14- The Genetic Code (Finished)

Question 1

How many bases codes for one amino acid

Answer 1

3

Question 2

Give and explain the 3 things used to describe the genetic code

Answer 2

Universal - the same triplets always code for the same amino acids in all species.

Degenerate- more than one triplet codes for each amino acid

Non-overlapping - each base is only part of one triplet, as they don’t overlap

Question 3

Explain how the nature of the genetic code provides evidence for evolution

Answer 3

As the genetic code used is the same in all organisms, this provides evidence for evolution

Question 4

Compare prokaryotic and eukaryotic DNA

Answer 4

• in prokaryotic cells, DNA molecules are shorter than in Eukaryotic cells.
• Prokaryotic DNA is circular but Eukaryotic DNA is linear
• Eukaryotic DNA is associated with proteins and Prokaryotic DNA isn’t
• Eukaryotic DNA has introns but prokaryotic DNA doesn’t

Question 6

What is an exon

Answer 6

Triplet base sequences that code for amino acids

Question 7

Define a genome

Answer 7

The complete set of all genes in a cell/organism

Question 8

Define a proteome

Answer 8

The full range of different proteins that a cell can produce, as coded for by it’s DNA

Question 9

Compare the structure of tRNA and mRNA

Answer 9

tRNA:
- single stranded base sequence
- very small
- Has an anticodon and an amino acid binding site
- clover shaped
- contains paired bases so has hydrogen bonds

mRNA:
- Longer,
- Straight, unbranched chain of base sequences
- no paired bases so no hydrogen bonds,

Question 11

Outline transcription

Answer 11

1) The DNA strands separate by breaking the hydrogen bonds using DNA helicase,
2) Only one of the DNA strands is used as a template strand to synthesise the mRNA against,
3) Free RNA nucleotides align next to the exposed bases via complimentary base pairing (Adenine to Uracil, Thymine to Adenine, Cytosine to Guanine and Guanine to Cytosine).
4) The enzyme RNA polymerase joins adjacent mRNA nucleotides together via condensation reactions, forming phosphodiester bonds,
5) introns are removed by splicing to form mature mRNA which leaves through a nuclear pore.

Question 12

Describe splicing

Answer 12

• in eukaryotes, transcription produces a strand of pre-mRNA which must be spliced to remove the introns (non-coding regions within the gene) before leaving the nucleus, forming mRNA.

Question 13

What type of cells DNA has introns

Answer 13

Eukaryotic DNA

Question 14

How does transcription in prokaryotes differ to in eukaryotes

Answer 14

Because prokaryotic DNA doesn’t contain introns, splicing doesn’t occur- transcription results directly in the production of mRNA from DNA

Question 15

Outline translation

Answer 15

1) mRNA associates with a ribosome
2) the ribosome moves to the start codon, AUG,
3) The tRNA brings the specific correct amino acid to the ribosome,
4) the anticodon on the tRNA binds to the complimentary codon on the mRNA,
5) The ribosome moves along one codon and a second tRNA carrying the specific complimentary amino acid binds, via complimentary base pairing between the second codon and anticodon.
6) The amino acids join together via a condensation reaction using ATP to provide energy, forming a peptide bond,
7) The ribosome moves along one codon, the first tRNA is recycled back to the cytoplasm and the process repeats until a stop codon is reached, forming a polypeptide which detached from the ribosome.

Question 16

Explain the 4 main things that cause genetic variation

Answer 16

1) mutations (change in DNA base sequence –> new alleles),

2) Meiosis- Independent segregation (chromosomes randomly move to different poles) and Crossing over (exchange of alleles between homologous chromosomes).

3) Random fertilisation of gametes - each gamete carries a unique combination of alleles- which sperm fertilises which egg is random, producing new allele combinations in the zygote,

4) Movement of alleles between populations via migration introduces new alleles to a population.

Question 17

Give and explain 2 types of mutations

Answer 17

1) Substitution mutations- one base pair is replaced with another. This doesn’t cause a frame shift. Therefore, this is likely to be a silent mutation

2) insertion - deletion mutations - insertion causes a frame shift to the right and deletion causes a frame shift to the right

Question 18

Define mutagenic agents

Answer 18

These are things that can increase the rate of gene mutation. They include high energy radiation (X rays and gamma rays) which damage the DNA molecule, and chemicals that alter the DNA structure or interfere with DNA replication.