chromosomes and cell division Flashcards
What is a Telomere and its function. And what happens to these parts during replication?
tips of the arms/natural ends of chromosomes
function is to protect the ends from DNA repair machinery
they are lost during replication
p arm
shorter arm
q arm
longer arm
heterochromatin
contains the DNA that the chromosome is not currently using- non-active part of the molecule
enables cell to keep it packaged and inaccessible
DNA replication
2 DNA strands are unzipped
DNA plymerases copy DNA info and synthesise complementary strand
which direction does DNA plymerase synthesis in
5’ to 3’
called continuous
3’ to 5’ synthesis
dis-continuous
completed in a series of fragments
called the lagging strand
what are the small fragments of DNA made during replication called and what do they need ot be initiated by
okazaki
RNA primer
End-replication problem
on the lagging strand there is a gap at the end where the primer has to stop- can’t fully replicate it
what is the answer to the end-replication problem?
telomerase
carries out reverse transcriptase
uses RNA template to template new DNA
chromosomes shorten during a number of division and telomerase detects this and extends them
kinetochore
protein complex that binds to microtubules
required for chromosome separation during cell division
kinetochore
protein complex that binds to microtubules
required for chromosome separation during cell division
Chromatin structure
DNA packaged with histone proteins packaged into units called nucleosomes
octamere nucleuosome structure- 8 histone proteins with DNA wrapped round them
locked in place by histone H1
Histone proteins
positively charged
small
solenoid structure
the structure of the 30 nm fibre. -secondary chromatin structure which helps to package DNA into the nucleus
- 6 nucleosomes in a turn
solenoid structure
the structure of the 30 nm fibre which ultimately becomes the chromosome
- secondary chromatin structure which helps to package DNA into the nucleus
- 6 nucleosomes in a turn
purpose of packaging DNA
- Negatively charged DNA neutralised by positive charged histone proteins
- DNA takes up less space
- Inactive DNA can be folded into inaccessible locations until required
mitosis
IPMAT
interphase
prophase - chromosomes condense, nuclear membrane disappears, spindle fibres form centrioles
metaphase- chromosomes align at equator (metaphase plate), max condensation of chromosomes
anaphase- sister chromatids separate at centromere, move to opposite ends of cell
telophase- new nuclear membranes form- each cell has 46 chromosomes (diploid)
cytokinesis- cytoplasm separates, 2 new daughter cells
fluorescent in situ hybridisation (FISH)
a powerful technique used in the detection of chromosomal abnormalities
- uses fluorescent probes that bind to only those parts of the chromosome with a high degree of sequence complementarity for the probes (the probes are complementary to specific parts of chromosome)
meiosis
2 rounds of cell division
cell division in germ cells
diploid cells (in ovaries and testes) divide to form haploid cells
chromosomes are passed on as re-arranged copies- allows re-assortment of paternal/maternal genes
meiosis process
PMAT-PMAT
essentially the same as in mitosis, except here the non-sister chromatids cross over, forming chiasmata, where they can exchange DNA resulting in variation
the secondary cycle is pretty similar to the first, except the chromosomes are not copied or crossed over. This results in 4 haploid cells being produced instead of 2 diploid cells
spermatogenesis (process of sperm formation)
four sperm cells are formed per meiotic cycle, over a course of around 60 days. They undergo many more divisions than eggs, meaning there are more chances for mutations to occur
oogenesis (process of egg formation)
Each meiotic cycle produces one ovum and 3 polar bodies and this process occurs over 10-50 years.
These polar bodies mature into ova.
x-inactivation
genes present on X chromosome- one needs to be switched off, random inactivation of one in early embryo
so that women don’t make twice as much of proteins encoded by X chromosome genes
What 3 types of chromosome abnormalities are there?
numerical
structural
mutational
numerical abnormalities and name the different autosomal and sex chromosome aneuploidy syndromes.
having too many or too little a number of chromosome pairs ie having more or less than 23
autosomal: Trisomy 21, Trisomy 18, Trisomy 13
sex chromosome: X and XXY
Trisomy 21
DOWN SYNDROME autosomal Trisomy 21 is a type of numerical chromosome abnormality 95% caused by non-dysjunction can cause an IQ of less than 50 Alzheimer's later in life
what is non-dysjunction?
homologous chromosomes or sister chromatids not separating
Trisomy 13
PATAU SYNDROME
autosomal
very few survive past first year of life
90% non-dysjunction
Trisomy 18
EDWARD SYNDROME autosomal 90% non-dysjunction most die within first month to a year of life cut 8 in half = E for edward
X
Turner syndrome
sex chromosome aneuploidy- affects females- 1 less X chromosome
results in neck webbing and widely spaced nipples
intelligence and lifespan are normal
about 97% of babies affects are miscarried or still born but quite rare syndrome 1 in 1500 to 2500
XXY
Klinefelter syndrome affects males- additional X chromosome long limbs infertile, small testes 50% develop breasts mild learning difficulties
Structural abnormalities include?
balanced and unbalanced rearrangements translocation deletion insertion inversion
translocation- balanced and unbalanced
balanced= piece of DNA or chromosome breaks off from two chromosomes and swaps to from two new structurally rearranged chromosomes unbalanced= only one breaks off and swaps
deletion
lost part of chromosome
insertion
addition of larger sequence of DNA into chromosome
inversion: peri-centric and para-centric
rearrangement
piece of chromosome breaks off, turns around and then is reinserted
peri-centric:-both sides of centromere
para-centric:- only at one side of centromere
Robertsonian translocation
fusion of two acrocentric (centromere is close to the end of chromosome) chromosomes
no loss of genetic info
loss of short arms
Mutational abnormalities: somatic and germline?
germline- mutations in germ cells- passed on to children
somatic- occurs after contraception in any of the cells in the body except germ cells
types of mutations?
non coding- doesn’t affect amino acid sequence/protein
coding- silent, missense, nonsense, frameshift
Silent mutation
no change in amino acid or protien
missense mutation
changes one of the RNA codons so amino acid sequence is altered- different protein
nonsense
stop codon is produced
frameshift
addition or deletion of base pair in DNA so translation is altered/ shifted from that point
Point mutation
only affects 1 or very few nucleotides in a gene sequence
Point mutation
only affects 1 or very few nucleotides in a gene sequence
Methods of detecting mutations
PCR DNA sequencing Gel electrophoresis ARMS RFLP
PCR
in vitro (out of the body, lab etc)
amplifies DNA
heating and cooling stages
gel electrophoresis
separates DNA fragments by size by applying electric field
DNA is negatively charged
agarose gel matrix
DNA sequencing
determining order of nucleotides in DNA sequence
very sensitive, robust
gold standard
however expensive
what type of mutation is down syndrome caused by
translocation
an unbalanced translocation of chromosome 21 can lead to having 3 copies of the long arm
It is more commonly caused by trisomy for the whole of chromosome 21
chromosome staining with dyes? what happens
When chromosomes are stained with dyes, they appear to have alternating lightly or darkly stained regions. The lightly stained regions are euchromatin and contain genetically active DNA. The dark regions are heterochromatin and are inactive DNA. (think dark, lights off, inactive)
when does gametogenesis in males occur? and how many divisions occur?
during puberty
when does gametogenesis in females occur? how many divisions occur?
in the fetus
which parent is mitochondrial DNA passed on by?
Mother
this is because the egg is bigger and has more space for mitochondria
when can you only see chromosomes?
when they are dividing
what are mismatch repair (MMR) genes?
they correct errors that spontaneously occur during DNA replication
who do Y-linked conditions occur to only?
only in males and in all their male descendants
