16. Structure and replication of DNA Flashcards
What is a gene?
The fundamental physical and functional unit of heredity which arrives information fro one generation to the next
- Segment of DNA
What are the three components of a nucleotide?
- Deoxyribose sugar
- Phosphate group
- Nitrogenous base
Which bases are purines?
A and G
Which are pyrimidines?
C and T
What are the bases joined by?
Hydrogen bonds
How may hydrogen bonds between C and g?
3
How many hydrogen bonds between A and T?
2
What is a locus?
The position of a gene on a chromosome is called a locus
Where are chromosomes found?
Cytosol in prokaryotes
Nucleus in eukaryotes
What is DNA and protein known as?
Chromatin
What is a nucleosome?
The structure formed when the DNA double helix coils around a core of eight histone molecules
When does DNA replication occur?
During the S (synthesis) stage of the cell cycle
What is the replication origin for eukaryotes?
- In yeasts specific sequence of DNA at replication origin (ARS)
- For mammals, a chromatin structure rather than DNA sequence at replication origin
- A complex of proteins that forms the origin recognition complex (ORC)
What is the sliding clamp?
A compels of proteins in the shape of a donut which keeps the DNA polymerase in place and increases efficiency of replication
For dna replication in E.coli what does helicase do?
It separates the strands of the dna
For DNA replication in E.coli what does DNA topoisomerase (gyrase) do?
It relieves supercoiling
- Single stranded binging proteins keep the strands apart
What does DNA polymerase III do in e.coli DNA replication?
It is responsible for DNA replication itself but needs a short double sanded region to start (primer RNA)
- Reads DNA 3’ to 5’ and can only add based to 3’ end of the growing strand so the strand grows 5’ to 3’
How does DNA replication occur on the lagging strand?
- Must open the strand a little and then prime it
- Short sequences replicated called Okazaki fragments
- Primers are removed and gaps filled to complementary bases
- Fragments joined with ligase
What are the differences between prokaryotic and eukaryotic DNA replication for Okazaki fragments?
Prokaryotes have 1000 bases while eukaryotes 100-200 bases
What is the difference between prokaryotic and eukaryotic DNA replication in terms of speed?
Humans are 3000 bases per minute while bacteria 30,000
What is the difference between prokaryotic and eukaryotic DNA replication in terms of polymerase?
Prokaryotes use DNA polymerase III while eukaryotes use lots of different DNA polymerases
What three things do chromosomes require to function?
Telomeres, centromeres, origins of replication
What do telomeres do?
They are repeated sequences that protect the ends of the chromosome by looping or recruiting protective proteins
Why do we need telomere sequences?
- Problem with replication of lagging strand as there is not enough DNA template for primer
- The overhang is not replicated
What is the function of telomerase?
- Present in germ cells, stem cells and cancer cells
- It extends the lagging strand so the primer attaches and DNA polymerase completes the replication
What does telomerase consists of?
RNA part which is the template for extension, complementary to the telomere repeat
What is werner syndrome?
Mutation in WRN gene which produces a protein which has a role in preserving the ends of chromosomes
