cell division part 2 meiosis - (flashcard mode only)

describe spermatogenesis

cytoplasm is divided equally during cytokinesis

what is a homolog

homologous chromosomes

homologous chromosomes

carry information for traits (genes)

humans are

diploid

meiosis

a reduction division occurs after interphase and consists of 2 rounds of division

what does meiosis produce

gametes from sexual reproduction

examples of haploid cells

sperm and egg (ova)
n=23

Interphase

2n - two copies of each DNA strand, one paternal and one maternal
- DNA is in chromatin form (loose not supercoiled)

G1

the metabolically active period and growth involves synthesis of cell components

What happens during S phase?

DNA replicates creating two identical daughter strands held together at the centromere.

What is a chromatid?

Each chromosome consists of two elongated DNA molecules (chromatids) held together until anaphase 2.

what happens in G2

cell prepares for meoisis

what is the function of meiosis 1

the separation of homologous chromosomes

prophase 1

nuclear envelope breaks down and fragments into vesicles
centrosomes form microtubules (mitotic spindle) + migrate to cell poles

explain bivalent, crossing over, non sister chromatids, alleles and chiasmata

Homologous chromosomes - a set of the same chromosomes that join to form a bivalent (tetrad). In doing so they allow for chiasmata to form and crossing over to occur.
Non sister chromatids cross over at random points (chiasmata) and exchange alleles making the sister chromatids no longer identical
End result is an increase in genetic variation

metaphase 1

1. kinetochores link and act as one
2. microtubules attach to the kinetochore
3. microtubules contract and align along the cell equator
- orientation of each bivalent along the equator is random increasing genetic variation

anaphase 1

1. cohesion proteins mean that sister chromatids are joined but the bivalents are separated
2. homologous chromosomes are separated - poles
3. kinetochores shorten microtubule
4. start of 2n-n

telophase 1

one of each homologous chromosome arrives at each pole
each chromosomes consists of 2 non identical sister chromatids
nuclear envelope reassemble + chromosomes decoil into loosely packed chromatin
microtubules depolymerise
contractile ring forms for cytokinesis 1
no interphase follows
both cells are haploid and genetically unique

what is the function of meiosis 2

the separation of sister chromatids - similar to mitosis

what is the result of meiosis

4 haploid daughter cells containing single chromosomes
n -> n

prophase 2

same as prophase 1 except no bivalent crossing over
Nuclear envelope breaks down and fragments into vesicles
centrosomes form microtubules and migrate to the cell poles
chromatin condenses into discrete chromosomes

metaphase 2

microtubules attach to kinetochore
sister chromatids within each chromosome are attached to the poles
contraction and tension from microtubules and microtubule motors align chromosomes along metaphase plate
Key difference noted here is that instead of 46 chromosomes along the plate there are only 23 since it is now haploid

anaphase 2

cohesin proteins at centromere are cleaved, separating chromatids from each other
chromatids are separated, moving towards the poles
kinetochore shortens microtubules and centrosomes move apart
chromatids now referred to as chromosomes

What happens during telophase 2?

Chromosomes arrive at the poles, nuclear envelope reassembles, chromosomes decondense, and microtubules depolymerize.

What happens during cytokinesis 2?

Contractile ring forms, cytoplasm divides, and each cell (gamete) ends up with a haploid nucleus, genetically unique from each other and the parent, and will specialize into sex cells (sperm or ovum).

Differences between mitosis and meiosis

Meiosis has two rounds of genetic separation and cellular division while mitosis only has one of each. In meiosis homologous chromosomes separate leading to daughter cells that are not genetically identical. In mitosis the daughter cells are identical to the parent as well as to each other.