DNA (CQC) Flashcards
What is the structure of DNA?
DNA is formed of two antiparallel nucleotide strands, forming a double helix . Nucleotides are made up of deoxyribose sugar, one phosphate and one base. The nucleotide is a subunit. There are four bases, Adenine Thymine, Guanine and Cytosine and Guanine, there is complementary base pairing between A-T C-G. Hydrogen bonds form between these pairs, two hydrogen bonds between A and T and three hydrogen bonds between C and G. There are covalent bonds between sugar and phosphate groups/ between sugar and bases. There is an alternating sugar-phosphate backbone
What are the differences between DNA and RNA?
- RNA has one polymer of nucleotides (single-stranded (mostly)), whereas DNA has two polymers of nucleotides (double-stranded)
- RNA has a ribose sugar whereas DNA has a deoxyribose sugar
- RNA has AUCG whereas DNA has ATCG
- RNA is single-stranded, DNA is a double helix
- RNA is synthesised and translated in the 5’→ 3’ direction whereas DNA is antiparallel (one runs in the 5’→3’ direction the other is 3’→5’)
what direction is the coding for transcription in?
5’→3’
What Did Chagraff do?
He used paper chromatography to separate the components of DNA and to measure the concentrations of the bases A,T,C and G.
what did Chagraff find?
- The number of Purine bases (A and G) always equalled the number of Pyrimidine bases (T and C)
- the number of A is always equal to the number of T (and the same for G and C)
What was the Wattson and Crick model?
- they fit perfectly as they are the correct length (A-T, C-G)
- purines are double ring bases, pyramidines are single ring bases
- therefore purines (A and G) will always pair with pyramidines (C and T)
What are the Purine Bases?
Adenine
Guanine
They are a double ring structure
What are the pyramidine structures?
Cytosine
Thymine
They are a single ring structure
What did Hershey and Chase do?
Hershey and Chase’s used bacteriophages (viruses) to perform experiments to see if DNA or proteins were the genetic material.
- they blended the experiment after infection to break up and separate the virus from the bacteria
- they centrifuged it to separate the bacteria from the virus, so they could study the bacteria
They found that the radioactive viral DNA entered the bacterial cell during infection (protein did not), therefore genetic material was DNA, not proteins.
What are nucleosomes
- They consist of DNA wrapped around histones;
- Histones are in an octamer (group of eight)
- These are held together by another histone in the linker region
- These help to supercoil chromosomes
- Needed to regulate gene expression
What is semi conservative replication of DNA?
each strand of DNA splits, both strands are copied as a template. The daughter DNA then has one new strand and the original parent strand
What is the process of semi-conservative DNA replication?
- Gyrase relieves strain and prepares for uncoiling
- DNA helicase unwinds the double helix and separates the two strands by breaking the hydrogen bonds at the replication fork
- Each exposed strand acts as a template for a new strand (semi-conservative replication)
- DNA primase adds RNA primer (short length of RNA nucleotides)
- This then enables the DNA polymerase III to bind and copy the DNA by adding nucleotides in a 5’→3’ direction using complementary base pairing (A+T, C+G)
- Replication has to happen in the 5’→3’ direction. Therefore, DNA polymerase moves towards the replication fork on one strand and away on the other
- DNA polymerase I replaces the primers with DNA
- Continuous on leading strand and fragments formed (discontinuous) on the lagging strand. On the lagging strand these short lengths of DNA that are formed with each primer called the Okizaki fragments
- DNA ligase joins the fragments together by forming sugar-phosphate bonds
- deoxynucleoside triphosphate provide energy to add nucleotides
What is PCR?
PCR- polymerase chain reaction, it is a process which amplifies the amount of DNA exponentially in a very short space of time
What is required for PCR?
- sample of DNA
- DNA polymerase
- Primers
- DNA nucleotides
Why are primers important for PCR?
A short single stranded length of DNA, about 20-30 nucleotides long. These complementary base pair with DNA allowing DNA polymerase to bind and DNA synthesis to occur as they show the enzyme where to start copying the DNA as the enzyme needs a starting strand to attach nucleotides to. They are also important as they keep DNA strands separate.
What are the steps for PCR?
- Add DNA sample, primers, nucleotides and DNA polymerase
- DNA heated to 90-95°C to break the hydrogen bonds and separate the double-stranded helix
- DNA is then cooled to 52°C, to allow for the bonding of primers
- It is then heated to 70-75°C to provide optimum temperature for DNA polymerase. N.B. this DNA polymerase is taken from bacteria that thrive in high temperature environments (it is Taq DNA polymerase)
- DNA polymerase forms new double stranded DNA by adding complementary base pairs- joining together forming phospho-diester bonds during condensation reactions
- The cycle is then repeated many times
What is DNA amplification? (in relation to PCR)
many genetically identical copies of a DNA molecule are made using PCR
What is gel electrophoresis?
method used to separate the DNA fragments according to their size by using electricity
Why do DNA fragments move to the anode in gel electrophoresis?
DNA fragments move from the cathode (-ve) to the anode (+ve). They diffuse towards the positive electrode due to the negative charge on the phosphate group in the DNA
How does gel electrophoresis work?
DNA fragments are placed in wells. Large enough samples are generated using PCR to amplify DNA first, before cutting it into fragments using a restriction endonuclease enzyme (which breaks covalent phosphodiester bonds)
A current is passed through the gel. Negatively charged DNA moves towards the positive electrode.
Why do different patterns appear on the gel electrophoresis?
Smaller fragments move more rapidly as they encounter less resistance from the gel. Fragments are separated due to size as small fragments move further.
what is genetic fingerprinting?
This is a unique DNA profile formed of non-coding DNA regions called minisatellites
What are minisatellites and tandem repeats? (for genetic fingerprinting)
Minisatellites are non-coding regions on DNA. These are formed from short repeating base sequences called tandem repeats. The number of times these tandem repeats are repeated determines the length of the minisatellite region
(this determines the distance travelled in gel electrophoresis)
what is a non overlapping triplet code?
Three consecutive bases in DNA is a triplet, each triplet codes for one specific amino acid, neither the second nor the third base of a triplet forms part of the next triplet
what does universal mean with regards to protein synthesis?
The same codon codes for the same amino acid in all species
what does degenerate mean with regards to protein synthesis?
One amino acid is coded for by more than one triplet (4 possible bases in 3 positions 4^3=64)
what does triplet mean with regards to protein synthesis?
Each amino acid is coded for by a mRNA codon, which is coded for by a sequence of three bases