Organization of the Genome Flashcards
What is a chromosome formed from?
2 chromatids joined at the centromere
Structural unit of Eukaryotic DNA
Nucleosomes
What is chromatin?
Repeating Nucleosome Units
The structure of chromatin is stabilized through …
the interactions between DNA and DNA-binding proteins
What are the classes of DNA-binding proteins?
Histones
Non-histones
What is chromatin composed of?
Nuclear DNA
Histone Proteins
Small amount of Non-Histone Proteins
Small amount of RNA (snRNA)
What are the forms of chromatin?
Heterochromatin
Euchromatin
What is heterochromatin?
Transcriptionally inactive chromatin
Darkly-stained regions
What is euchromatin?
Transcriptionally active chromatin
Lightly-stained regions
What are the types of heterochromatin?
Constitutive Heterochromatin
Facultative Heterochromatin
What is constitutive heterochromatin?
Always condensed and inactive
E.g. Chromosomal Centromere and Telomeres
What is facultative heterochromatin?
Sometimes condensed and sometimes uncondensed for transcription
E.g. one X chromosome in mamillian females
Histones are ____ proteins
basic
What makes histones basic?
Rich in lysine and arginine
How do histones interact with DNA?
Form ionic bonds with -vely charged acidic DNA phosphate group
What are the types of histones?
H1
H2A
H2B
H3
H4
What are the functions of histones?
Help DNA condensation
Protect DNA from exonuclease digestion
What is the structure of a nucleosome?
dsDNA wound twice around an octameric core
dsDNA is 150bp long
What forms the octameric core
Two molecules of each: H2A, H2B, H3 and H4
How are neighbouring nucleosomes linked?
By linker DNA (20-200bp long)
H1 histones bind linker DNA
What is the purpose of the H1 histone?
Facilitates packing of nucleosome
Protects DNA from exonuclease digestion
Nucleofilaments
Tightly stacked nucleosomes arranged as a coil
What is the purpose of histone modification?
Affect how tightly histones bind to DNA affecting gene expression
What are the methods of histone modification?
Acetylation
Methylation
Phosphorylation
Which parts of the histones are affected by histone modification?
N-terminals
Histone modification is an example of ____
epigenetics
How does histone acetylation affect the DNA?
Acetyl group binds to lysine residue of histones
Removes the +ve charge reducing affinity of DNA for histone making DNA transcriptionally active
Histone acetylation is catalysed by …
Histone Acetyltransferase (HAT)
Histone deactylation is calaysed by …
Histone Deacetylases (HDACs)
The histones in ____ are deactylated whereas in ____ it is acetylated
__Heterochromatin__
__Euchromatin__
What does Histone Methylation cause?
Inactivation of gene expression
Exon meaning
Coding sequences of genes
Introns/Intervening Sequence meaning
Non-coding sequence separating coding sequences of genes
Where is non-coding DNA found in prokaryotes?
Intergenic DNA (between genes)
Where is non-coding DNA found in eukaryotes?
Between and within genes
Mbp meaning
Million Base Pairs/Megabase Pairs
Stem and loop structure
RNA complementary intermolecular base pairing within the same molecule
NNN represents the unpaired bases
A Unique Sequence is …
a sequence which occurs twice in diploid genomes (or once in haploid genome)
Comprises >20% of total DNA of higher organisms
What are the possible distributions of repeat sequences?
Tandem Repeats
Interspersed Sequences
Categories of repeated DNA sequences
Moderately Repetitive Sequences
Highly Repetitive Sequences
What are Moderately Repetitive Sequences?
Present in hundreds or thousands of copies
Formed mostly from Long Interspersed Elements (LINEs)
E.g. ribosomal RNA genes
What are Highly Repetitive Sequences?
Present in hundreds of thousands to millions of copies
Formed mostly from Short Interspersed Elements (SINEs)
E.g. 300 bp Alu element
Types of Short Tandem Repeats (STRs)
Microsatellite Repeats
Variable Number of Tandem Repeats (VNTRs)
What are Microsatellite Repeats?
Non-coding DNA formed from repeats of 2-8 bp
How many times can microsatellite repeats appear?
100 times
Microsatellite repeats exist in long clusters of …
Short Tandem Repeats (STRs)
Diseases related to microsatellites
Microsatellite expansion in protein-coding genes causes Huntington’s Disease and Fragile-X Syndrome
How can genetic links be determined between family members?
PCR detects number of microsatellite repeats
What are VNTRs?
Segments of short tandem repeats
Fewer copies than satellites
What is the most common location of VNTRs?
Eukaryotic telomeres
Why is the number of repeats of VNTR variable between individuals?
Due to unequal crossing over
Variations between VNTRs are called alleles
What can VNTRs be used for?
DNA fingerprinting
Characteristics of Human Mitochondrial DNA
Circular
Double-Stranded
What does mitochondrial DNA encode?
Mt rRNA
Mt tRNA molecules
Proteins of respiratory chain
What are the methods of altering/rearranging genetic material?
Chromosomal Recombination
Chromosomal Integration of viruses
Transposition
Immunoglobin genes rearrange
How does Chromosomal Recombination occur?
Crossing-Over
How may Chromosomal Integration occur?
Bacteriophages recombine with bacterial DNA
How does transposition occur in eukaryotic cells?
Small DNA elements are capable of transposing themselves in and out of the host genome
Describe how immunoglobin genes rearrange
VL and CL genes are closer together in plasma cells than other cells
What are telomeres?
Short repeating sequences at the end of each chromosome
Nucleotide sequence of telomeres
TTAGGG
How do telomeres change with age?
Length of telomeres decreases over time
What can happen when telomeres shorten too much?
Cells become senescent, die or sustain genetic damage causing cancer
In which cells do telomeres no longer shorten?
Germ Cells
Stem Cells
Cancer Cells
Why do telomeres no longer shorten in germ, stem and cancer cells?
Enzyme called telomerase prevents telomeres from getting shorter
