Cell Division : Mitosis and Meiosis

Chapter 8

Asexual vs. Sexual reproduction Asexual reproduction – new

organisms/cells are genetically identical to parent cells/organisms

Sexual reproduction – offspring have a combination of genes from both parents.

Asexual Reproduction

Budding - plants Vegetative propagation -plants Binary fission -bacteria One cell dividing to become two –

mitosis

Hermaphroditic organisms are NOT asexual!!!

Cells only come from other cells To make more cells they must divide

Mitosis – one cell divides to make two genetically identical cells for asexual reproduction and growth and repair.

Meiosis – One cell divides twice to create 4 cells that are not genetically identical. These cells are eggs or sperm (gametes)

Cell Cycle

Interphase – cell does normal job, grows, and duplicates genetic material to prep for division, 90% of cell cycle G1, S, G2

Mitosis – division of genetic material

Cytokinesis- division of cytoplasm, usually occurs after mitosis

Cell Cycle

Interphase G1- first gap, growth, normal function,

makes proteins etc S phase- synthesis, cell copies DNA to

prepare for division G2- second gap, growth and final

preparation for division

Eukaryotic cells have complex genomes DNA in a non-dividing cell is

disorganized Called chromatin When a cell prepares to divide the

chromatin (DNA and proteins called histones) coil into chromosomes

Each chromosome is made of two identical halves called sister chromatids

Sister chromatids are connected by a centromere

Cytokinesis

In animal cells a cleavage furrow develops. The cell pinches from the outside

In plant cells a cell plate forms. A new cell wall develops from the inside and works out to the borders

Factors that affect cell division Must be attached to a surface Will stop dividing when they touch

each other- density dependent inhibition

Secretion of proteins called growth factors

Three key checkpoints in the cell cycle G1 G2 M-

Cancer

Do not respond normally to checkpoints in the cell cycle

Excessive cell division, wasting of cellular resources, form masses called tumors Benign- stays in original location Malignant- spreads from original

location- metastasis

Mitosis Summary 8.11 Occurs in somatic or non-sex cells Creates two genetically identical

cells from one cell The cells created are diploid (2n)–

having a full set of chromosomes. Used for repair and growth

Human life cycle

Diploid cells in ovaries and testes divide by meiosis to create haploid gametes.

Gametes are egg and sperm. Haploid cells have a half set of

chromosomes Haploid gametes combine to form

a diploid zygote Diploid zygotes divide by mitosis to

form Multicellular organisms

Homologous chromosomes 8.12 Humans have 22 pairs of

autosomes – non-sex chromosomes and one pair of sex chromosomes

Females have two X chromosomes Males have an X and a Y

chromosome

Meiosis- steps

Two divisions Meiosis I; meiosis II Major differences

Prophase I- homologous chromosomes pair into a tetrad. Sometimes the homologous pairs exchange small pieces – crossing over

Synapsis – the exchange of pieces

Meiosis increases genetic variation 1. Crossing over – creates

chromosomes that are mosaics of both maternal and paternal genes.

Is a random event and doesn’t happen for every meiotic cycle

Occurs in prophase I Called genetic recombination

Meiosis- increases genetic variation

2. Law Of Segregation Each haploid cell inherits only one

chromosome from each parent. Homologous chromosomes carry genes for the same trait but not necessarily the same gene – Law of Segregation.

The physical process that underlies this law occurs in Anaphase I

Genetic variation

3. Law of Independent assortment

Each homologous chromosome pair lines up side by side and separates randomly in metaphase I.

Creates many different random combinations of chromosomes in each egg or sperm

Different possibilities = 2 to the n power, where n= the haploid number

Genetic variation

4. Random fertilization increases genetic variability in a species

Why is variation needed?

Organisms with very similar genomes have no raw material for natural selection should the environment abruptly change

Genes

Are carried on chromosomes Each trait in your body is determined

by at least two genes on two different homologous chromosomes – one from dad, and one from mom

Mistakes occur in meiosis Crossing over Separation in anaphase I and/or

anaphase II

Nonreciprocal crossovers- exchange of pieces of DNA of different sizes

Inversion- pieces of chromosomes are reattached incorrectly

Non homologous crossovers Failure to separate –

nondisjunction

Crossing over mistakes

Chromosomes missing parts due to non reciprocal cross overs have deletions

Chromosomes with too much info have duplications.

Fragments reattached in the wrong sequence are inversions

Translocations occur when non-homologous chromosomes cross-over

Disorders caused by CO mistakesCri du chat- deletion on # 5, “cry of

the cat” in babiesDown Syndrome can be caused

when #21 attaches to another chromosomes

Chronic myelogenous Leukemia (CML)- non homologous cross over activates a cancer gene. Call it the “Philadelphia chromosome”

Nondisjunction

Can occur in anaphase I or anaphase II

Results in organisms with the wrong number of chromosomes for their species – aneuploid individuals

Most situations with missing or extra chromosomes lead to spontaneous miscarriage

Extra Autosomes

Trisomy Trisomy #21 – Down’s Syndrome Trisomy- # 18- Edward’s Syndrome Trisomy #13- Patau’s Syndrome

Extra or missing sex chromosomes Individual with only one X-

Turner Syndrome, female, sterile, can have other physical traits XO

Individual with two X chromosomes and one Y, - Klinefelter’s Syndrome male, sterile, some female characteristics, taller than normal XXY

Prenatal diagnosis of defectsAmniocentesis – performed at 14-

20 weeks, a needle is inserts into the uterus to extract amniotic fluid which contains fetal cells. Cells are cultured for a few weeks

Chorionic villi sampling- placental tissue is removed and cultured within 24 hours, can be performed at the 8th week

Both carry a small risk of miscarriage

Karyotyping

Fetal cells or blood and tissue samples are cultured and are used to make pictures of chromosomes called karyotypes

White blood cells are useful for karyotyping