DNA Genomics 2 (MJC)
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Explain why replication of human chromosome would take so long with only a single replication site.
Human chromosomes have large DNA, have more bases and are associated with histones. More time is needed to uncoil and unravel the human DNA which prolongs replication time.
Other than a mRNA that serves as a template and the two ribosomal subunits, what two molecules are needed to complete translation?
- 1) Guanosine triphosphate
- 2) Aminoacyl-tRNA complex
Explain why replication only occurs in a 5' to 3' direction.
This is because the elongation of the daughter strand requires the presence of a free 3' OH group of a previous nucleotide for the formation of the phosphodiester bond.
Replication of DNA can be explained through the semi-conservative hypothesis. Explain the hypothesis.
The hypothesis states that the two strands of the parental molecule separates and each strand serves as a template for the synthesis of a new complementary strand.
Briefly describe how the structure of RNA polymerase differs from a ribosome.
- 1) RNA polymerase consist of a singular unit while a ribosome contains a large and a small subunit.
- 2) RNA polymerase requires a single site for its action but a ribosome requires 3 sites (A, P and E sites) for its action.
- 3) RNA polymerase is composed of polypeptide sequences but a ribosome is composed of rRNA and ribosomal proteins.
How does DNA replicate itself to ensure that the genetic material it carries is passed on accurately?
- - DNA replicates by semi-conservative hypothesis
- - Each parental strand serves as a template for the synthesis of the daughter strand
- - Complementary base pairing occurs where adenine base pairs with thymine and cytosine base pairs with guanine
- - DNA polymerase I proof-reads DNA to ensure accuracy of replication
State the two functions of DNA polymerase I.
- 1) DNA polymerase I proofreads the previous region as the daughter strand elongates to ensure proper base pairing.
- 2) DNA polymerase I replaces the primers with deoxyribonucleotides.
State the two functions of RNA polymerase.
- 1) It reads the sense strand and takes up ribonucleotides which are complementary to the bases and match them via complementary base pairing.
- 2) It also catalyse the formation of phosphodiester bonds between adjacent ribonucleotides.
Define the term base pair substitution and explain what happens if this substitution is a silent mutation.
- -Base pair substitution is the replacement of one nucleotide
- pair with another pair of nucleotides.
- -A silent mutation means that the mutation is not expressed in the phenotype
- -A base pair substitution may be silent as the triple code is degenerate where more than one codon can still code for the same amino acid or one with similar properties
Describe three ways in which the process of transcription is similar to that of translation.
- 1) Both require a template molecule, DNA for transcription and mRNA for translation
- 2) Both involves enzymes, RNA polymerase for translation, aminoacyl-tRNA synthetase and peptidyl transferase for translation.
- 3) Both require a start and stop region. Promoter and terminator sequence for transcription and start and stop codon for translation.
Describe the termination of translation.
- Translation stops when a stop codon (UAG, UAA or UGA) reaches the A site of the ribosome.
- A protein known as a release factor will bind to the stop codon
- This protein causes the addition of a water molecule to the polypeptide chain, thus dissociating the translation complex and freeing the polypeptide chain.
Explain why 3, and not 2, bases codes for one amino acid in human DNA.
- There are 20 essential amino acids in a typical cell but only 4 different bases in the human genome.
- If 2 bases code for one amino acid, only 42 = 16 amino acids can be code, thus it is insufficient.
- If 3 bases code for one amino acid, 43 = 64 amino acids can be coded, which is more than sufficient.
The triplet code that codes for amino acids is degenerate.
Explain the term degenerate.
It means that more than one codon can code for one amino acid.
Etc Proline can be coded by CCU, CCC, CCA and CCG
Explain what happens if a base pair deletion is not in the multiples of three.
- -The reading frame (triplets of bases) of the genetic message is altered from the point of insertion or deletion.
- -The sequence of amino acids is completely different from the original protein.
- -This leads to a non-functional protein
- -The resulting mutation is known as a frameshift mutation.
Other than the DNA template, state three other factors that are required for DNA replication.
- 1) Enzymes such as DNA ligase and DNA polyerase I and III
- 2) RNA primers to provide the free 3' OH group for the formation of phosphodiester bond
- 3) Deoxyribonucleoside triphosphate for the synthesis of daughter strands
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