Chromosomes and Cell Division Flashcards
describe function of a telomere on a chromosome
protects the ends of the chromosomes to make sure they aren’t eroded
where is the centromere found?
typically in the centre of the chromosome (holds two chromatids together)
what is heterochromatin?
non-functional units
what is euchromatin?
functional units
why is DNA replication not symmetrical?
DNA polymerase can only synthesise from 5’ to 3’ direction, adding free nucleotides to the 3’ end of the strand
what happens over many generations to chromosomes?
they will get shorter
what can telomerase do?
enzyme which can add a DNA sequence using an RNA template to a DNA which has been eroded
what repeats does telomerase add to the ends of existing elongated chromosomes?
5’-TTAGGG-3’ repeats (hundreds of copies)
Most cells in humans don’t express telomerase, but which cells do?
- fetal tissues
- adult germ cells
- tumour cells
what occurs at G1 phase of cell cycle?
cellular components are duplicated, prepared for cell division
what occurs at S phase of the cell cycle?
DNA synthesis
what occurs at G2 phase of the cell cycle?
cell double checks chromosomes for error and makes any repairs
what occurs at prophase?
- chromosomes condense and become visible/compact
- nuclear membrane disappears
- spindle fibres form from the centriole
what occurs at metaphase?
- chromosomes align at the equator of the cell
- chromosomes attached by fibre by each centriole
- max. condensation of chromosome
what occurs at anaphase?
- sister chromatids separate at centromere by spindle fibres
- separate longitudinally
- move to opposite poles of the cell
what occurs at telophase?
- new nuclear membrane forms
- each cell now contains 46 chromosomes (diploid)
what occurs at cytokinesis?
- cytoplasm separates
- two new daughter cells formed
what do centromeres do?
hold two sister chromatids together
what can happen if centromere doesn’t work properly?
Irregular separation occurs leading to cellular problems
what forms centromeres?
satellite DNA (non-coding) forms heterochromatin
what is a kinetochore?
protein complex assembled on the centromere which binds to spindle fibres (microtubules)
how many genes does the human body approx. have?
20-30,000
in what two places is satellite DNA found?
- cenromeres
- telomeres
what 2 components are DNA sequences in our genome made of?
- SINEs ( short interspread nuclear elements)
2. LINEs (long interspread nuclear elements)
is majority of DNAm SINEs or LINEs?
LINEs
what structures make up chromatin?
histones
histone +DNA =
nucleosome
what charge is DNA and histone?
DNA: -ve
histone: +ve
how many nucleosomes per turn are there in wrapping?
6 nucleosomes
what structure is formed by compact DNA by a factor of 40?
solenoid structure
what are the 4 levels of packaging DNA to form a chromosome?
- nucleosome
- chromatin fibre
- fibre-scaffold complex (lots of heterochromatin)
- chromosome
what is the purpose of packaging DNA?
- negative DNA neutralised by positive histones
- DNA takes up less space
- inactive DNA can be folded into inaccessible locations until required
what is the short arm of the chromosome called?
p arm
what is the long arm of the chromosome called?
q arm
what is a submetacentric chromosome?
centromere is not quite at the centre of the chromosome
what is a metacentric chromosome?
centremere is at the centre of a chromosome
what does chromosome analysis by karyotyping require beforehand?
blood sample
what is a colchicine?
microtubule inhibitor, no mitosis occurs but chromosomes align (become visible)
what does karyotyping allow us to do?
look at individual chromosomes
what does FISH allow us to do?
used to look at sequence content of chromosomes (as karyotyping doesn’t let us do it)
how does FISH work?
- fragment of desired DNA sequence is isolated
- DNA sequence is labelled with fluorescent dye
- cytogenic technique that uses fluorescent probes bind only to the part of the chromosome with a high degree of sequence complementarity
- specific regions on the chromosomes can be looked at
what are the 4 different FISH probes?
- unique sequence probes
- centromeric probes
- for determining chromosome number - telomeric probes
- present in children with retardation diagnosis
- detect subtelometic rearrangements - whole chromosome probes
- detects translocations and rearrangements
what is meiosis?
- cell division in germ cells
- diploid cells divide to to form haploid cells
- recombination or rearrangment of genes creates diversity
- 4 haploid cells formed
what is oogenesis?
process of egg formation
what is speratogenesis?
process of sperm formation
do sperm go through more cell divisions than eggs?
Yes- therefore higher chance of mutations
what is fertilisation?
two haploid cells (sperm and egg) fuse to form a diploid cell (zygote)
who does the embryo get their mitochondria (and its DNA) from?
the mother (maternal inheritance)
why is X inactivation necessary in females?
- so only characteristics of one X chromosome are expressed (no double characteristics)
- deactivation is RANDOM
when does X deactivation happen?
in embryonic stage
