chromosomes Flashcards
features of chromosomes:
Telomere - ends of the chromosome
Centromere - DNA that links two sister chromatids
Heterochromatin - can’t be expressed
(inactive genes - cant be transcripted)
Euchromatin - ability to be expressed
(active genes - can be transcripted)
euchromatin and heterochromatin:
Euchromatin - an open form of chromatin that comprises active or potentially active genes
Heterochromatin - a condensed form of chromatin that comprises silenced genes
DNA synthesis:
DNA Polymerases
- can’t initiate DNA synthesis de novo, must use an existing RNA primer
Leading strand:
Synthesizes DNA in a 5’ to 3’ brine direction in one go
Lagging strand:
Synthesizes DNA in 5’ to 3’ brine direction using a series of short fragments [Okazaki fragments]
Stitched together by DNA ligase once completed
Uses RNA primer in between the series
replication:
when the ends of linear DNA cannot be replicated completely during lagging strand DNA synthesis
[1] Final RNA primer & Okazaki fragment at the end of DNA synthesis
[2] Final RNA primer removed - forms gap
[3] This leads to degradation and loss of DNA at the very ends of ‘telomeres’
Telomeres get shorter as metazoan organisms age
telomerase:
an enzyme that adds repeats and resynthesizes to fill in the ends of DNA during DNA replication.
cell cycle:
[1] Interphase
G1 - Cell growth (10-12 hrs)
S - DNA replication (6-8 hrs)
G2 - Division preparation (2-4 hrs)
[2]Mitosis (cell division - PMAT)
P - Prophase
M - Metaphase
A - Anaphase
T - Telophase
[3] Cytokinesis (formation of the new daughter cells)
mitosis (cell division in somatic cells - PMAT):
P - Prophase
Chromosomes condense
Nuclear membrane disappears
Spindle fibres form from the centriole
M - Metaphase
Chromosomes aligned at the equator of the cell
Attached by fibre to each centriole
Maximum condensation (10000) of chromosome
A - Anaphase
Sister chromatids separate at the centromere
Separate longitudinally
Move to opposite ends of the cell
T - Telophase
New nuclear membranes form
Each cell contains 46 chromosomes (diploid)
Cytokinesis
Cytoplasm separates
Formation of two new daughter cells
extragenic sequences:
[1] Tandemly repeated DNA sequences
– Satellite DNA
(14-500 bp repeats in 20-100 kb arrays,
at centromeres and telomeres
– Minisatellite DNA
(15-100 bp repeats in 1-5 kb arrays;
used for DNA fingerprinting)
[2] Interspersed repeated DNA sequences
~ 45% of the genome
(remnants of transposon)
– SINEs
(short interspersed nuclear elements, 100-400 bp,
most common are Alu elements)
– LINEs
(long interspersed nuclear elements, up to 6 kb,
most common are L1 elements)
histones:
beads around a string form nucleosomes
– 146 bp DNA wrapped (1.8 turns) around ca ore of 8 histone proteins
Formation of a solenoid structure that compacts DNA by a factor of 40
a bunch of DNA packaged with a bunch of histone proteins form chromatin
i.e. a bunch of beads around a string
Histones have a positive charge
Packaging DNA purpose:
Negatively charged DNA neutralised by positively charged histone proteins
DNA takes up less space
Inactive heterochromatic DNA folded into inaccessible locations until required:
- characterised by specific histone covalent modification (e.g. methylation)
metacentric, submetacentric and acrocentric:
Metacentric
centromere medially situated so that the two chromosomal arms are of roughly equal length
Submetacentric
centromere situated so that one chromosome arm is somewhat shorter than the other.
→ largest group of human chromosomes
Acrocentric
centromere situated so that one chromosomal arm is much shorter than the other.
→ p arm so small - no functional DNA
g banding:
a technique used in cytogenetics
produces a visible karyotype by staining condensed chromosomes.
Fluorescent in situ hybridisation (FISH):
a laboratory technique for detecting and locating a specific DNA sequence on a chromosome.
relies on exposing chromosomes to a small DNA sequence [probe]; it has a fluorescent molecule attached to it.
Two dots = corresponding to the replicated sister chromatids
Types of Fluorescent in situ hybridisation (FISH) probes:
Unique sequence probes
Centromeric probes
– Useful for determining chromosome number
Telomeric probes
– Useful for detecting subtelomeric rearrangements
(unexplained mental retardation)
Whole chromosome probes
– Cocktail of probes covering different parts of a particular chromosome
– Used with different fluorescent dyes & Spectral karyotype
– Useful for detecting translocations and rearrangements
Why is it better to look at the chromosomes of a person in a metaphase spread rather than interphase spread?
Interphase chromosome spread:
DNA is interspersed throughout the nucleus
Metaphase chromosome spread:
DNA is organised into sister chromatids
meiosis:
cell division in germ cells.
Diploid cells (in ovaries and testes) divide to form haploid cells
Chromosomes are passed on as re-arranged copies (crossing over)
→ allows genetic diversity
features of meiosis:
Homologous chromosome pairing - tetrads
Genetic recombination (crossing over)
- Two PMATs I & II
→ no centromere separation in 1st phase, homologous chromosome pairs separate
→ centromere separation in 2nd phase, :chromatids separate - Production of four haploid (n) daughter cells - gamete cells (genetically distinct)
gametogenesis:
[1] Oogenesis = process of egg formation
[2] spermatogenesis = process of sperm formation
- Both go through several stages, with different timing in males and females
- Sperms go through more cell divisions than eggs do – more chance of mutation
fertilisation:
occurs when two haploid cells (egg, sperm) form 1 diploid cell (zygote) – and develop into an embryo.
Whether the sperm contains an X or Y chromosome
→ determines if the embryo is female (XX) or male (XY)
Mitochondria (and their DNA) come only from the other via the egg - known as maternal inheritance
x inactivation:
the random inactivation of one X chromosome during early embryo in female mammals
→ prevents them from having twice as many X chromosome gene products as males,
who only possess a single copy of the X chromosome
e.g. tortoiseshell cats illustrate the mosaic nature of female mammals
