16. Structure and Replication of DNA Flashcards
What are the components of a nucleotide
- deoxyribose sugar
- a phosphate group
- a nitrogenous base
Purines and pyrimides
Purines - Adenine, Guanine
Pyrimidines - Cytosine, Thymine
What type of bonds hold bases together?
Hydrogen bonds
How many bonds between C and G
3 hydrogen bonds
How many bonds between A and T
2 hydrogen bonds
locus
The position of a gene on a chromosome
When are chromosomes visible
during cell division
Chromatin
DNA + protein = chromatin
What is a nucleosome made up of?
- histone molecule (protein) - dna double helix coils around this
- linker histone clamps DNA to the core
The whole thing makes up a nucleosome
Euchromatin
- “switched on”
- is a lightly packed form of chromatin (DNA, RNA, and protein) that is enriched in genes, and is often (but not always) under active transcription.
Heterochromatin
- “Switched off”
- chromosome material of different density from normal (usually greater), in which the activity of the genes is modified or suppressed.
What is meant by ‘DNA is semi-conservative’
New molecule of DNA has one strand which comes from the parent molecule and one strand which is newly
synthesised.
Why do eukaryotes have multiple replication origins?
•having one replication origin means it would take years to replicate DNA
- so humans have thousands
Replication origin in mammals
• chromatin structure rather than a DNA sequence
- a complex of proteins forms ORC (origin recognition complex)
Replication origin in yeast
• specific sequence of DNA at the replication origin with 120 bases
What is the sliding clamp?
Complex of proteins in the shape of a donut which keeps the DNA polymerase in place and increases the efficiency of replication
Describe the steps of DNA replication in E.coli
- Helicases separate strands of DNA
- DNA topoismerase (gyrase) relieves supercoiling
- single stranded binding proteins keeps the strands apart
- DNA polymerase III adds bases 3’ -> 5’
What does DNA polymerase III need to start replication
- needs a short double-stranded region to start DNA replication
- short primer RNA
In what direction can bases be added
From 3’ prime to 5’ end
What are okazaki fragments?
One of the RNA-primed short segments of DNA synthesised during replication
of the lagging strand of the double helix.
Laggin strand replication
- primers are added to start replication
- primers are removed and the gaps are filled with complementary bases
- okazaki fragments are joined
(Bases added from 5’ to 3’ end)
Speed of replication bw eukaryotes and prokaryotes
Prokaryotic DNA replication is a lot faster
Leading strand vs the lagging strand
•The leading strand is that which is synthesised continuously
•The lagging strand is synthesised in a discontinuous fashion during DNA
replication.
What 3 elements are needed for a chromosome to function and why?
- telomeres - structural stability, replication, positioning
- centromeres - essential for segregation at cell division (acentric fragments get lost on the spindle)
- origins of replication - DNA replication starting point
What type of bond links adjacent nucleotides together?
Phosphodiester bond
5’ (PO4) end
The end of the polynucleotide that terminates with a phosphate attached to the
5’- carbon of the sugar.
3’ (OH) end
The end of a polynucleotide which terminates with a hydroxyl group attached
to the 3’- carbon of the sugar.
DNA heliCase
An enzyme that uses energy from ATP to unwind the DNA
What is Topoisomerase
gyrase
An enzyme which can relax DNA by cutting and reforming the polynucleotide
backbones
Single-strand binding protein function
One of the proteins that attaches to single stranded regions of DNA and
prevents the reformation of base pairs, particularly in the region of the
replication fork.
primase
an enzyme which synthesises the RNA primer
DNA polymerase I
Enzyme which adds nucleotides to gaps when primers removed
DNA polymerase III
Main polymerase which adds nucleotIdes to template in a complementary way
RNA primer
Sequence of RNA to make a stretch of double stranded DNA on which the
DNA pol III can work
Ligase
Enzyme which joins sequences of DNA
What are telomeres
- A compound structure at the end of a chromosome.
* protect ends of chromosome by looping or recruiting protective proteins
Relation bw telomeres and ageing
- problems with replication of lagging strand
- unreplicated DNA is degraded so telomeres shorten with age
- lose bases after each round of replication
- genes on the ends of chromosomes are lost so cell dies
Telomere replication
• leading strand replication is continuous from 3’ to 5’ end
• lagging strand a problem since there is not enough DNA template for primer
- there is an overhang which isn’t replicated (not enough sequence for primer)
•enzyme telomerase extends the lagging strand and primer attaches so DNA polymerase can complete the replication
What are the effects of having shorter telomeres?
- premature aging
* shorter telomeres
