3.3 meiosis

Question 1

define meiosis

Answer 1

the reduction division of a diploid cell to produce 4 haploid daughter cells

Question 2

identify the main differences between meiosis 1 and meiosis 2

Answer 2

meiosis 1:

-involves the separation of homologous chromosomes (bivalent/ tetrads)
-is a reduction division (diploid to haploid)
-promotes genetic variation (crossing over random assortment)

meiosis 2:
-involves the separation of sister chromatids
-is a mitotic division (haploid to haploid)
-does not promote genetic variation

Question 3

differentiate between homologous chromosomes and sister chromatids

Answer 3

Homologous chromosomes are the maternal and paternal copies of a given chromosomes

homologous chromosomes have the same structure and the same genes at the same loci positions

sister chromatids are the duplicated copies of the chromosome’s DNA (copied during S phase of interphase)

Question 4

compare the processes of meiosis and mitosis

Answer 4

meiosis:
type of cell produced: sex cells (gametes)
number of cells produced: 4
number of divisions: 2
ploidy of daughter cells: haploid
genetics of daughter cells: shows genetic variation

mitosis:
body somatic cells
2
1
diploid
are genetically identical

Question 5

outline how crossing over and independent assortment give rise to infinite genetic variety

Answer 5

crossing over:

-crossing involves the exchange of genetic material between non sister chromatids of a bivalent

-bivalents are connected at points called chiasma during the process of synapsis (during prophase 1)

-it is at these chiasma that recombination occurs

independent assortment:

bivalents will line up at the cell’s equator in a random orienatation
during metaphase 1

meaning there is equal probability of a gamete containing the maternal OR paternal copy for any chromosome pair

Because human cells have 23 chromosome pairs, there are 2^23 possible chromosomes combinations
(equates to over 8 million different gamete combinations)

Question 6

explain how random gamete fusion promotes variation within a species

Answer 6

when two haploid gametes fuse, they form a diploid zygote which can grow into a new organism

because gamete fusion is random, each successive offspring will be composed of a distinct combination of maternal and paternal chromosomes

this means every member of a species is unique (promoting biodiversity)

Question 7

explain how non disjunction can give rise to aneuploidy

Answer 7

non disjunction describes the failure of chromosomes to separate during cell division

if non disjunction occurs in anaphase 1, all 4 gametes will be affected

if non disjunction occurs in anaphase 2, only 2 of the gametes are affected

if a gamete with one extra chromosome fuses with a normal gamete, the offspring will have trisomy

Question 8

list the genetic condition that causes Down syndrome

Answer 8

is caused by trisomy 21
increased maternal age increases the risk of non disjunction

Question 9

describe the method by which cells are obtained for karyotyping

Answer 9

cells are isolated and treated with drugs to promote cell division (makes chromosomes visible to microscope)

cells are arrested during mitosis and then chromosomes are isolated and visualized

Question 10

compare the benefits and risks associated with amniocentesis and chorionic villi sampling

Answer 10

amniocentesis:

cells are extracted from the amniotic fluid

occurs later in pregnancy (~15 weeks) but has a slightly lower risk of miscarriage (~0,5%)

chorionic villi sampling:

cells are extracted from the placenta (chorionic villus)

occurs earlier in the pregnancy (~11 weeks) but has a slightly higher risk of miscarriage (~1%)