protein synthesis

Question 1

Define the term exon.

Answer 1

. Base/nucleotide/triplet sequence coding for polypeptide/sequence of amino acids/primary structure;

Question 2

Describe Transcription in Eukaryotes

Answer 2

1. Hydrogen bonds (between DNA bases) break;
2. (Only) one DNA strand acts as a template;
3. (Free) RNA nucleotides align by complementary base pairing;
4. (In RNA) Uracil base pairs with adenine (on DNA) OR (In RNA) Uracil is used in place of thymine;
5. RNA polymerase joins (adjacent RNA) nucleotides;
6. (By) phosphodiester bonds (between adjacent nucleotides);
7. Pre-mRNA is spliced (to form mRNA) OR Introns are removed (to form mRNA);

Question 3

Describe Translation

Answer 3

1. (mRNA attaches) to ribosomes OR (mRNA attaches) to rough endoplasmic reticulum;
2. (tRNA) anticodons (bind to) complementary (mRNA) codons;
3. tRNA brings a specific amino acid;
4. Amino acids join by peptide bonds;
5. (Amino acids join together) with the use of ATP;
6. tRNA released (after amino acid joined to polypeptide);
7. The ribosome moves along the mRNA to form the polypeptide;

Question 4

Give two structural differences between a molecule of messenger RNA (mRNA) and a molecule of transfer RNA (tRNA).

Answer 4

. mRNA does not have hydrogen bonds / base pairing, tRNA does; OR mRNA is linear / straight chain, tRNA is cloverleaf;
. mRNA does not have an amino acid binding site, tRNA does;
. mRNA has more nucleotides;
. (Different) mRNAs have different lengths, all tRNAs are similar / same length;
. mRNA has codons, tRNA has an anticodon;

Question 5

What is an allele?

Answer 5

. (Different) form/type/version of a gene / different base sequence of a gene;

Question 6

Explain how selection occurs in living organisms

Answer 6

. Variation due to mutation;
. Different environmental/abiotic/biotic conditions / selection pressures;
. Selection for different/advantageous, features/characteristics/mutation/ /allele;
. Differential reproductive success / (selected) organisms survive and reproduce;
. Leads to change in allele frequency;
. Occurs over a long period of time;

Question 7

Define degenerate code

Answer 7

. More than one codon for one amino acid

Question 8

Explain what the term non-overlapping DNA means.

Answer 8

. A base from one triplet cannot be used in an adjacent triplet

Question 9

A mutation in a gene coding for an enzyme
could lead to the production of a non-functional enzyme. Explain how

Answer 9

. Change in base sequence (of DNA/gene);
. Change in amino acid sequence / primary structure (of enzyme);
. Change in hydrogen/ionic/ disulphide bonds;
. Change in the tertiary structure/active site (of enzyme);
. Substrate not complementary/cannot bind (to enzyme / active site) / no enzyme-substrate complexes form;

Question 10

Not all mutations in the nucleotide sequence of a gene cause a change in the structure of a polypeptide. Give two reasons why.

Answer 10

. Triplets code for same amino acid
. Occurs in introns /non-coding sequence;