Introduction To DNA Flashcards
Why is the euchromatin (beads on a string) DNA expressed as genes whereas heterochromatin is not?
Euchromatin has a more open structure so can be accessed for transcription
What are the minor and major grooves of a DNA molecule?
Major grove = where the backbones are far apart
Minor groove = where the backbones are close together
What is a nucleosome?
A length of dna coiled around a histone
What is the diameter of a solenoid fibre?
30nm
Are genes expressed when DNA is in the chromosome form? Give a reason for your answer.
Chromosomes - genes not expressed, DNA very tightly wound
How many genes are there roughly in the human body?
~25000
What is a genome?
How many chromosomes is the human genome?
The entire dna sequence of an organism
24 chromosomes (22 autosomes, 2 sex chromosomes)
Give two examples of nucleic acids
DNA
RNA
What is a nucleotide? What is a nucleoside?
Nucleotide = phosphate + sugar + base Nucleoside = sugar + base
What is a similarity between the sugars that make up DNA and RNA?
They are both pentose sugars
What is the sugar in a DNA nucleotide?
2-deoxyribose
What is the sugar of an RNA nucleotide?
Ribose
At which carbon does the nitrogenous base of a nucleotide connect to the sugar?
Carbon-1
To which carbon is the phosphate group of a nucleotide connected to the sugar?
Carbon-5
What are the two classes of nitrogenous bases?
Purines and pyrimidines
What are the two purines?
Adenine, guanine
What are the two pyrimidines (+ 1 in RNA)?
Thymine, cytosine (and uracil)
How are nucleotides joined to one another?
By phosphodiester bonds between Carbon-3 and Carbon-5
Why can nitrogenous bases form hydrogen bonds?
The =O of the base is delta-ve
The -NH2 of the base is delta+ve
What type of helix is shown by DNA?
A right-handed double helix
In what way do the two strands of a DNA molecule run?
Antiparallel, one 5’-3’ and one 3’-5’
What are the 5 stages of the cell cycle?
G1, S, G2, Mitosis (and G0)
What happens in G1 of the cell cycle?
Cell content replication, e.g. Increase in organelle no.
What happens in the S phase of the cell cycle?
DNA Replication
What happens in the G2 phase of the cell cycle?
Double-checking/repair of the DNA
What is the G0 phase of the cell cycle?
Cell cycle arrest
Which enzyme catalyses the extension of the DNA nucleotide sequence in DNA replication? In which direction does it always work and extend the chain?
DNA polymerase
Reads in the 3’->5’ direction, producing a new 5’->3’ strand
What are the 3 stages of DNA replication?
Initiation
Elongation
Termination
Which enzyme provides the ‘kick-start’ for DNA replication? What does it do?
Primase
Synthesises a short RNA primer to begin DNA replication
What is the name given to the fragments that make up the lagging strand? Which enzyme joins these fragments?
Okazaki fragments
DNA ligase
Which enzyme unwinds the DNA double helix?
DNA helicase
What causes DNA replication to stop?
When the two replication forks come together
What stages make up the interphase of the cell cycle?
G1, S, G2
What is cytokinesis? When does it occur?
Division of the cytoplasm. After telophase.
How does DNA wrap around histones?
Loops around 8 histones twice to form a nucleosome
Further packaging of the dna by forming coils of nucleosomes produces what?
What happens to these chromatin fibres in mitosis?
Chromatin fibres
Condense into chromosomes
What are some clinical applications of nucleosides as drugs?
Often used as antiviral/anti cancer drugs
Why are nucleosides often used instead of nucleotides as drugs?
Nucleosides can be phosphorylated in the cell anyway and don’t carry the -ve charge that a phosphate group has making them able to pass through the plasma membrane more easily
What is an example of an antiviral nucleoside based drug? How does it work?
AZT
Lacks carbon-3 OH- group, no phosphodiester bonds formed, no dna elongation
What are exonucleases?
Enzymes that cleave nucleotides from the END of a polynucleotide chain
What are endonucleases?
Enzymes that cleave the polynucleotide chain at any point other than the end 2
How does DNA polymerase repair mismatched nucleotides discovered during proofreading?
Using its exonuclease 3’->5’ proofreading domain
What is the function of the enzyme topoisomerase?
Releases tension in the DNA helix as the helix is unwound
When does replication slippage occur in dna replication?
When there is respective sequences of DNA
What is replication slippage? What can it result in?
Results in the looping out of the strand and in the next S phase there will be an extra or omitted nucleotide
Can lead to the expansions/contractions of the DNA sequence
By which method are single strand DNA breaks repaired?
Base excision repair
Double stranded DNA breaks are the most dangerous type of DNA damage. How do mutations in the BRCA gene affect the repair of DSBs?
Can result in the persisting DSBs and defect in repairing —> cancer
What gene is responsible for Huntington’s? In what way is this gene inherited?
HTT Gene
Autosomal Dominant
Which mutation results in Huntington’s?
A triplet repeat expansion (CAG - glutamine)
What is the normal amount of CAG repeats and the disease amount of CAG repeats?
Normal = 6-39 Disease = 35-121
High levels of DNA damage result in either…
Cancer or Ageing
Mutations in which gene are often seen as the final step between pre-malignant to malignant cancer?
P53 mutations
What is the result of Werner Syndrome? What inheritance does it show?
Premature ageing
Autosomal recessive
What are the differences between the histone proteins in heterochromatin and euchromatin?
Heterochromatin = histones are deacetylated Euchromatin = histones are acetylated
