MEIOSIS

MEIOSIS

KEY CONCEPT

Gametes have half the number of chromosomes

that body cells have.

MEIOSIS : CELL TYPES

You have Body cells and Gametes

Body cells are also called somatic cells.

Germ cells develop into gametes. Germ cells are located in the ovaries and testes.

Gametes are sex cells: egg and sperm.

Gametes have DNA that can be passed to offspring.

MEIOSIS : ALL ABOUT CHROMOSOMES

• Your body cells have 23 pairs of

chromosomes.

– Homologous pairs of

chromosomes have the same

structure and genetic

material.

– For each homologous pair,

one chromosome comes from

each parent.

• Chromosome pairs 1-22 are

autosomes.

• Sex chromosomes, X and Y,

determine gender in mammals.

Your cells have autosomes & sex chromosomes

MEIOSIS : DIPLOID AND HAPLOID

Diploid (2n) cells have two copies of every chromosome.

Body cells are diploid.

Half the chromosomes come from each parent.

In humans this means we have 2 sets of 23

chromosomes (total of 46)

MEIOSIS : DIPLOID AND HAPLOID

– Gametes are haploid.

– In humans gametes have 22 autosomes and 1 sex

chromosome. (23 total)

Haploid (n) cells have one copy of every chromosome.

MITOSIS REVIEW

• Mitosis and meiosis are types of nuclear

division that make different types of cells with

different number of chromosomes.

• Mitosis makes more diploid cells.

• Number of Chromosomes in parent is Equal to

the number of chromosomes in offspring.

Goal of Mitosis is Chromosome number must be

maintained.

INTRODUCTION TO MEIOSIS

Meiosis makes haploid cells from diploid cells.

This happens because cells go through TWO

rounds of division during meiosis.

Meiosis reduces chromosome number and creates genetic

diversity.

Meiosis Occurs in Sex cells

Meiosis Produces gametes.

MEIOSIS DETAIL KEY CONCEPT

During meiosis, diploid cells undergo two cell

divisions that result in haploid cells.

Meiosis I and Meiosis II

Meiosis I and meiosis II each have four phases, similar to

those in mitosis.

Biggest difference is the how the DNA organizes for each

division.

In Meiosis I Pairs of homologous chromosomes form tetrads.

homologous chromosomes

sister

chromatids

sister

chromatids

MEIOSIS DETAIL KEY CONCEPT

During meiosis, diploid cells undergo two cell

divisions that result in haploid cells.

Meiosis I pairs of homologous chromosomes (tetrads)

separate.

Homologous chromosomes are similar but not identical

genetically

In Meiosis II SISTER CHROMATIDS separate.

MEIOSIS I DETAILS

• Meiosis I occurs after DNA has been replicated.

• Meiosis I divides homologous chromosomes in four phases.

Prophase I

Metaphase I

Anaphase I

Telophase I

PROPHASE I

Each chromosome pairs with its corresponding homologous

chromosome to form a tetrad.

There are 4 chromosomes in a tetrad.

The pairing of homologous chromosomes is the key to

understanding meiosis.

Crossing-over may occur here

Crossing-over is when chromosomes overlap and exchange

portions of their chromatids.

CROSSING OVER !!!!!

They do the genetic TANGO!

PROPHASE I (PICTURE)

Homologous Chromosomes

(called Tetrads) are held together

by a synapse

METAPHASE I

Tetrads line up along the equator

Spindle fibers attach to the

centromeres of each chromosome in

the tetrad.

ANAPHASE I

The cell begins lengthening.

The spindle fibers pull the homologous

chromosomes apart and toward opposite

ends of the cell.

Sister chromatids attached at

centromeres move together

Disjunction Occurs – process of

separation

There are ½ as many chromosomes as

in the original cell but the chromosome is

double stranded.

TELOPHASE I

Movement of homologous chromosomes continues until there is

a haploid set at each pole

Each chromosome = linked sister chromatids

At each pole, now, there is a complete haploid set of

chromosomes (but each chromosome still has two sister

chromatids).

Nuclear membranes reforms.

A cleavage furrow appears.

The cell separates into 2 daughter cells.

TELOPHASE I

IMPORTANT NOTE

IN BETWEEN MEIOSIS I and

MEIOSIS II

there is no INTERPHASE!!!!

PROPHASE II

• Meiosis I results in two

haploid (N) cells.

• Each cell has half the

number of

chromosomes as the

original cell.

• Spindle Reforms

• Sister Chromatids are

present

METAPHASE II

The Sister Chromatids

line up .along the

equator (middle) of the

cell similar to

metaphase in mitosis

ANAPHASE II

Sister chromatids

separate and move to

opposite ends of the

cell.

TELOPHASE II

• Nuclear membranes form

around each set of

chromosomes at opposite ends

of the cell

• Spindle fibers break apart

• Cell undergoes cytokinesis

• End result is four HAPLOID cells

with genetically unique

information.

COMPARE AND CONTRAST

–Meiosis has two cell divisions while

mitosis has one.

– In mitosis, homologous chromosomes

never pair up.

–Meiosis results in haploid cells; mitosis

results in diploid cells.

• Meiosis differs from mitosis in significant

ways.

GAMETOGENESIS

In Males:

SPERMATOGENESIS

– Sperm become streamlined and motile.

– Sperm primarily contribute half of the DNA to an

embryo.

Haploid cells develop into mature gametes.

•Gametogenesis is the production of gametes.

•Gametogenesis differs between females and

males.

GAMETOGENESIS

In Females:

Oogenesis

– Eggs contribute half of DNA, all of the cytoplasm,

and all organelles to an embryo.

– During meiosis, the egg gets most of the

contents; the other cells form polar bodies.

– So only one functional gamete per cycle of

meiosis