GCD 2024 REVIEW EXAM QUESTIONS WITH ALL CORRECT ANSWERS
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GCD 2024
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GCD 2024
GCD 2024 REVIEW EXAM QUESTIONS WITH ALL CORRECT ANSWERS
Heterochromatin - Answer-Tightly compacted regions of chromosomes
Transcriptionally inactive (in general)
Radial loop domains compacted even further
The 30-nm fibers are organized into loops - Answer-Two proteins responsible for loops...
GCD 2024 REVIEW EXAM
QUESTIONS WITH ALL CORRECT
ANSWERS
Heterochromatin - Answer-Tightly compacted regions of chromosomes
Transcriptionally inactive (in general)
Radial loop domains compacted even further
The 30-nm fibers are organized into loops - Answer-Two proteins responsible for loops
are CTCF and SMC protein
In cells, nucleosomes are in 30-nm fibers - Answer-In conditions where a histone H1 is
present in the chromatin, a thicker chromatin fiber is seen: 30-nm in diameter. This is
how the chromatin exists in the cell.
Nucleosomes - Answer-In conditions where the 30-nm fiber is partially disrupted, you
see a structure that looks like a zig-zag in places.
It turned out that images like the one on the right were obtained in conditions where a
protein called histone H1 was missing from the chromatin. - Answer-Nucleosomes
DNA vs Histones - Answer-In real life, the DNA is larger in relation to the histones
8 histone molecules make up a nucleosome - Answer-There are five types of histones
H2A, H2B, H3 and H4 are the core histones
Two of each make up the octamer
H1 is the linker histone
Binds to linker DNA
Also binds to nucleosomes
But not as tightly as are the core histones
A nucleosome is composed of double-stranded DNA wrapped around the octamer of
histone proteins - Answer-146 bp of DNA make 1.65 negative superhelical turns around
the octamer
The nucleosomes "beads on a string" structure - Answer-shortens the DNA length about
seven-fold
The proteins in nucleosomes are called histones - Answer-Histone proteins are basic ie.
they contain many positively-charged amino acids (lysine and arginine)
These bind with the negativelycharged phosphates along the DNA backbone
Histone proteins have a globular domain and a flexible, charged amino terminus or 'tail'
Nucleosomes - Answer-Eukaryotic cells were broken open and the DNA was seen in
this form. The round things were named 'nucleosomes'
, Transposable elements have a variety of effects on chromosome structure and gene
expression - Answer--heterochromatin can spread from a transposable element and
silence a nearby gene-heterochromatin can be unstable:-heterochromatin can spread
and contract influencing nearby genes differently in different people
-if a transposable element loses its heterochromatin this can expose DNA sequences
that may alter the expression of nearby genes-for example, some transposons have
DNA sequencesthat function as enhancers of promotors
Transposable elements can rapidly enter the genome of an organism and proliferate
quickly: - Answer-Drosophila melanogaster A transposable element known as the P
element was introduced into the species in the 1950s
Remarkably, in the last 50 years, the P element has expanded throughout D.
melanogaster populations worldwide
The only strains without the P element are lab stocks collected prior to 1950
The original interpretation of retroelements is that they exist because they simply can! -
Answer-In other words they are like parasites
They can proliferate within the host as long as they do not harm the host to the extent
that they significantly disrupt survival
This has been termed the selfish DNA theory
Retrotransposition - Answer-A single retroelement can be copied into many RNA
transcripts
Therefore, retroelements may accumulate rapidly within a genome
The Alu family of retroelements does not share any sequence similarity with viruses -
Answer-It is derived from a single ancestral gene known as the 7SL RNA genethat is
part of a RNA-protein complex that transports proteins carrying a specific sequence
('signal') to the endoplasmic reticulum (while they are still being synthesized
This Alu gene sequence has been copied by retrotransposition to greater than
1,000,000 copies in humans
More information on the most common retroelements in humans (these do not have
LTRs): - Answer-LINE-1
Long interspersed nuclear elements
Up to 6-kb long
Found in ~900,000 copies per genome
SINEs
Short interspersed nuclear elements
<500-bp in length
Example: Alu sequence
Present in >1,000,000 copies in the human genome
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