7.1 dna structure and replication understanding: -dna structure suggested a mechanism for dna...
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7.1 DNA structure and replication
Understanding:- DNA structure suggested a mechanism for
DNA replication- Nucleosomes help to supercoil the DNA- DNA replication is continuous on the
leading strand and discontinuous on the lagging strand
- DNA replication is carried out by a complex system of enzymes
- DNA polymerase can only add nucleotides to the 3’ end of the primer
- Some regions of DNA do not code for proteins but have some other important function
Applications:
- Rosalind Franklin’s and Maurice Wilkins’ investigation of DNA structure by X-ray diffraction
- Tandem repeats are used in DNA profiling
- Use of nucleotides containing dideoxyribonucleic acid to stop DNA replication
Skills:
- Analysis of results of the Hershey and Chase experiment providing evidence that DNA is the genetic material
- Utilisation of molecular visualisation software to anaylyse the association between proteins and DNA within a nucleosome
Nature of science:
- Making careful observations: Rosalind Franklin’s X-ray diffraction provided crucial evidence that DNA is a double helix
Hershey-Chase experiments
Is DNA the genetic material?
Alfred Hershey and Martha Chase used radioisotopes to help prove this.
Radioisotopes
Forms of elements that decay over time at a predictable rate.
Particles given off in this decay allow detection of the specific isotope used
Hershey-Chase experiments
Grew bacteriophage viruses in two different types of cultures.
Radioactive phosphorous-32- Detectable phosphorous in DNARadioactive sulphur-32- Detectable sulphur found in outer coat of virus
Allowed to infect bacteria (E.coli)
Hershey-Chase experiments
As DNA contains phosphorous and not sulphur, they concluded that DNA was the
genetic material not protein.
Hershey-Chase experimentsDescribe the Hershey-Chase experiment:
DNA structure3 components:
– Pentose sugar (deoxyribose in DNA)
– Phosphate – Organic base (always contains nitrogen)
Phosphate
sugarbaseStay the same
ChangesContains nitrogen & carbon
Pentose sugar (5 Carbon atoms)
Single strand of DNA
Backbone is made of alternating phosphate and deoxyribose sugar
They are held together by a phosphodiester bond (just ester at SL!)
Condensation reaction – producing water
This produces a chain of DNA (as this links the single nucleotides together)
Phosphodiester bond
Where does the phosphodiester bonds occur?
Phosphate group reacts on the 5’ carbon
Hydroxyl group reacts on the 3’ carbon
Double strand of DNA
The strands of DNA run in opposite directions to each other
There is a 5’ carbon free to bond on this end…
…and a 3’ carbon free to bond on this end
There is a 3’ carbon free to bond on this end…
…and a 5’ carbon free to bond on this end
Double strand of DNA
Each end is either the 5-prime or 3-prime end5’
3’
3’
5’
Label the phosphodiester bond, hydrogen bonds, 3’ end and 5’ end for both strands, bases, ribose sugar, phosphate group, 3’ carbon and 5’
carbon in ribose.
DNA packaging
DNA molecules are paired with a protein called histone
Histones help to package DNA
Essential as DNA can be 4cm long, it must fit into a microscopic nucleus
DNA packagingUnfolded DNA looks like beads on a string
These beads are nucleosomes
8 histones (proteins) make up a nucleosome core
DNA wraps round these histones twice(slight negative charge on DNA attracts to
positive charge of histones)
Between the nucleosomes are strings of DNA
DNA sequencesFrom the Multinational Genome Project we have learnt
that less than 2% of human DNA codes for proteins.
What does the other 98% do?
- Regulate gene expression- Telomeres (protects DNA)
- Introns (interruptions in coding region)
Types of DNA sequencesFind out what the following are:
- Protein-coding genes- Highly repetitive sequences
- Structural DNA- Short tandem repeats
Protein-coding genes
Highly repetitive sequences
Structural DNA
Short-tandem repeats
Protein coding genesGenes that have coding functions
Provide base sequence to produce proteins
Genes will have interruptions of non-coding regions in between them then must be spliced out before proteins
are made.
Coding regions = ExonsNon-coding regions = Introns
Highly repetitive sequences3-500 base pairs long in a repetitive sequence
It could go up to 100,000 repeats of a single unit
If it is clustered together – satellite DNA
So far not discovered any coding function (not genes)
Transposable – can move from one location to another
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
TTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTT
Structural DNAHighly coiled DNA with no coding function.
Near centromere and telomeres
Could have lost their function due to mutations involving a base sequence change
Short Tandem RepeatsYour DNA is almost identical to the person next to you
Specific regions are varied – polymorphisms
Analyse these using DNA profiling
Usually look at short tandem repeats
Short repeating sequences of DNA – 2-5 base pairs
T T T C C C T C A T C A T C A T C A T C C C GA A A G G G A G T A G T A G T A G T A G G G C
DNA replicationReplication starts as a bubble
Helicase breaks the hydrogen bonds between nucleotides
Two strands separate
At each end of the bubble there is a replication fork – where DNA strands open.
Bubbles enlarge in both directions – bidirectional
Bubble eventually fuse with one another to produce two identical daughter DNA molecules
DNA replication
1. Primer (short piece of RNA) is produced at the replication fork by primase
2. Primers match exposed DNA bases, marks the start of replication
3. DNA polymerase III allows addition of nucleotides in 5’ to 3’ direction
4. DNA polymerase I removes the primers from the 5’ end
Energy to create the bondsEach nucleotide molecule is a deoxynucleoside triphosphate (dNTP) molecule
Contains - Deoxyribose - A base- Three phosphate groups
As the molecules are added together to form nucleotides, two phosphates are lost
This provides energy needed for the nucleotides to bind
DNA replication
When DNA is replicated – it assembles 5’ to 3’ due to the action of polymerase III.
5’
3’
3’
5’
DNA replicationThis means that there is a difference in the assembly of DNA from the
templates
Leading strand – continuous and fast (5’ to 3’)Lagging strand – slowly (3’ to 5’)
DNA replication1. The leading strand assembled continuously in 5’ to 3’ direction2. Lagging strand is assembled by fragments produced moving away
from the replication fork – in the 5’ to 3’ direction3. Fragments of the lagging strand called Okazaki fragments 4. Primer, primase and DNA polymerase III are needed to begin both
the leading and lagging strand 5. Once Okazaki fragments assembled, DNA ligase enzyme attaches
them together to make a single strand
Formation of the lagging strand
1. 2.
3.
4.
5.
6.
Proteins in replication
What do they do?
Protein Role
Helicase
Primase
DNA polymerase III
DNA polymerase I
DNA ligase
Proteins in replication
What do they do?
Protein Role
Helicase Uniwnds double helixBreaks H bonds
Primase Synthesises RNA primer
DNA polymerase III Synthesises new strand by adding nucleotides onto the primer (5’ to 3’
direction)DNA polymerase I Removes primer
DNA ligase Joins Okazaki fragments
Complete for your homework
DNA sequencing
- What is DNA sequencing?- What is the process?- What is it used for?
DNA profiling
- What is DNA profiling?- What is the process?- What is it used for?- How is it different to DNA sequencing?