2.7 Replication, transcription and translation

Question 1

mRNA (messenger RNA)

Type of RNA

Answer 1

• Formed during transcription
• Formed in the nucleus
• Carries protein-building information by rewriting the sequence of bases of a section of DNA (every 3 bases is called a codon)

Question 2

tRNA (transfer RNA)

Answer 2

• Found in the cytoplasm
• Picks up and delivers amino acids to ribosomes during translation

Question 3

-

What is DNA?

Answer 3

A molecule that holds our genetic information. It stores genetic codes for cells.

Question 4

In what phase of interphase does DNA replication occur

Answer 4

During S-phase, prior to cell division, DNA is replicated to make daughter cells identical to the parent cell.

Question 5

What is the process of DNA replication?

Answer 5

• The enzyme helicase unzips the double helix, separating the two DNA strands by breaking the hydrogen bonds between the bases.
• Then the enzyme DNA polymerase can start making new strands of DNA using the two ‘old parent’ strands as templates by joining free nucleotides with their matching complementary bases in the DNA.

Its important to note that the rule of complementary base pairing is followed to ensure that the two strands of DNA end up being identical

Question 6

Is DNA replication semi-conservative?

Answer 6

DNA replication 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

Question 7

Proposals as to how DNA might replicate

Answer 7

• Dispersive Model – New molecules are made of segments of new and old DNA
• Conservative Model – An entirely new molecule is synthesized from a DNA template (which remains unaltered)
• Semi-conservative Model – Each new molecule consists of one newly synthesized strand and one template strand

Question 8

Meselson and Stahl experiment (1958)

Supporting the theory that DNA replication is semi-conservative

Answer 8

The experiment:
- They cultured E. coli bacteria in the presence of a heavy nitrogen isotope, ¹⁵N.
- DNA contains nitrogen in its nitrogenous bases (ATCG), so the radioactive ¹⁵N would end up in the DNA of the bacteria.
- The result was that all bacterial DNA had ¹⁵N in its bases.
- They then transferred the bacterial culture into a fresh medium with ¹⁴N, a lighter isotope, and the bacteria were allowed to grow for several generations.
- DNA samples were then separated via centrifugation as it allowed the DNA to move to different positions on the tube depending on its density.
- DNA containing one or two strands with ¹⁵N was heavier and showed lower bands than those containing two strands with ¹⁴N (DNA with ¹⁵N in both strands was heaviest).

Results:
- After one replication (generation 1), DNA molecules were found to contain a mix of 15N and 14N, disproving the conservative model
- After two replications (generation 2), some molecules of DNA were found to consist solely of 14N, disproving the dispersive model

Question 9

What happens in transcription

What does DNA become?

Answer 9

DNA → mRNA
mRNA is made by a template provided by DNA

Question 10

Detailed explanation of transcription

Answer 10

1. RNA polymerase separates the DNA strands by breaking the hydrogen bonds between the bases.
2. The enzyme RNA polymerase joins complementary bases of free RNA nucleotides to one of the DNA strands (used as a template)
3. Weak hydrogen bonds are formed between bases and phosphodiester bonds are formed between the RNA nucleotides to form the mRNA strand. (until the terminator sequence is transcribed)
4. The mRNA strand peels of the DNA and moves to the cytoplasm

Notes:
- In mRNA U (uracil) replaces T (thymine)
- The strand that is transcribed is called the antisense strand and is complementary to the RNA sequence
- The strand that is not transcribed is called the sense strand and is identical to the RNA sequence (with T instead of U)

Question 11

Codons

Answer 11

Triplets of bases
- Codons of three bases on mRNA correspond to one amino acid in a polypeptide
- The order of the codons in an mRNA sequence determines the order of amino acids in a polypeptide chain

Question 12

What happens in translation

What does mRNA become?

Answer 12

mRNA → polypeptides (protein)
Proteins are made by the instructions provided by mRNA

Question 13

Detailed explanation of translation

Answer 13

Translation is the process where the mRNA molecule forms the template for the synthesis of polypeptides on ribosomes.
1. The mRNA, which is in the cytoplasm, attaches to a ribosome - Initiation
2. tRNA molecules bring the appropriate amino acid to the ribosome due to complementary base pairing, its anticodon has to match the codon on the mRNA. - Elongation
3. Peptide bond between amino acids to form the polypeptide - Elongation
4. Translation halts when a stop codon is reached and the polypeptide is released from the ribosome - Termination

tRNA is reused and collects another specific amino acid
mRNA can also be reused by moving to another ribosome or it can be broken down into nucleotides to be reused

Question 14

What is the start codon on mRNA?

Answer 14

AUG - which codes for methionine

Question 15

Is the genetic code universal?

Answer 15

Yes. Almost every living organism uses the same genetic code.

Question 16

The use of human insulin gene within bacteria

Application

Answer 16

• Insulin is an important protein (hormone) in the body that helpes regulate blood sugar
• As the same codons code for the same amino acids in all living things, genetic information is transferrable between species.
• The ability to transfer genes between species has been utilised to produce human insulin in bacteria.

Procedure:
1. The gene responsible for insulin production is extracted from a human cell
2. It is inserted into the genome of a bacterial cell via a plasmid
3. When the bacterial cells reproduce they make copies of their DNA and plasmid DNA, meaning the insulin gene will be copied and passed on to their offspring
4. The bacteria now produce human insulin, which is harvested, purified and packaged for human use (i.e. by diabetics)

Question 17

RNA bases

Answer 17

adenine (A), uracil (U), cytosine (C), and guanine (G)

U replaces T which would be in DNA

Complementary base pairs:
- A and U
- C and G