BIO302 Midterm Solved Papers

20 MCQs.

Q1. How DNA replication initiates? 2 marks

Answer: The proteins that initiate DNA replication bind to DNA sequences at a replication origin to catalyze the formation of a replication bubble with two outward-moving replication forks. The process begins when an initiator protein–DNA complex is formed that subsequently loads aDNA helicase onto the DNA template.

Q2. How and what cross linkage by alkylating agents? Exact wording yad nahi 2 marks

Answer: Alkylating agents utilize selective alkylation by adding the desired aliphatic carbon chain to the previously chosen starting molecule. This is one of many known chemical syntheses. Alkylating agents are a highly reactive group of electrophiles which transfer methyl, ethyl or alkyl groups to the electron-rich atoms in the DNA and demage it.

Q3. Why wrongly incorporated nucleotide while DNA replication results in permanent

Answer: If DNA polymerase incorporates the wrong nucleotide into a newly synthesized DNA strand a single base pair substitution will occure. Another kind of error called strand slippage happens when DNA polymeraseadds an extra nucleotide or skips a nucleotide in the newly synthesized DNA strand, which results respectively in an insertion or deletion. Strand slippage often occures in regions of the genome with repeatitive DNA sequence.

Q4. Why C-N bond of peptide bond can't rotate freely? 3 marks

Answer: The stability of the peptide bond is due to the resonance of amides. With resonance, the nitrogen is able to donate its lone pair of electrons to the carbonyl carbon and push electrons from the carbonyl double bond towards the oxygen, forming the oxygen anion. This resonance effect is very stabilizing because the electrons can be delocalized over multiple atoms, with one especially stable resonance structure containing the highly electronegative oxygen as an anion. The double bond character of the C-N bond results in a relatively short bond The double bond resonance form of the peptide bond helps to increase stability and decrease rotation about that bond. The partial double bond character is either strengthened or weakened depending upon the environment that it is in.

Q5. How many standard amino acids are there? Name any 5. 5 marks

Answer: In eukaryotes, there are only 21 proteinogenic amino acids, the 20of the standard genetic code, plus selenocysteine. Humans can synthesize 12 of these from each other or from other molecules of intermediary metabolism.

Examples:

• Alanine
• Tryptophan
• Glutamine
• Aspartic acid
• Histidine

Q6. Write the mechanism of CPD photolyases in repair of DNA.. 5 marks

Answer: CPD photolyase can bind to DNA in the dark by recognizing the altered DNA structure caused by a CPD formation rather than a specific nucleotide sequence.

This binding is about 10 5 tighter when a DNA segment contains a CPD than when it does not.

• Half of the binding energy appears to come from interactions between enzyme and the DNA back bone.
• The other half of energy comes from interactions between the FADH at the active site and the CPD.
• However, the light harvesting antenna pigment does not influence binding.
• Once the enzyme – DNA complex is formed, the CPD is flipped out of the DNA double helix and into the enzyme’s active site.
• After the CPD flips into the enzyme’s active site, the energy of an absorbed photon is

transferred from the light harvesting antenna pigment to the FADH

• FADH then transfers an electron to the CPD to induce cyclobutane ring cleavage.
• The catalytic cycle is completed when the electron is transferred back from the repaired thymine to the FADH cofactor.

Bio302

Q1. Salient feature of DNA polymerase

Answer: The DNA polymerases are enzymes that create DNA molecules by assembling nucleotides, the building blocks of DNA.

This region of DNA polymerase binds two divalent metal ions (Mg2+ or Zn 2+).

DNA polymerase is required to help duplicate the cell’s DNA, so that a copy of the original.

DNA molecule can be passed to each of the daughter cells. In this way, genetic information is transmitted from generation to generation.

DNA polymerase adds new free nucleotides to the 3’ end of the newly-forming strand, elongating it in a 5’ to 3’ direction.

The DNA is therefore “proofread” by DNA polymerase after it has been copied so that misplaced base pairs can be corrected.

Q2. Structure of influnza virus,

Answer: The structure of the influenza virus is somewhat variable, but the virion particles are usually spherical or ovoid in shape and 80 to 120 nanometers in diameter. Sometimes filamentous forms of the virus occur as well, and are more common among some influenza strains than others.

Q3. Name three proteins that catalyse DNA

Answer: 1) O6 -alkylguanine DNA alkyltransferase I

2. O 6 -alkylguanine DNA alkyltransferase II