Nucleotides and Nucleic Acids

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What is the function of DNA polymerase?
It catalyses the formation of new strands. It uses each parent strand as a template 'reading' the sequence of bases and adding complementary bases to form new strands. Joins together the new DNA strands
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What is the function of DNA ligase?
It joins together the Okazaki fragments to form a continuous strand.
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How is the lagging strand copied?
Discontinuously as it runs in the 3' to 5' direction and DNA polymerase can only work in the 5' to 3' direction
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What can happen when a virus is incorporated into the DNA in the nucleus of an infected cell?
This can cause RNA to be copied into DNA by reverse transcriptase
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Why are ribosomes necessary for photosynthesis?
mRNA binds to the ribosome, as does tRNA, the tRNA is positioned to allow binding of amino acids. the rRNA catalyses the formation of peptide bonds
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What percentage of RNA is rRNA?
80%
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what are the Okazaki fragments?
the small fragments copied from the lagging strand
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What is the replication fork?
the point at which the two DNA strands are separated
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What are pyrimidine bases? which bases are pyrimidine bases?
Smaller bases with single carbon ring structures. Thymine, uracil and cytosine are pyrimidine bases
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What are purine bases and which bases are purine?
Larger bases with double carbon ring structures. Adenine and guanine are purines
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How many hydrogen bonds form between thymine and adenine?
2
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How many hydrogen bonds form between cytosine and guanine?
3
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What is the arrangement of complementary nucleotides in the double helix?
Anti-parallel
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What would be the impact in the following errors in DNA replication- substitution, insertion, deletion?
Subsitutution- usually only a minor mutation, affects a maximum of one amino acid so primary strucutre only changes very slightly, if at all. Insertion/deletion has a knock-on effect on all other triplets altering the primary structure-could be fatal
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What are single stranded binding proteins?
Molecules which act as buffers to keep the DNA strands separated during replication
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What is a cistron?
Another name for a gene
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What is semi-conservative replication?
The mechanism of how DNA replicates. Each new DNA molecule contsins one parent strand and one new strand as an original DNA molecule will unzip and unwind and the parent strands will be used as templated to synthesise the new strand
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Why did scientists doubt the semi-conservative replication theory?
They didn't think DNA would be able to unzip or unwind due to the strength of the hydrogen bonds
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Compare the structure and function of an ATP nucleotide with a DNA nucleotide.
ATP has ribose sugar+DNA has deoxyribose/ATP always has Adenine base+ DNA has A,T,G,C/ ATP has 3 phoshates+DNA has 1/ATP involved in energy transfer+DNA forms part of genetic code
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What is the anticodon?
The end of the tRNA molecule which binds to the complementary codon on mRNA, making temporary hydrogen bonds and brings the complementary amino acid
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where do the amino acids attach to the tRNA?
Attach to the exposed region called the acceptor arm. This always has the base sequence CCA
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Roughly how many nucleotides make up and tRNA molecule?
80
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What percentage of RNA is mRNA?
about 5%
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Why is prokaryotic DNA described as 'naked'?
Because it is not associated with (wound around) any histone proteins- it is free within the cytoplasm
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Where is DNA without histone proteins found in eukaryotic organisms?
In the mitochindria and chloroplasts
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What do we mean when we say that the genetic code is degenerate?
There are 64 possible base triplets but only 20 amino acids meaning that most amino acids are coded for by more than one triplet
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Why is the genetic code being degenerate an advantage?
It reduces the effect in point mutations because if there was a change in one base triplet there is a chance that the same amino acid could be coded for anyway
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Why is the genetic code describes as near universal?
Because the same base triplets code for the same amino acids in almost all orrganisms
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why is the genetic code described as non-overlapping?
Each triplet is read separately- there is no sharing of bases. The single start codon ensures the codons are read in sequence.
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Give the defenition of a gene
A sequence of DNA nucleotides that codes for a polypeptide
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Why did the Meselson and Stahl experiment rule out the dispersive replication theory?
because the 2nd generation of bacteria grown in the 14N (3rd generation overall) half were made of all light N and half were made of a mixture. The dispesrion theory would cause each DNA molecule to have fragments of both heavy and light nitrogen
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What is the sense strand?
The strand which codes for the protein to be synthesised. It runs from the 5' to 3' direction
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What is the antisense strand?
The strand which runs in the 3' to 5' direction and is a complementary copy of the sense strand. It acts as a template strand during transcription
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How do ribosomes form?
The 2 subunits are sythesised separately in the nucleus, they pass separately out of the nucleus through a nuclear pore and then join together in the cytoplasm
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What helps the two ribosome subunits bind together?
Magnesium (Mg2+) ions
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We could liken protein synthesis to the chicken and egg theory as we need enzymes (polymerase and helicase) to synthesise protiens but these are proteins themselves and must also be sythesised. So how do we obtain these enzymes?
From our mother during foetal development
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What are plasmids?
Circular loops of prokaryotic DNA that can be transferred between bacterial cells
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Why is DNA kept within the nuclear envelope?
It is too large to pass out the nucleus, but this also prevents it from getting damaged in the cytoplasm
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What is the terminator region?
The point at which the DNA stops being copied, and the mRNA strand breaks off
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During translation where does the mRNA attach to?
The start codon (AUG) at the 3' end of the mRNA strand
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What are ribosomes made of?
Almost equal amounts of rRNA and protein
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Why is rRNA important?
It maintains the strucutral stability of the protein synthesis sequence- it holds the mRNA strand in place while it is decoded into a sequence of amino acids and plays a biochemical role in catalysing the reaction
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Where is tRNA made?
In the nucleus
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Adding urea to a solution of double-stranded DNA will severely disrupt the hydrogen bonding in the DNA. Explain what affect this will have on the DNA's structure.
It will lose its double helix structure- the two DNA strands will unravel because the two DNA strands are held together by hydrogen bonding between the base pairs
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What could be the effect of random spontaneous mutation in a gene during replication have on the protein produced?
The amino acid sequence of DNA could be affected. This can cuase an abnormal protein to be produced- this abnormal protein may function better than the normal protein or it might not work at all.
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Describe DNA replication
H bonds break/DNA double helix unzips by DNA helicase/ Each original strand acts as a template/ Free-floating DNA nucleotides join to the exposed bases complementary base pairing/nucleotides on new strand joined by DNA polymerase/ H bonds form
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Describe translation
tRNAs carry amino acids to ribosome/tRNA with complementary anticodon to mRNA start codon attaches to mRNA/2nd tRNA attaches in same way and rRNA in ribosome catalyses formation of peptide bond/1st tRNA moves away leaving its amino acid/ti
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When does translation end?
When a stop codon is reached.
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What is the start codon and what amino is coded for?
AUG and this codes for methionine
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An error occurs in transcrition that accidentally inserts a stop codon in the middle of the mRNA sequence. What effect would this have on the protein that is eventually produced?
It would be shorter and so could be a different protein. Translation of the mRNA sequence only continues until a stop codon is reached. Any codons after the stop codon would not be translated into amino acids
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Suggest one way in which Meselson and Stahl could have made sure their results were valid
Could have controlled all variables e.g. the other nutrients in the broth. Could have used a negative control e.g. bacteria grown in broth without any nitrogen
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Describe how you could purify DNA from a sample of cells using a precipitation reaction
Macerate cells in blender/mix with detergent, salt+distiled H2O/Incubate mix in water bath at 60degrees for 15mins and put in ice bath. When cool transfer to boiling tube+ protease/Slowly dribble cold ethanol down side-layer/DNA= white precipitate
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Scientists analysed a section of DNA 74 bases in total (36 pairs). 18 bases were guanine- how many of the others?
18 cytosine, 19 adenine 19 thymine
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How do viruses replicate?
By attaching to the host cell with attachment proteins on their surface/inject their nucleic acid into host cell/genetic info in nucleic acid provides 'instructions' for the host cell to start producing and assembling proteins.
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what joins the phosphete group to the pentose sugar?
An ester bond
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What joins the pentose sugar to the organic base?
A glycosidic bond
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Which enzyme in the ribosome catalyses the formation of a peptide bond?
Peptide synthetase
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How much energy is released when ATP is hydrolysed into ADP?
30,6 kJ mol-1
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