DNA replication, transcription and translation

Question 1

Semi conservative (DNA)

Answer 1

DNA is a semi conservative process, because when a new double stranded DNA molecule is formed:
- one strand will be from the original template molecule
- one strand will be newly synthesised
this happens since each nitrogenous base can only pair with its complementary partner.

Question 2

Meselson-Stahl experiment

Answer 2

It confirmed the theory that DNA replication was semi conservative.
Meselson and Stahl were able to experimentally test the validity of these three models using radioactive isotopes of nitrogen.
DNA molecules were prepared using the heavier 15N and then induced to replicate in the presence of the lighter 14N
DNA samples were then separated via centrifugation to determine the composition of DNA in the replicated molecules

Question 3

DNA replication

Answer 3

DNA replication is a semi conservative process whereby pre-existing strands act as templates for newly synthesised strands.
The process of DNA replication is coordinated by two key enzymes – helicase and DNA polymerase.

Question 4

Helicase

Answer 4

Helicase unwinds the double helix and separates the two polynucleotide strands. It does this by breaking the hydrogen bonds between complementary base pairs

Question 5

DNA polymerase

Answer 5

DNA polymerase synthesises new strands from the two parental template strands. Free deoxynucleoside triphosphate align opposite their complementary base partners. DNA polymerase cleaves the two excess phosphates and uses the energy released to link the nucleotide to the new strand

Question 6

Polymerase chain reaction (PCR)

Answer 6

The PCR is an artificial method of replicating DNA under laboratory conditions.
The reaction occurs in three steps:
1) Denaturation – DNA sample is heated (~90ºC) to separate the two strands
2) Annealing – Sample is cooled (~55ºC) to allow primers to anneal (primers designate sequence to be copied)
3) Elongation – Sample is heated to the optimal temperature for a heat-tolerant polymerase (Taq) to function (~75ºC)

Question 7

Taq polymerase

Answer 7

It is an enzyme isolated from the thermophilic bacterium. It is used in PCR since it can function at high temperatures without denaturing.
Taq polymerase extends the nucleotide chain from the primers – therefore primers are used to select the sequence to be copied

Question 8

Transcription

Answer 8

transcription is the process by which an RNA sequence Is produced from a DNA template
1) RNA polymerase separates the DNA strands and synthesises a complementary RNA copy from one of the DNA strands
2) When separated, ribonucloside triphosphates align opposite the complementary base partner
3) RNA polymerase removes the additional phosphate groups and uses the energy from this cleavage to join the nucleotide to the growing sequence
3) Once the RNA sequence has been synthesised, RNA polymerase detaches from the DNA molecule and the double helix reforms

Question 9

Gene

Answer 9

The sequence of DNA that is transcribed into RNA is called a gene.
The strand that is transcribed is called the antisense strand.
The strand that is not transcribed is called the sense strand

Question 10

Where does transcription of genes occur

Answer 10

Transcription of genes occur in the nucleus, before the RNA moves to the cytoplasm

Question 11

Codons

Answer 11

The mRNA sequence is read by the ribosome in triplets of bases called codons. Each codon codes for one amino acid with a polypeptide chain.

Question 12

Genetic code

Answer 12

The genetic code is the set of rules by which information encoded within mRNA sequences is converted into amino acid sequences (polypeptides) by living cells. The genetic code identifies the corresponding amino acid for each codon combination.
Typically the genetic code shows the codon combinations expressed on an mRNA molecule

Question 13

Translation

Answer 13

Translation is the process of protein synthesis in which the genetic information encoded in mRNA is translated into a sequence of animo acids on a polypeptide chain.

Question 14

key components of Translation

Answer 14

Messenger RNA
Ribosome
Codons
Anticodons
Transfer RNA
Amino acids
Peptide bonds
Polypeptides

Question 15

Universality

Answer 15

The genetic code is universal (almost every living organism uses the same code)

Question 16

production of Insulin

Answer 16

The ability to transfer genes between species has been utilised to produce human insulin in bacteria:
1) The gene responsible for insulin production is extracted from a human cell
It is spliced into a plasmid vector (for autonomous replication and expression) before being inserted into a bacterial cell
2) The transgenic bacteria (typically E. coli) are then selected and cultured in a fermentation tank (to increase bacterial numbers)
3) The bacteria now produce human insulin, which is harvested, purified and packaged for human use (i.e. by diabetics)

Question 17

mRNA to DNA

Answer 17

mRNA is a complementary copy of a DNA segment and consequently can be used to deduce the gene sequence.
Cytosine is replaced with Guanine
Uracil is replaced with Adenine
Adenine is replaced with Thymine