Genetics - Meiosis

Question 1

What is meiosis?

Answer 1

a reduction division of nuclei to form haploid gametes

Question 2

What are most somatic cells?

Answer 2

diploid (2n)

Question 3

What are homologous chromosome pairs?

Answer 3

chromosomes of similar size, banding pattern and shape that form pairs

one is maternal and the other paternal

if 2 chromosomes have same sequence of genes = homologous

Question 4

What are the phases of meiosis?

Answer 4

-Interphase

Meiosis I (separates homologous pairs / reduction division):

• prophase I
• metaphase I
• anaphase I
• telophase I
• cytokenesis I

Meiosis II (separates sister chromatids):

• prophase II
• metaphase II
• anaphase II
• telophase II
• cytokenesis II

Question 5

What are the products of meiosis?

Answer 5

4 haploid daughter cells

each has unique mix of 1/2 genetic info.

Question 6

What is a locus?

Answer 6

the position of a gene on a homologous chromosome

each gene has specific locus

Question 7

What happens in interphase?

Answer 7

preparation for division:

• chromosomes replicate
• each single chromosome becomes a pair of sister chromatids, joined at equator

Order: Growth Phase 1 (G1), Synthesis Phase (S) & Growth Phase 2 (G2)

G1 - rapid cell growth, routine metabolic processes, proteins for DNA replication made, copies of organelles made (longest cell cycle phase)

S - DNA replicated

G2 - final preparations for division (e.g. additional proteins & organelles)

Question 8

What happens in prophase I?

Answer 8

homologous structures associate wt each other

crossing-over btw sister chromatids take place (BTE HOMOLOGOUS CHROMOSOMES)
therefore, recombination of alleles & source of GENETIC VARIATION in gametes

nuclear membrane breaks down

centrioles migrate to poles

chromosomes visible

Question 9

What happens in metaphase I?

Answer 9

bivalents (homologous chromosomes) meet at equator

random orientation of pairs = MASSIVE GENETIC VARIATION in gametes

2^23 possible orientations (over 81 million)

Question 10

What happens in anaphase I?

Answer 10

spindle fibres contract

homologous pairs separated & pulled to poles = REDUCTION DIVISION

non-disjunction here will affect chromosomes (chromosomes don’t seperate properly)

Question 11

What happens in telophase I?

Answer 11

new nuclei form & cytoplasm begins division

nuclei no longer diploid

each contain 1 pair of sister chromatids

Question 12

What happens in cytokenisis I?

Answer 12

cell splits into 2 haploid cells

Question 13

What happens during prophase II?

Answer 13

nuclei breakdown

NO CROSSING-OVER

chromosomes condense & become visible

Question 14

What happens during metaphase II?

Answer 14

pairs of sister chromatids aline at equator

spindle fibres form & attach to centromeres

random orientation AGAIN = increase VARIATION (not to the same extent as metaphase I)

Question 15

What happens during anaphase II?

Answer 15

spindle fibres contract

centromeres broken

sister chromatids to poles

non-disjunction may occur = 2 gametes wt WRONG # chromosome

chromosome/pole

Question 16

What happens during telophase II?

Answer 16

new haploid nuclei formed

cytokenisis begins = 4 haploid gamete cells

fertilisation of haploid gametes will produce a diploid zygote

Question 17

What is non-disjunction?

Answer 17

in anaphase (I & II) chromosome / sister chromatids can be pulled to wrong pole (fail to seperate) = change in chromosome number (45 or 47)

is a chromosome mutation

Question 18

What is a trisomy?

Answer 18

abnormal # chromosomes

fertilisation adds the homologous chromosomes resulting in a zygote & somatic cells wt a trisomy of one chromosome = FATAL / DISORDERS

extra chromosome to one pole = 24 chromosomes
bc 2 or >2 homologous chromosomes stuck together

Question 19

What is down syndrome?

Answer 19

trisomy 21 (3 chromosomes on chromosome 21)

symptoms: mental & growth retardation, malformations of digestive system, hearing loss, heart / vision disorders

Question 20

What are karyotypes?

Answer 20

a pre-natal test to check gender & trisomy disorders

refers to # & type of chromosome nucleus contains

examined during metaphase

Question 21

How are karyotypes made?

Answer 21

1. extract fetal cells through amniocentesis / chorionic villus sampling
2. culture cells & stimulate mitosis
3. stop division at metaphase
4. take photo under light microscope / scan wt computer
5. arrange chromosomes in pairs based on size, banding pattern & centromere positions
6. check gender (chromosome 23 - XX / XY) / trisomy disorders

Question 22

What is Klinefelter’s syndrome?

Answer 22

when chromosome 23 is XXY, producing an abnormal male

physical appearance may not appear until after puberty

decreased fertility, decreased muscles

Question 23

How is the procedure for amniocentesis conducted?

Answer 23

amnio = amniotic fluid centesis = puncture

needle through mother’s abdominal wall, ultrasound to guide needle

withdraw sample of amniotic fluid

cells cultured & prepare a karyotype

done at 15 - 20 weeks

Question 24

How is the procedure for chorionic villus sampling conducted?

Answer 24

2% miscarriage

samples cells from placenta, specifically the chorion (membrane part of placenta)

can be done earlier than amniocentesis (10 - 12 weeks)

entry via vagina

Question 25

What is nuchal translucency scan?

Answer 25

non-invasive

btw 11-13 weeks

ultrasound looks at nachal fluid btw neck of fetus

large fluid amounts = down syndrome (then advised to do amniocentesis / chorionic villus sampling)

Question 26

What are some ethical issues associated with karyotype tests?

Answer 26

allows parents to abort foetuses wt chromosome abnormality

both sampling are risky & can cause miscarriage

some countries, abortions occurs on basis of gender