Topic 11: Meiosis

Question 1

what is meiosis

Answer 1

special cell division
chromosome number reduced - one round of replication, two rounds of division
made of meiosis I and II (cell division)

Question 2

what are some key features of meiosis

Answer 2

for germ line cells (multicellular organisms that pass on genetic material to offspring)
results in 4 non identical daughter cells with half the chromosome content of the parental cell
so diploid (2n) to haploid (n)
production of gametes (sperm and eggs)

Question 3

what happens during meiosis 1 prophase 1

Answer 3

• centrosomes separate
• nuclear membrane disintegrates
• homologous pairs find each other (2 sister chromatids of maternal copy and 2 sister chromatids of its homologue). these cross over (recombination of genetic material). each cross over is chiasmata

Question 4

what happens during meiosis 1 metaphase 1

Answer 4

• homologous pairs line up on metaphase plate to form a tetrad
• random assortment of chromosomes occurs (maternal or paternal copy on left or right side), if two chromosomes pairs, 2 to the power of 2 so 4 possibilities, if 3 then 2 to the power of 3. 23 pairs = 2 to power of 23 = 8388608 diff gametes from each parent

Question 5

what happens during meiosis 1 anaphase 1

Answer 5

• homologous pairs of chromosomes will separate to each pole

Question 6

how does genetic variation occur

Answer 6

1. crossing over of genetic material in prophase 1

2. random assortment of chromosomes in metaphase 1

Question 7

what happens during meiosis 1 telophase 1

Answer 7

• nuclear envelope appears
• 23 chromosomes in each nuclei so two daughter cells form
• centrosomes duplicated ready for next division

Question 8

what happens during meiosis II prophase 11

Answer 8

• centrosomes separate

- nuclear memrbane disintergrates

Question 9

what happens during meiosis II metaphase 11

Answer 9

• chromosomes line up

Question 10

what happens during meiosis II anaphase 11

Answer 10

• chromatids are separating

Question 11

what happens during meiosis II telophase 11

Answer 11

• form two daughter cells for each daughter cell of meiosis 1 so form 4 daughter cells (haploid)

Question 12

what are the consequences of meiosis

Answer 12

• generates genetic diversity
• maintains constant chromosome number from generation to generation
• male and female meiosis to form sperm (spermatogenesis) and egg (oogenesis)

Question 13

how does spermatogenesis occur

Answer 13

spermatogonium(2n) -> primary spermatocyte(2n) -> spermatids (4 lots of n) -> mature sperm (4 lots of n)
so diploid to haploid

Question 14

how does oogenesis occur

Answer 14

oogonium (2n) -> primary oocyte (2n) -> 4 cells called polar bodies (4 lots of n) -> mature ovum -> egg plus 3 polar bodies (n)

Question 15

what happens in nondisjunction

Answer 15

missegregation of chromosomes - leads to aneuploidy - which leads to monosomy (extra copy of a certain chromosome) or triosomy (3 copies of a certain chromosomes, usually 2 copies) in a zygote

Question 16

what are the consequences of faulty meiosis

Answer 16

• 1/3 of identified miscarriages
• infertility
• mental retardation

Question 17

what occurs during non disjunction in meiosis 1

Answer 17

one of the daughter cells will get both copies of chromosomes, the other cell will one less chromosome
gametes will have either two copies of that chromosome, other will have one less of chromosome
when fertilised by a normal gamete, the chromosome of the complementary gamete will add, so zygotes will have either 3 copies of that particular chromosome (triomy which is 2 zygotes) or one copy, which came from the complementary zygote so monosomy in two zygotes

Question 18

what occurs during non disjunction in meiosis 11

Answer 18

meiosis 1 occurs normally
both chromatids into one gamete, other gamete lacking on chromosome
when fertilised by a normal, complementary gamete, results in trisomy in one zygote, monosomy in one zygote or 2 normal zygotes

Question 19

how is downs syndrome diagnosed

Answer 19

mosaic downs syndrome

if 5 cells = 46 chromosomes, 15=47 then positive