Ch26 Molecular Genetics

Question 1

Degenerate (definition)

Answer 1

most amino acids are coded by more than one codes

Question 2

Universal (definition)

Answer 2

the same triplet codes code for the same amino acids in all organisms

Question 3

Process of transcription
(DNA -> mRNA)

Answer 3

1. RNA polymerase binds to the DNA at the start of a gene. The two DNA strands unwind.
2. As RNA polymerase moves along the DNA, free ribonucleotides are attached to the template strand through complementary base pairing. RNA polymerase catalyses the joining of the adjacent ribonucleotides to form an mRNA.
3. Once the whole gene has been transcribed, the mRNA separates from the template strand of DNA. The DNA winds back into a double helix again.

Question 4

Process of translation
(mRNA -> polypeptides)

Answer 4

1. The mRNA attaches to a ribosome. A specific amino acid is carried to the ribosome by a tRNA molecule. The anticodon on tRNA binds to the first codon on the mRNA as they are complementary to each other. Another tRNA molecule carrying another amino acid binds to the next codon.
2. The two amino acids are joined by a peptide bond.
3. The ribosome moves along the mRNA to add amino acids to the growing polypeptide chain, one at a time.
4. The ribosome reaches a stop codon. The polypeptide formed is released from the ribosome.

Question 5

From polypeptides to proteins

Answer 5

After translation, the polypeptide chain coils and folds into a protein due to the attraction forces between different amino acids in the polypeptide.

Question 6

Types of gene mutations

Answer 6

1. substitution
2. inversion
3. deletion
4. insertion

Question 7

Substitution / Inversion

Answer 7

• change of one triplet code only
• if the new code specifies the same amino acid -> no effect
• if the new code specifies different amino acid -> one different amino acid, may alter the shape of the protein and the resulting protein is non-functional
• if the new code specifies a stop signal -> production of polypeptide will stop prematurely, resulting protein is non-functional

Question 8

Deletion / insertion

Answer 8

• shift the reading frame
• the whole amino acid sequence after the point of mutation is altered, resulting protein is non-functional

Question 9

Causes of sickle-cell anaemia

Answer 9

• caused by a substitution of a base in the gene that codes for a polypeptide chain in haemoglobin
• base T on the template strand of DNA is replaced by base A at one position
• the polypeptide produced has one different amino acid (valine instead of glutamic acid)
• affects the shape of the polypeptide formed and affects the properties of the chain formed

Question 10

Effects of sickle-cell anaemia

Answer 10

When oxygen levels in the body are low, the abnormal haemoglobin molecules stick together and form long fibres. The fibres distort the shape of red blood cells, causing them to become sickle-shaped. The sickled-shaped cells cannot transport oxygen efficiently. They clump together and block capillaries causing damage in organs.

Question 11

Chromosome mutations - types of changes in chromosome structure

Answer 11

1. deletion
2. duplication
3. inversion
4. translocation

Question 12

Chromosome mutations - change in chromosome number

Answer 12

Non-disjunction: members of a homologous pair of chromosomes (meiosis I -> n+1, n+1, n-1, n-1) or sister chromatids (meiosis II -> n+1, n-1, n, n) fail to separate

Question 13

Disorders caused by chromosome mutations

Answer 13

Down syndrome: an extra chromosome 21
Turner’s syndrome: one X chromosome missing (XO)
Klinfelter’s syndrome: an extra X chromosome for male (XXY)