3.3

Question 1

What is meiosis

Answer 1

process by which sex cells (gametes) are made in the reproductive organs

Question 2

Meiosis 1

Answer 2

The 1st meiotic division is a reduction division*(diploid→haploid)in which homologous chromosomes are separated

meiosis I: the number of cells is doubled but the number of chromosomes is not. This results in 1/2 as many chromosomes per cell.

Question 3

Meiosis 2

Answer 3

The 2nd division separates sister chromatids (these chromatids may not be identical due to crossing over in prophase I)

Question 4

Prophase 1

Answer 4

• homologous chromosomes undergo a process called*synapsis→ synapsis: chromosomes pair up to form abivalent
  (or tetrad)

homologous chromosomes are held together at points calledchiasmata(singular: chiasma)

• Crossing over of genetic material between non-sister chromatids occurs at chiasmata
  
  • result of exchanging genetic material → new gene combinations are formed on chromatids (recombination)
• Once chiasmata are formed, the homologous chromosomes condense as bivalents and then are separated in meiosis
  
  • If crossing overoccurs then all 4 haploid daughter cells will be geneticallydistinct (sister chromatids are no longer identical)

Question 5

where does crossing over occur

Answer 5

at the chiasmata

Question 6

What promotes genetic variation

Answer 6

• crossing over
• random orientation
• fusion of gametes from different parents

Question 7

In what phase does crossing over occur?

Answer 7

prophase 1

Question 8

When does random assortment of chromosomes occur

Answer 8

metaphase 1

Question 9

The failure of chromosomes to separate may occur via:

Answer 9

• Failure of homologues to separate in Anaphase I (resulting in four affected daughter cells)
• Failure of sister chromatids to separate in Anaphase II (results in only 2 daughter cells being affected)

Question 10

synapsis

Answer 10

when chromosomes pair up to form a bivalent

Question 11

what does crossing over result in

Answer 11

new gene combinations are formed on chromatids (recombination)

Question 12

monosomy

Answer 12

chromosomal abnormality - when there is 1 less chromosome

Question 13

nuchal translucency

Answer 13

• non-invasive

- ultrasound looking at nuchal fluid behind the neck of the foetus - large about is strongly correlated w Down syndrome

Question 14

chronic villus sampling

Answer 14

• invasive
• sample of chronion (villi - placenta tissue) via tube inserted through the cervix
• cells are cultured & then karyotypes
• 1% risk of miscarriage

Question 15

amniocentesis

Answer 15

• invasive
• sample of amniotic fluid (contains foetal cells) with a needle guided by camera/ultrasound
• 0.5% of miscarriage

Question 16

why is chromosome number halved in gametes?

Answer 16

to prevent the total chromosome numbers doubling each generation

Question 17

Metaphase 1

Answer 17

• Spindle fibres connect to bivalents which align along the equator
• random orientation occurs - chromosomes line up in 1 of 2 arrangements
  
  • maternal copy left/right I paternal copy left/right
• this orientation is random, as is the assortment of chromosomes into gametes
• final gametes will differ depending on whether they got the maternal or paternal copy of a chromosome
• random assortment will occur for each homologous pair - number of possible gamete combinations depends on number of homologous pairs, therefore 2^23 possible gamete combinations
  This means an allele on one chromosome has an equal chance of being paired with, or separated from, any allele on another chromosome (their inheritance is independent of one another)

Question 18

Random assortment

Answer 18

describes how pairs of alleles separate independently from one another during gamete formation

• According to independent assortment, inheritance of one gene/trait is independent to the inheritance of any other gene/trait
• Independent assortment is due to the random orientation of pairs of homologous chromosomes in meiosis I
• Independent assortment willnot occur if two genes are located on the same chromosome (linked genes)

Question 19

Random orientation vs independent assortment

Answer 19

• random orientation is the behaviour of chromosomes in meiosis
• independent assortment is the behaviour of alleles of unlinked genes as a result of gamete productions - due to random orientation, the alleles of these unlinked have been independently assorted into the gametes

Question 20

Anaphase 1

Answer 20

• spindle fibres contract and split the bivalent
• homologous chromosomes move to opposite poles of the cell
• reduction division - homologous pair is separated but sister chromotids remain attached

Question 21

Telophase 1

Answer 21

• Chromosomes decondense
• nuclear membranemayreform
• cell divides (cytokinesis) forming two haploid daughter cells each containing one pair of sister chromatids

Question 22

Prophase 2

Answer 22

• Chromosomes condense
• nuclear membrane dissolves
• centrosomes move to opposite poles (perpendicular to prophase I)
• no crossing-over occurs

Question 23

Metaphase 2

Answer 23

spindle fibres attach to chromosomes (at centromere) and align them along the equator

Question 24

Anaphase 2

Answer 24

• spindle fibres contract - centromeres split
• sister chromatids are separated.
  
  • now called chromosomes
  • pulled to opposing poles

Question 25

Telophase 2

Answer 25

- chromosomes ***decondense*** - nuclear membrane reforms - ***cells divide*** (cytokinesis) to form four haploid daughter cells - fertilisation of these haploid gametes will produce a diploid zygote