Chapter 2 Flashcards
what is the function of chromatin?
functions of chromatin:
- packing DNA into smaller volumes
- strengthening DNA for meiosis and mitosis
- prevents damaging DNA
- controls gene expressio and replication
what are two different types of chromatin?
- heterochromatin
> found in peripheral areas of nucleus
> responsible for gene regulation and protection of chromosomal integrity
- euchromatin
> less intense than heterochromatin
> tightly packed DNA, RNA and protein
> rich in gene expression and often under active transcription
what is the centromere?
centromere:
> where sister chromatids are joined together
> essential for segregation during meiosis/mitosis
what is the telomere?
telomere
> end of chromosomes
> long array of tandem repeats
>>> 5’- TTAAGGG-3’
what is a problem during replication?
the final okazaki fragment cannot be printed
> there is no primer site to attach to
> DNA molecule would get shorter with every replication
> even if primer binds exactly at the end of the chromosome, there is no “replacement” of primer
replication problem at telomeres
> solution?
solution:
> telomerase: used as a template to extend the 3’ end
> is only active in the early embryo and after birth in reproductive cells and stem cells
what happens with cells that lack telomerase?
cells that lack telomerase activity undergo chromosome shortening every time they divide
> critical factor: maintain a protein cap
> cancerous cells are able to divide continously and often show overactive telomerase
somatic cells are …?
germ cells are …?
somatic cells are diploid
> 2n chromosomes
germ cells are haploid
> n chromosomes
what is the difference between mitosis and meiosis?
mitosis:
> normal cell division
meiosis
> cell division to form germ cells
> 4 haploid gametes are formed
sex chromosomes men vs women
problem in men?
women: each chromosome has a fully homologous partner
> 22 autosomal pairs
> 2 x chr
men problem:
> X and Y are diferent
> they do pair however during prophase 1, due to short regions of homology between X and Y
chr Y, how many base pairs?
how much of total DNA?
how many genes?
how many proteins?
chr Y
> 58 million bp
> 0.38% of total DNA
> 86 genes
> 23 distinct proteins
what does gene SRY do?
gene SRY
> triggers testis development, determines sex
chr Y cannot do what?
> exeption?
> implications?
chr Y cannot recombine with X
> exeption: short homologous regions
> rest are non-recombining regions
> SNPS in non recombining regions are used for tracing direct paternal ancestral lines
what is X-inactivation?
why does it happen?
X-activation is a special form of imprinting
> one of the female X chr is silenced in every cell
> about 12 days after conception, half of female cells express paternal X and half maternal X -> genetic mosaics
> choice which X is random, but permanent
why:
> so females do not have twice as many X chr gene products as males (dosage compensation)
how many basepairs in entire human genome?
entire genome:
> 3.146 billion bp
what is a karyogram?
karyogram:
> set of chromosomes of an organism (banded appereance)
what is a critical characteristics of heterochromatin?
in heterochromatin, transcription is limited
what is the difference between
consitutional and somatic chromosomal abnormalities?
consitutional
> present in all cells of the body
> results of abnormal sperm or egg
somatic (acquired)
> present in only certain cells or tissues of an individual (mosaic)
what are 2 categories of chromosomal abnormailites?
A. copy number is altered
B. structure of chromosome is abnormal
what are 3 classes of numerical abnormalites?
numerical abnormalites
- polyploidy: more than two paired homologous sets of chromosomes
- aneuploidy: abnormal number of chr, e.g. 45/47 when 46 expected
- mixoploidy: two or more genetically different cell lineages within one individual
> from same zygote: mosaicism
> different zygotes: chimerism
what is nondisjunction?
during
- meiosis I
- meiosis II/mitosis
nondisjunction:
meiosis I: failure of paired chr to separate
meoisis II/mitosis: failure of sister chromatids to disjoin
consequences of nondisjunction
> during meiosis
> during mitosis
nondisjunction
meiosis: produces trisomic/monosomic individual
mitosis: produces a mosaic individual
what is ploidy?
what are the general consequences of any ploidy higher than diploid?
almost any ploidy (extra copies of all chr) higher dan diploid results in early death in utero
what is trisomy?
in which chr (partly) compatible with life?
trisomy: extra copy of a single chr
chr13/18/21
> 13/18 result in early death
> 21 result in down syndrome
47, XXY: name?
45, X: name?
47, XXY: Kleinfelter syndrome
> trisomy of sex chr (at least one extra X)
45, X: Turner syndrome
> monosomic, one X is missing
how do abnormalities in chromosomal structure develop?
- dmage to DNA due to radiation/chemicals etc
- errors in the recombination proces
structural abnormalities
> 2 breaks in the same arm, consequences?
2 breaks in one arm
> deletion
> paracentric inversion (within arm)
structural abnormalities
> breaks in 2 diff. arms, consequences?
breaks in 2 diff. arms
> join broken ends results in ring chr
> pericentric inversion (involving both centromere)
what is reciprocal translocation?
reciprocal translocation
> exchange of fragments between two non-homologous chr
robertsonian translocation
> which chr?
> results?
robertsonian translocation: centric and acentric fragments exchanges
> between chr 13,14,15,21,22 (acrocentric chr), short arm of those is very similar in DNA content
> result: 1 chr stable at mitosis
1 chr lost at mitosis
result of translocation between chr 9 and chr 22?
association with cancer?
t(9;22)
- Der 9: chr9 longer than normal
- philadelphia: chr 22 shorter than normal
> creation of a fusion gene “bcr-abl”
> speeding up white blood cell divison
> philadelphia chr. is often found in leukemic cells of patients with chronic myelogenous leukemia
what is chromosome condensation?
chromosome condensation
> long and fragile DNA strands resilient to breakage during cell division
> gene expression is impossible during this state
what is a zygote?
zygote:
> first diploid cell, are cells are derived from this cell
what is a spiral/solenoid arrangement?
spiral/solenoid:
> DNA is wrapped around nucleosomes, which are in turn wrapped in a spiral/solenoid structure
> 6-8 nucleosomes per turn
which histone characteristics are associated with euchromatin DNA?
euchromatin
H1: weak binding
> extensive acetylation of all other histones
what are two different types of heterochromatin?
consitutive heterochromatin
> repetative DNA, genetically inactive
facultative heterochromatin
> some times condensed - genetically inactive
> some times decondensed - genetically active
in which phase of the cell cycle do the sister chromatids separate?
anaphase
what are acentric chromosome fragments?
> what happens with those fragments during cell division?
> why?
acentric fragments: abnormal chromosome fragments that lack a centromere
> during cell division: those fragments cannot attach to the spindle and fail to be segregated correctly
what happens with telomere length each life year?
telomere ~ 27bp shorter each year
what are the 7 phases in a cell cycle
> what happens?
- interphase - normal cell activity
- prophase - centreoles form (first stage of mitosis)
- prometaphase - condensed chrom become attached to array of microtubules
- metaphase - chr aligned with mitotic spindle
- anaphase - sister chromatids separate
- telophase - nuclear envelope folds again, chr decondensate
- cytokinesis - final stage, two daughter cells produced
why is meiosis called “reductive division”?
meiosis involves 2 cycles of cell division but only one cycle of DNA replication
what is euploidy?
what is aneuploidy?
euploidy: having complete chromosome sets
> e.g. n, 2n, 3n etc.
aneuploidy: extra copies of single chrom.
what are two general mechanisms that can lead to aneuploidy?
- nondisjunction
- anaphase lag
> chromosome/chromatid is delayed in movement during anaphase
what happens with acentric/dicentric chrom. fragments during mitosis?
> are there exeptions?
acentric/dicentric chrom. fragments are typically unstable during cell division
> exeption: robertsonian translocation
why are some chrom. fragments resulting from robertsonian translocation stable at cell division?
robertsonian translocation involved acrocentric chromosomes
> short arm of those chr. is very small and very similar in DNA content
> translocation results in two fragments
- acentric fragment which is lost in cell division
- dicentric fragment with two centromeres in close proximity which is stable at cell division
