Genetic Control of Protein Structure and Function

Question 1

mRNA

Answer 1

mRNA is a long strand that is arranged in a single helix. Mirror copy of part of one of DNA’s two strands. mRNA leaves the nucleus via nuclear pores in the nuclear envelope and enter the cytoplasm where it relays the information to ribosomes. Acts as a template upon which proteins are built. It possesses the correct sequences of many triplets of organic bases that code for specific polypeptides. It’s easily broken down so exists only when it’s needed to manufacture a given protein.

Question 2

tRNA

Answer 2

tRNA is a single-stranded chain folded into a clover leaf shape, with one end of the chain extending beyond the other. This is where amino acids attach to. There are several types of tRNA and each can carry a single amino acid. The opposite end of the tRNA molecule is a sequence of 3 other organic bases called the anticodon (complimentary to the codon). tRNA lines up amino acids on the mRNA template during protein synthesis.

Question 3

codon

Answer 3

Sequence of triplet bases on mRNA that codes for single amino acid.

Question 4

anticodon

Answer 4

Sequence of triplet bases on tRNA that is complimentary to the codon.

Question 5

degenerate code

Answer 5

Most amino acids have more than one codon.

Question 6

stop-codons

Answer 6

3 codons do not code for any amino acid. They mark the end of a polypeptide chain.

Question 7

non-overlapping

Answer 7

Each base in the sequence is read only once.

Question 8

universal code

Answer 8

Same codon codes for the same amino acid in all organisms.

Question 9

transcription

Answer 9

A complementary section of part of DNA sequence is made in the form of pre-mRNA

Question 10

translation

Answer 10

mRNA is used as a template to which complementary tRNA molecules attach and the amino acids they carry are linked dot form a polypeptide

Question 11

Introns

Answer 11

Non-coding sections of DNA

Question 12

Exons

Answer 12

Sections of DNA that code for proteins

Question 13

Splicing

Answer 13

Removing introns and re-combining exons

Question 14

Outline Translation

Answer 14

1. A ribosome becomes attached to the starting codon on mRNA
2. The tRNA molecule with complimentary anticodon sequence moves to the ribosome and pairs up with the sequence on the mRNA, carrying with it a specific amino acid.
3. The ribosome moves along the mRNA, bringing together two corresponding tRNA molecules at any one time, each paring up with the corresponding two codons on the mRNA
4. Amino acids on the tRNA are joined together by a peptide bond catalysed by an enzyme and ATP
5. As the ribosome moves on, the first tRNA is released from its amino acid and is free to collect another amino acid from the amino acid pool in the cell
6. The synthesis of the polypeptide chain continues until a stop codon is reached. At this point, the ribosome, mRNA and last tRNA will separate and the polypeptide chain is complete.

Question 15

Why is splicing important?

Answer 15

DNA is made of exons which code for proteins and introns which don’t. Intervening introns would interfere with the synthesis of a polypeptide. Splicing removes intervening non-function introns and join function exons together. The remaining exon sections can be rejoined in a variety of different combinations, allowing the coding of up to a dozen different proteins from a single section of DNA. Mutations affect the splicing of pre-mRNA.

Question 16

Outline Transcription

Answer 16

1. DNA helicase acts on a specific region of DNA molecule to break hydrogen bonds between bases causing strands to separate and expose the nucleotide bases in that region
2. RNA polymerase moves along one of the two DNA strands known as the template strand, causing the nucleotides on this strand to join with the individual complementary nucleotides
   Base pair links are formed -> Uracil instead of thymine is formed
3. RNA polymerase adds the nucleotides one at time to build a strand of pre-mRNA, DNA strands rejoin behind it. Only 12 base pairs on DNA are exposed at any one time.
4. When RNA polymerase reaches a particular sequence of bases on the DNA, it recognises a ‘stop’ triplet code and detaches -> pre-mRNA production complete

Question 17

3 differences between DNA and mRNA + tRNA

Answer 17

1. Double polynucleotide chain (RNA = single)
2. Largest molecule of the three, double-helix molecule
3. Deoxyribose

Question 18

3 differences between mRNA and tRNA

Answer 18

1. mRNA = Single-helix molecule, tRNA = Clover-shaped
2. mRNA is larger
3. mRNA is less chemically stable

Question 19

4 similarities between mRNA and tRNA

Answer 19

1. Single polynucleotide chain
2. Pentose sugar ribose
3. Manufactured in nucleus, found throughout cell
4. Uracil base

Question 20

A nonsense mutation

Answer 20

Base change results in the formation of 1/3 stop codons

Question 21

A mis-sense mutation

Answer 21

Base change results in a different amino acid being coded for

Question 22

A silent mutation

Answer 22

Substituted base codes for the same amino acid - degenerate

Question 23

Deletion/ Addition

Answer 23

Frameshift due to triplet code

Question 24

2 Mutagenic agents

Answer 24

1. high-energy radiation

2. chemicals that alter the DNA structure or interfere with transcription

Question 25

Role of Proto-oncogenes

Answer 25

Stimulate cell division. Growth factors attach to a receptor protein on cell-surface membrane via relay proteins in cytoplasm, ‘switch on’ the genes necessary for DNA replication.

Question 26

What happens if Proto-oncogenes mutate?

Answer 26

They can mutate into oncogenes which can either:

1. Permanently activate the receptor protein on the cell-surface membrane so cell division is switched on even in the absence of growth factors
2. Code for a growth factor that is then produced in excessive amounts, stimulating excessive cell division

Question 27

Role of tumour suppressor genes

Answer 27

Maintains normal rates of cell division and prevents the formation of tumours.

Question 28

What happens if tumour suppressor genes mutate?

Answer 28

1. Inactivated -> most will die

2. Any that survive are able to make clones of themselves forming tumours