nucleic acids and chromatin Flashcards
When are chromosomes visible in the cell cycle?
Metaphase
What is the ordered display of chromosomes called?
Karyotype
How do you count chromosomes and chromatids?
Chromosomes: number of centromeres
Chromatids: number of arms
What are telomeres?
Ends of a chromosome
what is the constricted region of a chromosome?
centromere
what is the role of the centromere?
act as the site of assembly of the kinetochore
What is the kinetochore?
Highly compex multiprotein structure that is responsible for the actual events of chromosome segregation
What is mitosis?
Cell division
Mother cell divides to produce 2 new daughter cells that are genetically identical itself
what is the 3 components of a nucleotide?
- nitrogen-containing base
- 5 carbon sugar
- phosphate group
Nitrogen containing bases in DNA / RNA
Cytosine
Guanine
Thymine / Uracil
Adenine
Bonding between nitrogen containing bases?
Hydrogen bonds
C ≡ G
A = T/U
Molecular shape of double stranded DNA?
alpha helix
Describe the structure of a DNA molecule
2 linear strands
Held together by hydrogen bonds between base pairs
Run antiparallel to each other
5’ to 3’ and 3’ to 5’
Twist together
How are adjacent nucleotides of RNA and DNA linked?
What is formed?
Phosphodiester bonds
Sugar-phosphate backbone
Describe the positioning of the phosphodiester bond between 2 nucleotides
Phosphate group attached to 5’ C forms bond with OH group on 3’ of the adjacent nucelotide
In which direction does the sugar-phosphate backbone grow?
5’ to 3’ direction
How does the OH group on the ribose in RNA nucleotides affect it? (not present in DNA nucleotides)
Responsible for secondary structure
More susceptible to alkaline degredation
What is a histone?
Protein that associates with DNA in the nucleus
helps condense DNA into a smaller volume
What is chromatin?
Complex of DNA and proteins
Forms chromosomes within the nucleus of eukaryotic cells
Describe 2 forms of chromatin
- Euchromatin
*less condensed
*can be transcribed - Heterochromatin
*more condensed
*not usually transcribed
Describe 2 forms of chromatin
- Euchromatin
*less condensed
*can be transcribed - Heterochromatin
*more condensed
*not usually transcribed
Most of the chromosome body is which type of chromatin?
euchromatin
What are nucleosomes?
Approximate diameter?
Fundamental subunit of chromatin
Where chromatin has wrapped around a histone
bead on a string
11nm
what is a histone octamer
8 proteins called histones
What charge do histone core proteins have?
Positive - binds tightly to the negatively charged DNA backbone
Each nucleosome has how many turns of DNA?
What are they wrapped around?
1.65 turns
8 histones - histone octamer
What makes up the histone octamer?
2 copies of each histone proteins:
H2A
H2B
H3
H4
Which histone is not part of the histone octamer?
Where is it found?
H1
found in Nucleosome linker DNA
linker DNA is between nucleosomes
Nucleosomes fold around each other to form…
Chromatin fibre
What does chromatin fibre formation require?
What does it do?
H1
Binds linker DNA between nucleosome cores
Forms a chromatosome
What does chromatin fibre formation require?
What does it do?
H1
Binds linker DNA between nucleosome cores
Forms a chromatosome
what is chromatin remodelling?
when DNA “opens” using remodellers for gene expression to take place
Cell’s chromatin ‘opens’ using remodellers
Allows access to the DNA for transcription
Describe ATP-dependent chromatin remodelling
ATP-dependent chromatin remodelling complex
Moves, ejects, or restructures nucleosome
Exposes DNA to transcription proteins
Two ways of chromatin remodelling
Using ATP-dependent chromatin remodelling complexes
Enzymes modify the histone proteins in the nucleosomes
by adding / removing covalently bonded moieties
Describe covalent histone modifications
Add/remove covalently bonded moieties to histone
Done by enzymes
HATs (histone acetyltransferases) - adds acetyl group
kinases - transfer phosphate groups
deacetylases - remove acetyl group
methyltransferases add methyl group
How does remodelling chromatin mediate the activation of genes?
Exposes gene promoter sequences
Increases frequency of transcription
What are histones rich in?
In which part?
Positively charged lysine residues
N terminus tails
extend out of nucleosome between DNA turns
How can histone tails be modified?
Where does each modification occur?
Methylation of lysine and arginine
*inactivates
Acetylation of lysine
*activates
Phosphorylation of serine
*activate / deactivate
What can modified histone tails act as?
Signals / Binding sites for regulatory proteins
some proteins promote condensation
some promote decondensation
easier to access dna
What can modified histone tails act as?
Signals / Binding sites for regulatory proteins
some proteins promote condensation
some promote decondensation
easier to access dna
Does methylation activate or deactivate chromatin?
deactivate
Where in the chromosome is methylation common?
Heterochromatin areas ( places unlikely to contain any genes )
telomeres
centromere
Where in the chromosome is methylation common?
Heterochromatin areas ( places unlikely to contain any genes )
telomeres
centromere
Chromatin remodelling, DNA modification and histone modification influences the ____ but does not alter the ____
Influences phenotype
Not alter genotype
Chromatin remodelling, DNA modification and histone modification influences the ____ but does not alter the ____
Influences phenotype
Not alter genotype
Can modifications to DNA and histones be replicated?
yes
