DNA replication, transcription, and translation

Question 1

Why is DNA replication a semi-conservative process?

Answer 1

DNA replication is a semi-conservative process, because when a double-stranded molecule is formed:

• One strand will be from the original template molecule
• One strand will be newly synthesized

Question 2

Why are the two DNA molecules produced during replication identical to each other and to the parent DNA molecule?

Answer 2

The two strands of the DNA molecule are separated by breaking the hydrogen bonds between their bases. The base sequence of the new stand of DNA will be determined by the base sequence on the template strand. This occurs because nitrogenous bases can only pair with its complementary partner.

Question 3

What were the alternative theories to the semi-conservative replication theory?

Answer 3

1. Conservative replication: an entirely new molecule is synthesized from a DNA template which remains unaltered
2. Dispersive replication: every molecule produced by DNA replication has a mixture of old and new sections in both of its strands

Question 4

How did Meselson and Stahl prove the semi-conservative model of DNA replication?

Answer 4

• Nitrogen is a key component of DNA and can exist as a heavier N15 or a lighter N14
• DNA molecules prepared in N15 medium are transferred to N14 medium
• DNA samples were then separated via centrifugation to determine the composition of DNA in the replicated molecules
• In the first generation, the DNA strand composed of N14 and N15
• In the future generations, strands of N14 grew and N14/N15 molecules decreased in quantity

Question 5

What are the stages in DNA Replication?

Answer 5

1. Helicase unwinds the double helix and separates the two strands by breaking hydrogen bonds (uses ATP)
2. DNA polymerase links nucleotides together to form new strands, using the two pre-existing strands as templates
   
   • DNA polymerase builds the new strand in the 5’ to
     3’ direction (3’ to 5’ direction of the parent strand)
3. The daughter DNA molecules each rewind into a double helix

Question 6

What is polymerase chain reaction (PCR)?

Answer 6

The PCR is an artificial method of repeating DNA under laboratory conditions

Question 7

What is the advantage of PCR?

Answer 7

It can be used to amplify large quantities of a specific sequence of DNA from an initial minute sample

Question 8

What are the steps of PCR?

Answer 8

1. Denaturation: DNA sample is heated (to 90 degrees) to separate the two strands
2. Annealing: Sample is cooled to allow primers to anneal
3. Elongation: Sample is heated to the optimal temperature for a heat-tolerant Taq polymerase to function

Question 9

What enzyme is used during PCR and why?

Answer 9

Taq polymerase is used to replicate DNA through PCR as it can resist the brief period at 90 degrees during denaturation. Its optimal temperature is 72 to 75 degrees.

Question 10

What is transcription?

Answer 10

Transcription is the process by which a base sequence of DNA is copied onto an RNA

Question 11

What is a gene?

Answer 11

The sequence of DNA that is transcribed into RNA is called a gene

Question 12

What is the antisense strand?

Answer 12

The strand that is transcribed is called the antisense strand (complimentary to RNA sequence)

Question 13

What is the sense strand?

Answer 13

The strand that is not transcribed is called the sense strand (identical to RNA sequence)

Question 14

What is the process of transcription?

Answer 14

1. RNA polymerase binds to a site on the DNA at the start of a gene and separates DNA into single strands
2. When the DNA strands are separated, RNA nucleotides are assembled along one of the two strands
3. RNA nucleotides are linked together by covalent bonds
4. The RNA strand separates from the DNA strand and is released completely when the end of the gene is reached
5. The DNA strands pair up again and twist back into a double helix

Question 15

What is translation?

Answer 15

Process of protein synthesis in which the genetic information encoded in mRNA is translated into a sequence of amino acids on a polypeptide chain

Question 16

What is mRNA?

Answer 16

mRNA is the RNA that carries the information needed to synthesize a polypeptide (i.e. protein)

Question 17

What are codons?

Answer 17

• The mRNA sequence is read by the ribosome in triplets of bases called codons.
• Each codon codes for one amino acid.
• Therefore, the order of codons in an mRNA determines the order of amino acids (and thus the function of the protein)

Question 18

What is the genetic code?

Answer 18

• The genetic code is the set of rules by which information encoded within mRNA sequences is converted into amino acid sequences
• Three bases code for one amino acid (codon)
• 64 different codons possible

Question 19

What is tRNA?

Answer 19

The type of RNA which carries amino acids to the ribosome

Question 20

What is the process of translation?

Answer 20

1. mRNA binds to a sire on the small sub-unit of the ribosome
2. Anticodons on tRNA, carrying a specific amino acid, align opposite appropriate codons on mRNA according to complementary base pairing
3. Ribosomes catalyze the formation of peptide bonds between adjacent amino acids
4. The ribosome moves along the mRNA molecule, synthesizing a polypeptide chain until it reaches a stop codon, at which point the polypeptide chain is released

Question 21

What is the significance of the universality of the genetic code?

Answer 21

Since the genetic code is universal (almost every organism uses the same code), genetic information is transferable between species

Question 22

What is an application of the universality of the genetic code?

Answer 22

The gene that codes for insulin has been transferred from humans to E coli and other organisms. The bacteria produce human insulin, which is harvested, purified, and packaged for human use.

Question 23

What is the process for producing human insulin in bacteria?

Answer 23

1. The gene responsible for insulin production is extracted from a human cell
2. It is spliced into a plasmid vector before being inserted into a bacterial cell
3. The bacteria then is cultured in a fermentation tank
4. The bacteria produce human insulin, which is harvested, purified, and packaged for human use