Genetics - Meiosis Flashcards
What is meiosis?
a reduction division of nuclei to form haploid gametes
What are most somatic cells?
diploid (2n)
What are homologous chromosome pairs?
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
What are the phases of meiosis?
-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
What are the products of meiosis?
4 haploid daughter cells
each has unique mix of 1/2 genetic info.
What is a locus?
the position of a gene on a homologous chromosome
each gene has specific locus
What happens in interphase?
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)
What happens in prophase I?
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
What happens in metaphase I?
bivalents (homologous chromosomes) meet at equator
random orientation of pairs = MASSIVE GENETIC VARIATION in gametes
2^23 possible orientations (over 81 million)
What happens in anaphase I?
spindle fibres contract
homologous pairs separated & pulled to poles = REDUCTION DIVISION
non-disjunction here will affect chromosomes (chromosomes don’t seperate properly)
What happens in telophase I?
new nuclei form & cytoplasm begins division
nuclei no longer diploid
each contain 1 pair of sister chromatids
What happens in cytokenisis I?
cell splits into 2 haploid cells
What happens during prophase II?
nuclei breakdown
NO CROSSING-OVER
chromosomes condense & become visible
What happens during metaphase II?
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)
What happens during anaphase II?
spindle fibres contract
centromeres broken
sister chromatids to poles
non-disjunction may occur = 2 gametes wt WRONG # chromosome
chromosome/pole
What happens during telophase II?
new haploid nuclei formed
cytokenisis begins = 4 haploid gamete cells
fertilisation of haploid gametes will produce a diploid zygote
What is non-disjunction?
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
What is a trisomy?
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
What is down syndrome?
trisomy 21 (3 chromosomes on chromosome 21)
symptoms: mental & growth retardation, malformations of digestive system, hearing loss, heart / vision disorders
What are karyotypes?
a pre-natal test to check gender & trisomy disorders
refers to # & type of chromosome nucleus contains
examined during metaphase
How are karyotypes made?
- extract fetal cells through amniocentesis / chorionic villus sampling
- culture cells & stimulate mitosis
- stop division at metaphase
- take photo under light microscope / scan wt computer
- arrange chromosomes in pairs based on size, banding pattern & centromere positions
- check gender (chromosome 23 - XX / XY) / trisomy disorders
What is Klinefelter’s syndrome?
when chromosome 23 is XXY, producing an abnormal male
physical appearance may not appear until after puberty
decreased fertility, decreased muscles
How is the procedure for amniocentesis conducted?
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
How is the procedure for chorionic villus sampling conducted?
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
What is nuchal translucency scan?
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)
What are some ethical issues associated with karyotype tests?
allows parents to abort foetuses wt chromosome abnormality
both sampling are risky & can cause miscarriage
some countries, abortions occurs on basis of gender
