4.2 DNA and protein synthesis

Question 1

What is a genome?

Answer 1

The complete set of genes in a cell.

Question 2

What is a proteome?

Answer 2

The full range of proteins that a cell is able to produce.

Question 3

Give two differences between the structure of mRNA and the structure of tRNA.

Answer 3

1. mRNA usually longer (also mRNA length depends on polypeptide length, whereas all tRNAs are the same length)
2. mRNA is a straight molecule but tRNA is folded into a clover-leaf shape;
3. mRNA contains no paired bases / hydrogen bonds but tRNA has some paired bases / hydrogen bonds;
4. tRNA has an amino acid binding site, mRNA does not;
5. tRNA has anticodon, mRNA has codon;

Question 4

Describe how mRNA is formed by transcription in eukaryotes.

Answer 4

Hydrogen bonds between DNA bases break (NOT hydrolysis) (mark schemes will ignore ‘using DNA helicase’ because some RNA polymerases can do the unzipping themselves).

Only one DNA strand acts as a template (the other being referred to as the coding strand, given that it carries the same code as the transcript, only with T instead of U).

Free RNA nucleotides align to the template strand by complementary base pairing, but with uracil used in place of thymine.

RNA polymerase joins adjacent RNA nucleotides by catalysing the condensation of phosphodiester bonds between the adjacent nucleotides, to form pre-mRNA

Pre-mRNA is then spliced to form mature mRNA). Splicing involves removal of introns and joining of exons.

Question 5

Describe how a polypeptide is formed by translation of mRNA.

Answer 5

1. mRNA attaches to ribosomes;
2. tRNA anticodons bind to complementary mRNA codons;
3. each tRNA carries a specific amino acid;
4. Amino acids join by peptide bonds, catalysed by the ribosome;
5. Using energy from the hydrolysis of ATP;
   [You probably haven’t seen many diagrams which actually show ATP being used directly by the ribosome, and that’s because the truth is slightly complicated. However protein synthesis does use a LOT of energy, costing the equivalent of between 4 and 5 ATPs per peptide bond, so credit will be given for mentioning ATP]
6. tRNA released (after amino acid joined to polypeptide);
7. The ribosome moves along the mRNA to form the polypeptide;

Question 6

Give the two types of molecule from which a ribosome is made.

Answer 6

One of RNA / ribonucleic acid(s) / nucleotide(s)/nucleic acid(s) / rRNA / ribosomal RNA / ribosomal ribonucleic acid

and

one of protein(s) / polypeptide(s) / amino acid(s) / peptide(s) / ribosomal protein;

Question 7

Describe the role of a ribosome in the production of a polypeptide.

Answer 7

1. mRNA binds to ribosome;
2. Idea of two codons / binding sites;
   [There are actually three tRNA sites in the ribosome, but only two are occupied at the same time - as one tRNA enters, another leaves]
3. tRNA with anticodons bind to codons;
4. Ribosome catalyses condensation of peptide bond between amino acids (held by tRNA molecules);
5. Ribosome moves along the mRNA to the next codon.

Question 8

Contrast the structures of DNA and mRNA molecules.

Answer 8

1. DNA double stranded/double helix and mRNA single-stranded;
2. DNA (very) long and RNA short (accept ‛RNA shorter’ or ‛DNA longer’)
3. Thymine/T in DNA and uracil/U in RNA;
4. Deoxyribose in DNA and ribose in RNA;
5. DNA has base pairing and mRNA doesn’t/ DNA has hydrogen bonding and mRNA doesn’t;
6. DNA has introns/non-coding sequences and mRNA doesn’t;

Question 9

Starting with mRNA in the nucleus of a cell, describe how a molecule of protein is synthesised.

Answer 9

1. mRNA leaves (nucleus) through nuclear pore;
2. mRNA attaches to ribosome;
3. tRNA molecules bring specific amino acids (to ribosome);
4. Anticodon of tRNA complementary to codon on mRNA;
5. Sequence of codons on mRNA determines order of tRNA molecules and therefore order of amino acids;
6. Peptide bonds form between amino acids;
7. tRNA detaches and collects another amino acid;
8. Ribosome moves along mRNA;