Chapter 11: Meiosis and Sexual Reproduction

Eukaryotes

have several chromosomes

Homologous chromosome pairs

Inherited from parents and contain identical genes

they may differ in sequence

Autosomes

matched pairs

Sex chromosomes

mismatched pair

Ploidy

number of copies of chromosomes

Karyotype

number and type, matched up

chromosomes, homologs, alleles

Homologous chromosomes replicated

gene for eye color(red)————Gene for eye color (purple eyes)

Sister chromatids

identical genes, identical alleles

Homologs

identical genes, different alleles

Meiosis

Nuclear division where 4 haloed gametes are created; chromosome number in each cell reduced; these cells become gametes

germ cells are the diploid parent cells

daughter cells receive one of each homologous chromosomes from parent cell

Interphase

Prior to meiosis where chromosomes replicate forming sister chromatids in an uncondensed state

Early Prophase I

chromosomes condense and nuclear envelope breaks up

spindle apparatus forms

synapsis of homologous chromosomes

Tetrad forms where 4 chromatids are from homologous chromosomes

Late prophase I

Crossing over of non-sister chromatids

often multiple cross over between the same chromatids

Metaphase I

Tetrads migrate to metaphase plate

Anaphase I

Homologs separate and begin moving to opposite sides of cell

Telophase I and Cytokinesis

chromosomes move to opposite sides of the cell, then cell divides

Prophase II

centrosomes replicate. Spindle apparatus forms

Metaphase II

chromosomes line up at middle of cell(metaphase plate)

Anaphase II

sister chromatids separate and begin moving to opposite sides of cell

Telophase II and cytokinesis

chromosomes move to opposite sides of cell, then cell divides

Importance and paradox of sex

Sexual reproduction extremely common among taxa

sex must be favored by evolution but come with some disadvantages

Asexual reproducing species

leave more copies of their genes in the next generation

well adapted organisms

make clones and keep good combinations of genes together

no wasted effort making useless males

Cost of asexual reproduction

If a gene becomes altered and doesn’t work all asexually produced offspring inherit the bad gene

Natural selection needs variation to weed out bad genes

What If a gene becomes altered and doesn’t work in sexual reproduction?

Half of sexual produced offspring will inherit the bad gene

natural selection favors good variants over bad ones(purifying selection)

Mullers Ratchet

mutations will build up in asexual populations in the absence of purifying selection

Testing mullers ratchet

predicts that deleterious alleles are higher in asexual populations

Concludes that sex id advantageous over the long run because purifying selection can remove alleles

Natural selection

favors variants best suited to environment

Genetically identical offspring

susceptible to same pathogens and environmental stresses

Genetic variation

increases chance that some variants survive in new environments

Uncertain or variable environments may favor producing variable offspring

Outcrossing rates increased with a pathogen then compared to without a pathogen

Sex contribution to variation

Meiosis with production of haploid gametes in the crossing over in prophase I

Independent assortment of maternal and Paternal chromosome sin anaphase I

Fertilization: random fusion of gametes from 2 parents

Crossing over via synapsis(Chiasma)

In prophase I

synapsis attaches homologs

synaptonemal complex forms special proteins

gene for gene lineup

Key events of Prophase I

Condensation

Pairing

synapsis(bivalent formation)

Partial separation of homologs

Gene swapping

Crossing over involves gene exchange

recombines alleles along homologous chromosomes and contributes to genetic variation by shuffling traits along chromosomes

Independent Assortment

chromosomes inherited from each parent randomly lined up at metaphase plate

mix and match allele combinations across chromosomes and generates genetically distinct gametes

During meiosis I

tetrads line up two different ways before homologs separate

Non disjunction

Meiosis will start normally and the tetrads line up in the middle one cell

One set of homologs won’t separate and meiosis occurs normally

all gametes have abnormal number of chromosomes with one too many or one too few

Aneuploidy

variation from typical chromosome number

feels with 22 pairs of autosomes and xx sex chromosomes

males with 23 pairs of autosomes ad XY chromosomes

autosomes

Trisomy 21- down’s syndrome

Duplication/deletion of other chromosomes rare, lethal

Mosaicism

two groups of cells that differ in number of chromosomes

2.4% of Down’s syndrome

Sexual Life cycles

Testis and Ovaries go through Meiosis and create sperm and eggs that go through fertilization haploid 1n, become zygotes, become diploid 2n and go through cell division and mitosis

Where is the SRY gene located?

short arm of Y chromosome

What does the SrY gene do

The SRY gene encodes a transcription factor that triggers male sex determination by initiating the formation of testesand suppressing female characteristics.

How does the SRY gene contribute to male development?

The SRY gene activates SOX9, which promotes testis formation and suppresses female reproductive structures. AMH (Anti-Müllerian Hormone) is produced, causing the regression of Müllerian ducts.

What happens in the absence of the SRY gene?

Without SRY, the gonads develop as ovaries, and female sexual characteristics form

What are the consequences of mutations in the SRY gene?

Mutations can cause XY females (male chromosome pattern, but female traits) or XX males (female chromosome pattern, but male traits)

How does SRY relate to sex-reversal syndromes?

XY females occur if the SRY gene is missing or mutated. XX males can develop if SRY is translocated from the Y chromosome to the X chromosome.

What is the role of the SRY gene in sex determination?

The SRY gene is the key factor in determining male sex by initiating testis development and suppressing female characteristics via the SOX9 gene.