CH 13: Homologous chromosome and ploidy , Meiosis

Meiosis

when cells are produced with half the number of chromosomes to make gametes (sperm, egg cells) for sexual reproduction

(aka haploids which have 1 set of chromosomes)

Gametes

sperm and egg cells (found in animals as plants) that fertilized to create a zygote (/diploid)

fertilization

when the sperm and egg gametes find eachother during sex and the sperm fertilizes the egg

fusing their gametes and making a diploid

gene

a section of the DNA, which affects 1 or more hereditary traits

code for different verisons of the same characteristics

Alles

different versions of the same gene

(ex. Hair colour: black, brown, red, blonde)

Homologous chromosomes

chromosomes of the same type:

they have the same genes in the same posistion along the chromosomes

but they may have different alles so not identical

How many chromosomes in organisms

number of chromosomes depends on the organsim (each has their own #)

Sex chromosomes

chromosomes which determine sex

(ex. xx, xy)

Autosomes

any chromosome thats not a sex chromosome

Ploidy (1n, 2n, etc)

the number of sets of chromosomes in a cell (of a species)

(ex. haploid, diploid, polypoid)

karyotype

the complete set of chromosomes in a species

(they are sorted by shape, size and number)

Diploid

two sets of chromosomes (ex. humans)

diploid number for a human cell

(number of diploids)

23 pairs of chromosomes, 46 tota (one from each parent)

how many autosomes would you find in one of your skin cells

22 autosomes, 1 sex chromosome (pair)

44 chromosomes total, 2 total

Haploid number (n)

the number of different chromosomes in a species

as only 1 type of each chromosome, and 1 alle for each gene

can be distinct for each species (each species have their own # of haploids)

Meisosis 1

seperate the homologous pair of replicated choromosomes (2) in a parent cell (diploid)

Meiosis II

Seperates the sister chromatids of each chromosome seperate into 2 daughter cells

Meiosis II

the two daughter cells divides into 4 halpoid cells (1 chromatid each)

Stages of Meiosis I

1. Interphase I

2. Early prophase I

3. Late prophase I

4. Metaphase I

5. Anaphase I

6. Telophase I

Interphase I

uncondensed chromosome replicate in parent cell (whole, cant distinguish chromosome)

Early prophase I

• Chromosomes condense (visible) - chromatids stays together with cohesin proteins

• spindle aparatus forms

• nuclear envelope begins desintegration

• homologous chromosomes synapsis pairing

Prophase I: explain homologous chromosomes synapsis pairing

two pairs of sister chromatids (2 homologous chromosomes from each parent) are binded together by proteins. this protein attachment creates synaptonemal complex:

this makes two homologous pairs grow togeher

synaptonemal complex:

a bridge of proteins between 2 homologous chromosome pairs which push them together

non sister chromatid

non identical chromatids

Prophase I: How does crossing over occur with synapsis: explain with chiasma

breaks are made in the DNA and in corresponding breaks between the non-sister chromatids,

this is where: cross over/swap of DNA (swaps same gene but swaps different version of genes) only linked by chiasma

Bivalent

a pair of homologous chromosomes which are conneted in Meiosis I. since they are duplicated it has 4 chromatids total. later on their genetic material will be mixed up “crossing over” which increases genetic variation.

4 chromatids from 2 homologous chromoomes

Late prophase

the chromosomes become bivalent: swap pairs forming non-sister chromatids

nuclear envelope disintegrated,

multiple chiasmata between non-sister chromatids

chiasmata

crossover between non-sister (maternal and paternal) chromatids

Explain The synapsis and corssing over in Prohase 1

1. Condensation

2. Synapsis formation

3. Crossing over and plasma formation

4. Partial seprationof homologs

synaptonemal complex gone after crossing over

homologs seperate, only linked by the chiasma

Metaphase I

the bivalents move to make a metaphase plate across middle of cell (random order)

Anaphase I

the homologs seperate and move to opposite sides (poles) of the spindle apparatis

Telophase I

the homologs finish moving to opposite poles and the spindle disintegrates

cytokenesis occurs creating 2 haploid cells with replicated chromosomes

Meiosis II notes

has the same phases ( prohase II, Telophase II etc) using the (genetically mixed) replicated chromosomes, making. iteven more mixed and varied

Recap: List all parts of the spindle aparatus and their function

MTOC: microtubule organzing center: is the where microtubules come out/grow by de/polymerize

Astral microtubule: attaches MTOC to plasma membrane (secure) (astral/outside)

Kinteochore microtubule: attaches to kinetochore in centromere creating pulling force that spilts sister chromatids, or homologous pairs

Polar microtubule: partially overlaps with opposite polar microtubule, to contribute to pulling force by elongating the cell pole