10.1 - meiosis

Question 1

what happens in meiosis?

Answer 1

• Meiosis follows a period of interphase in the cell cycle: G1, S and G2
• During the S phase of interphase, DNA is replicated so that each chromosome consists of two chromatids
• start of meiosis, chromosomes condense and are visible as two chromatids - sister chromatids

Question 2

what is synapsis?

Answer 2

• a pairing process where homologous chromosomes align beside each other
• This combination is called tetrad as it consists of 4 chromatids or bivalent for the homologous pairs

Question 3

what happens in prophase I of meiosis?

Answer 3

• all of the chromatids of two homologous chromosomes become tightly associated during synapsis to form bivalents/tetrads

Question 4

what happens in crossing over (part of meiosis)

Answer 4

• Breaks in the DNA occur and the non-sister chromatids (maternal and paternal chromosomes) can exchange corresponding sections of DNA
• when complete the non-sister chromatids remain adhered at the site of crossing over called the chiasma.
• these connections via chiasmata are essential to the successful completion of meiosis

Question 5

what do chiasma do?

Answer 5

• X-shape structure that is formed between non-sister chromatids
• chiasma represents a physical sign of crossing over
• 1-3 chiasmata form per homologous pair
• chiasmata persist through metaphase I and play a role in the prevention of nondisjunction
• presence of chiasmata results in increased stability of bivalents
• increase genetic variability too due to crossing-over events as alleles may be exchanged
• Crossing over can decouple linked combinations of alleles and lead to independent assortment

Question 6

what is recombination?

Answer 6

• new combinations of alleles are created by crossing over
• process by which offspring possess a combination of alleles different from that of either parent is called recombination
• The resulting genetic variation is why meiosis is important as crossing over produces new combinations of alleles

Question 7

what happens in meiosis I?

Answer 7

• sister chromatids remain associated with one another
• prophase I, metaphase I, anaphase I and telophase I.
• Meiosis I is a reduction division where it reduces the number of chromosomes by half to produce haploid cells (n) from diploid cells (2n)
• meiosis I is responsible for the
  process of generating genetically variable gametes

Question 8

what happens in prophase I?

Answer 8

• homologous chromosomes behave in a coordinated way where they exchange DNA leading to genetic recombination during crossing-over events

Question 9

what happens in metaphase I?

Answer 9

• homologous chromosomes line up on the metaphase plate (synapsis) formed
• Homologous chromosomes are segregated during anaphase I which results in two haploid cells each with only one copy from a homologous pair of chromosomes

Question 10

what happens in Telophase I?

Answer 10

• separate the cell into two haploid
  cells from one diploid cell

Question 11

what does the law of independent assortment mean?

Answer 11

• any one of a pair of characteristics may combine with either one of another pair
• means alleles of genes located on different chromosomes will line up
  on metaphase independently of another pair of homologous chromosomes
• chromosome pair orients independently of all the others during metaphase I.

Question 12

how does independent assortment happen?

Answer 12

• homologous chromosomes pair up and then separate to opposite poles during anaphase
• pole that the chromosome moves to will depend on the way the pair is facing - random
• one pair orientation does not effect another
• each combination has equal chance
• punnet square

Question 13

how was sex-linkage
to explain the discrepancies found?

Answer 13

• William Bateson and Reginald Punnett conducted crosses with sweet peas
• they didn’t get the expected ratio so found discrepancies.

Question 14

what happens in Meiosis II?

Answer 14

• the haploid daughter cells (n) enter meiosis II without passing through interphase
• Sister chromatids are separated but likely to be non-identical sister chromatids due to crossing-over events during prophase I.
• so one cycle of meiosis, a diploid cell is separated into four haploid cells called gametes
• Each gamete will have different chromatids bc of crossing over (prophase I) or independent assortment (metaphase I).