Transcript DNA - Moodle

DNA
Past Paper Questions
1. Draw as simple diagram of the
molecular structure of DNA. 5 marks
1. Draw as simple diagram of the molecular structure
of DNA. 5 marks
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two sugar-phosphate backbones shown
A with T and C with G
double helical shape shown
antiparallel nature of strands indicated
ten base pairs per turn of helix
correct hydrogen bonding shown (A=T and
C=G)
2. Describe the genetic code. 6 marks
2. Describe the genetic code. 6 marks
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composed of mRNA base triplets
called codons
64 different codons
each codes for the addition of an amino acid to a growing
polypeptide chain
the genetic code is degenerate
meaning more than one codon can code for a partiuclar
amino acid
the genetic code is universal
meaning it is the same in almost all organisms
(AUG is the) start codon
some (nonsense) codons code for the end of translation
3. Explain the relationship between
genes and polypeptides. 5 marks
3. Explain the relationship between
genes and polypeptides. 5 marks
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genes code for proteins/ polypeptides
one gene one polypeptide
(one) gene is transcribed into (one) mRNA
mRNA is translated by a ribosome to synthesize a
polypeptide
• if the information on a gene is changed/ mutated this
may alter the structure of a protein
• genetic information transcribed by eukaryotes is edited
before it is translated
• polypeptides may be altered before they become fully
functional proteins
4. Living organisms use DNA as their
genetic material. Explain how DNA is
replicated within the cells of living
organisms 8 marks
4. Living organisms use DNA as their genetic material.
Explain how DNA is replicated within the cells of living
organisms 8 marks
• helix is unwound
• two strands are separated
• helicase (is the enzyme that unwinds the helix separating the two
strands)
• by breaking hydrogen bonds between bases
• new strands formed on each of the two single strands
• nucleotides added to form new strands
• complementary base pairing
• A to T and G to C
• DNA polymerase forms the new complementary strands
• replication is semi-conservative
• each of the DNA molecules formed has one old and one new strand
5. Explain the significance of
complementary base pairing in DNA
replication 4 marks
5. Explain the significance of complementary base
pairing in DNA replication 4 marks
• when 2 DNA strands separate, each acts as a
template for DNA synthesis
• DNA polymerase adds free nucleotides to the
parent/template strand
• DNA polymerase adds nucleotides according
to the complementary base pairing rules
• A=T, G=C
• the two double-stranded DNA molecules
produced are identical to the parent DNA
6. Compare DNA & RNA structure:
3 marks
6. Compare DNA & RNA structure: 3 marks
DNA
sugar
deoxyribose
pyrimidines thymine
strands
double
RNA
ribose
uracil
single
7. Outline DNA transcription. 7 marks
7. Outline DNA transcription. 7 marks
initiation
• RNA polymerase is an enzyme complex
• which unwinds and unzips DNA double strand
• RNA polymerase attaches to promoter region of gene, which marks the
beginning point for transcription
elongation
• RNA polymerase uses DNA the anti-sense strand as a template
• RNA polymerase synthesizes a complementary RNA strand using base
pairing rules
A=U T=A
G=C C=G
termination
• RNA polymerase reaches termination region of the gene, which marks the
end of the coding sequence
• terminates transcription by releasing both DNA and RNA
• maximum 3 marks from each of initiation, elongation, termination
8. Explain the process of translation. 8
marks
8. Explain the process of translation. 8
marks
Initiation
• mRNA binds to the small subunit of the ribosome
• tRNA anticodon binds to mRNA codon by complementary base pairing
• large ribosomal subunit binds, completing ribosomal structure,
Elongation:
• tRNA with anticodon complementary to second mRNA codon binds to ribosome
• with appropriate amino acid attached to tRNA
• enzymes in ribosome catalyze formation of peptide bond between 1st and 2nd amino acids
• 1st tRNA (separated from amino acid) exits ribosome
• ribosome moves one codon (3 nucleotides) along the mRNA
• tRNA with anticodon complementary to next mRNA codon binds to ribosome with appropriate
amino acid attached
• enzymes in ribosome catalyze formation of peptide bond between 2nd and 3rd amino acids
• repetition of process until stop codon is reached
Termination:
• when ribosome reaches a stop codon, no tRNA has a complementary anticodon
• release factor protein binds to ribosome stop codon
• polypeptide and mRNA are released
• large and small ribosomal subunits separate
• maximum 3 marks from each of initiation, elongation, termination