error in meiosis. Mutations and Evolutionary events

Compare diploid and haploid number

Diploid: cell that contains two of each kind of chromosome (2N)

Body cells(somatic cells) are diploid

Haploid: cell with only one kind of chromosome (N)

Gametes (sex cells) are haploid

Meiosis I

The sexual life cycle in animals involves meiosis.

Meiosis produces gametes. When gametes combine in fertilization, the number of chromosomes is restored.

This is often called alternation of generations, the cycling between haploid and diploid organisms (cells)

Meiosis I Prophase I- each chromosome pairs with

its corresponding homologous chromosome to form a tetrad(4 chromatids)

The nuclear envelope breaks down. Spindles form.

*Crossing-over- chromosomes will switch some genes. This gives us genetic variation. Crossing-over is more common the farther it is located from the centromere. Remember map units in Sordaria

Crossing Over

Metaphase I Homologous pairs of chromosomes, still in their tetrad, line up together at the middle of the cell

Chromosome centromeres attach to spindle fibers.

Anaphase I

homologous chromosomes separate (the tetrad seperates) & move to opposite ends. (centromeres do not split)

Telophase I & Cytokinesis

spindle disappears and the cell divides

Meiosis I results in 2 daughter cells, each with the same number of chromosomes as the original cell however sister chromatids are still attached to each other

Meiosis IIDuring the second meiotic division, nothing is replicated, everything is going to be just divide

Prophase II

spindle apparatus reforms and the chromosomes condense, while the nuclear membrane dissolves again

Metaphase II chromosomes still attached at the centromere line up at the equator (middle)

Anaphase II sister chromatids are pulled apart at the centromere & move to opposite poles

Telophase II (includes cytokinesis) when the 2 daughter cells divide into 4 new daughter cells, each new daughter cell has 23 chromosomes. This is half the number of the original parent cell

Difference in meiosis

In females: oogenesis 1 mature egg, 3 polar

bodies which break down

Women born with all eggs they will have, meiosis I occurs before birth, meiosis II occurs once a month

Much larger Have all X

chromosomes Has no method of

movement

In males: spermatogenesis

4 mature sperm Males begin to

produce sperm after puberty, produced constantly until death; meiosis II occurs immediately after meiosis I

Much smaller than egg May have X or Y

chromosomes Have flagella to move

Meiosis/gamete production

Females have XX as their 23rd pair of chromosomes

Males have XY which are nonhomologous

Since all eggs are X, father determines the sex of the child since the sperm may be X or Y

Meiosis There are 2 divisions of meiosis- Meiosis I and Meiosis II. It must be this way in order to end up with the sex cells (sperm & egg) only having 23 chromosome.Meiosis occurs in the testes of the male

and is called spermatogenesis (4 viable sperm)

And in the ovaries of the female it is called oogenesis. ( produces 1 egg and 3 polar bodies)

23 + 23 = 46 chromosomes

Mitosis/Meiosis

MitosisCell division

producing identical somatic cells

Results in 2 daughter cells

One divisionDaughter cells

have same number of chromosomes (2N)

MeiosisCell division

producing unique gametic cells

Results in 4 daughter cells

Two divisionsDaughter cells

have half the number of chromosomes (N)

Cell division cell type # of daughter # of

cells chromosomes

Mitosis somatic(body) 2 46(diploid)

Meiosis gametes(sex) 4 23(haploid)

www.pbs.org

How cells divide:Mitosis vs. meiosis