Process of Meiosis

What is Meiosis? • Form of nuclear division

• Creates FOUR haploid cells from ONE diploid cell

– Diploid=2N=46 chromosomes (somatic cell)

– Haploid=1N=23 chromosomes (sex cells)

• Two rounds of cell division=Meiosis I & Meiosis II

– Each round=four phases, similar to phases in Mitosis

What is Meiosis? • reduces chromosome number

• creates genetic diversity

• Takes place only at certain times in organism’s life cycle

• Involved in sexual reproduction

– Makes sperm and egg cells

• Because human cells have 23 chromosomes, meiosis may result in 223, or 8,388,608, possible combinations of chromosomes.

Review!! • This is a

chromosome…

• Each chromosome is a long, continuous thread of DNA

• DNA stores genetic information in all organisms – DNA determines your

physical appearance

Review!! • DNA plus proteins is called

chromatin.

• One half of a duplicated chromosome is a chromatid.

• Sister chromatids are held together at the centromere.

• Telomeres protect DNA and do NOT include genes.

telomere

Paired Up…

• Each pair of chromosomes is known as a homologous pair.

• Homologous means “having the same structure”

Homologous Chromosomes • Two separate

chromosomes

– one from mother, one from father

• Very similar to each other

– Same length

– Carry the same genes

• Are NOT copies of each other

• Divided in Meiosis I

Chromatids • Each half of a

duplicated chromosome

• Sister chromatids=two chromatids together

• Divided in Meiosis II

Homologous Chromosomes vs. Sister Chromatids

homologous chromosomes

sister

chromatids

sister

chromatids

2 Types of Chromosomes

Autosomes

&

Sex Chromosomes

Autosomes • Pairs 1-22 are your

Autosomes

• These chromosomes contain genes for characteristics that are NOT related to the sex of an organism

Sex Chromosomes • Sex chromosomes make

up the 23rd pair

• Sex chromosomes control the development of sexual characteristics

– Determine male or female

• Humans have two sex chromosomes: X and Y

• XX=female

• XY=male

• NOT Homologous

X and Y Chromosomes

• X chromosome

– Larger sex chromosome

– Carries the most genes

– Determines characteristics other than sex of the organism

• Y chromosome

– Smallest sex chromosome

– Carries the fewest genes

– Determines the development of males

Two Types of Cells

• Your body is made up of Somatic cells and Gametes.

• These are the two major groups of specialized cells in your body.

Somatic Cells

• Also known as body cells

• Make up most of your body tissues and organs – For example: your spleen, kidneys, and eyeballs

are made entirely of body cells.

• DNA in body cells are NOT passed on to your children.

Gametes • Sex cells

– Ova, or eggs, in the female

– Spermatozoa, or sperm cells, in the male

• DNA in your gametes CAN be passed on to your children

• Gametes are formed when germ cells (cells found in the ovaries or the testes) undergo meiosis

Diploid Cells • Somatic, body, cells are

diploid

• Diploid means a cell has two copies of each chromosome: one copy from the mother, and one from the father.

• Diploid cells can be represented as 2n.

• In humans, the diploid chromosome number is 46.

Haploid Cells

• Gametes are haploid cells

• Haploid means that a cell has only one copy of each chromosome

• Haploid cells are represented as n.

Meiosis I • Meiosis I occurs after DNA has

been replicated.

• Meiosis I divides homologous

chromosomes in four phases.

–Prophase I

–Metaphase I

–Anaphase I

–Telophase I

Meiosis I-Prophase I • Nuclear membrane breaks down

• Centrosomes and centrioles move to opposite sides if the cell

• Spindle fibers start to form

• Homologous chromosomes pair up, including sex chromosomes

Meiosis I-Metaphase I • Spindle fibers randomly align the

homologous chromosomes along the cell equator.

• Meta=Middle – Each side has chromosomes from both parents

• This arrangement mixes up the chromosomal combinations

• Helps create and maintain genetic diversity

Meiosis I=Anaphase I • Paired homologous chromosomes

separate from each other and move toward opposite sides of the cell.

• Sister chromatids remain attached.

Meiosis I=Telophase I • Nuclear membrane forms again

• Spindle fibers disassemble

• Cell undergoes cytokinesis=cell completely divides

• Results in two cells that are genetically different – Each have a unique combination of 23 duplicated

chromosomes coming from both parents

Meiosis II • DNA is not replicated between meiosis I and

meiosis II.

• Meiosis II divides sister chromatids in four phases.

– Prophase II

– Metaphase II

– Anaphase II

– Telophase II

• Process applies to BOTH cells produced in Meiosis I

• Results in undoubled chromosomes

Meiosis II=Prophase II • Nuclear membrane breaks down

• Centrosomes and centrioles move to opposite sides of the cell

• Spindle fibers assemble

Meiosis II=Metaphase II • Spindle fibers align the 23

chromosomes at the cell equator.

• Meta=Middle

• Each chromosome still has two sister chromatids at this stage.

Meiosis II=Anaphase II

•Spindle fibers pull the sister chromatids apart from one another and move them to opposite sides of the cell.

Meiosis II=Telophase II • Nuclear membranes form again

around the chromosomes

• Spindle fibers break apart

• Cell undergoes cytokinesis

• Results in FOUR haploid cells – Each with a combination of genes from both the

mother and the father

Next step for haploid cells…

• Haploid cells are the end result of meiosis.

• These cells must undergo changes to become mature gametes before they are ready for fertilization.

Gametogenesis • Production of gametes

–Gametes=sex cells

• ova, or eggs in the female

• spermatazoa, or sperm cells in males

• Gametogenesis differs between females and males

Gametogenesis in Males

– Sperm become

streamlined and

motile.

– Sperm primarily

contribute DNA to

an embryo.

Gametogenesis in Females

– Eggs contribute DNA,

cytoplasm, and

organelles to an embryo.

– During meiosis, the egg

gets most of the

contents; the other cells

form polar bodies=cells

with little more than DNA

that are eventually

broken down.

Meiosis vs Mitosis Meiosis

• Two cell divisions

• Homologous chromosomes pair up long the equator

• In anaphase I, sister chromatids are still together.

• Results in haploid cells that are genetically different.

Mitosis • One cell division

• Homologous chromosomes NEVER pair up

• In anaphase, sister chromatids separate.

• Results in diploid cells that are genetically identical.