3.3 + 10.1 Meiosis

Question 1

Define meiosis

Answer 1

MEIOSIS: division of diploid germ line cells to produce haploid gametes

Question 2

Main steps in meiosis

Answer 2

Question 3

Sister chromatids vs homologous chromosomes

Answer 3

Sister chromatids - genetically identical

Homologous chromosomes - analogous chromosomes of different organisms containing different genes

Question 4

Meiosis I sequence

Answer 4

Meiosis starts after interfase

P1: chromosomes condense, nuclear envelope dissolves, homol. chrom. form bivalents - chiasmata, crossing over

M1: spindle fibers connect to bivalents (at centromeres) and align at equator

A1: spindle fibers contract, separate homol. chrom. to opposite poles

T1: chrom decondense, nuclear envelope MAY reform, cytokinesis

Question 5

Meiosis II sequence

Answer 5

Meiosis starts after interfase

P2: chrom. condense, nuclear envelop dissolves (if formed), centrosomes (centrioles in division) move to opposite poles

M2: spindle fibres from opposite cetrosomes attach to chrom, align at equator

A2: spindle fibers contract, sister chromatids to opposite poles

T2: chrom. decondense, nuclear membrane reforms, cytokinesis => 4 haploid cells

Question 6

Explain crossing over

Answer 6

CROSSING OVER in prophase 1

between homologous chromosomes at chiasmata - genetic exchange - new gene combinations - recombination - all 4 gametes genetically distinct

Question 7

Explain random assortment in meiosis

Answer 7

RANDOM ASSORTMENT in metaphase I

• homolg. chrom. align ast equator - form bivalents in two possible ways: maternal (left) - paternal (right) or paternal (left)-maternal (right)
• this orientiation is random - chrom. assortment into gametes is also random - gamete has only one chrom. - maternal or paternal
• number of combinations in diploid organisms 2ⁿ where n - number of chrom.

Question 8

Describe sexual life cycle

Answer 8

Question 9

Main sources of enetic variation in sexual reproduction

Answer 9

1. Crossing over: reconbination of genetic material in homolog. chrom.
2. Random assortment of chromosomes: line up at equator in M1 - orientiation to opposite poles is random - different comninations of maternal/paternal chrom.
3. Random fusion of gametes from different parents: random fertilisation

Question 10

Why non-disjunction of chrom. might occur?

Answer 10

• failure of homolog. chrom. so separate in A1
• failure of sister chrom. separate in A2

Non-disjunction causes genetic conditions: Down, Patau, Turner’s syndromes

Question 11

What factor increases the chnance of non-disjunction of chromosomes?

Answer 11

Maternal and paternal age

• especially in maternal age - positive correlation between increase in age and chance of Down syndrome

Question 12

Types of sampling of fetus for karyotyping

Answer 12

Question 13

Chromosome number throughout meiosis

Answer 13

• interphase replicates DNA: I to X
• the first division seprataes homolg. pairs XX to X, X
• second division separates sister chrom. X to I, I -> single chromatid chromosomes

Question 14

Germ-line vs somatic mutation

Answer 14

Question 15

Explain synapsis in meiosis

Answer 15

Homologues (both of chrom 4) - red maternal, blue paternal - in prophase 1 - replicated homologues overlap - form synapsis - crossing over (swap genes) can happen - points of exchange - chiasmata - tetrad of four connected chromosomes

Question 16

Mitosis vs meiosis

Answer 16