DNA and Protein Synthesis

Question 1

What are three bases in the sequence of a gene called?
How many combinations are there?

Answer 1

A triplet and it codes for a specific amino acid.

There are four different nucleotides which means there are 4^3= 64 possible combinations of triplets to code for 20 amino acids

Question 2

What are the 3 characteristics of a triplet?

Answer 2

1. Degenerate - because most ammo acids are coded for by more than one triplet
2. Universal - the same in all organisms
3. Non-overlapping - each base is read only once, each base is part of only one triplet

Question 3

What does the first triplet always code for?

Answer 3

Methionine

Question 4

Which triplets don’t code for amino acids?

Answer 4

There a three triplets which don’t code for amino acids - stop triplets, a stop triplet is found at the end of each gene

Question 5

What is a codon?

Answer 5

3 mRNA bases complementary to a triplet if DNA bases

Question 6

How do use the universal genetic code chart?

Answer 6

e.g if the DNA triplet is GCA
The mRNA codon will = CGU
Which codes for the amino acid ARG

Question 7

What types of RNA are needed to synthesise proteins?

Answer 7

mRNA and tRNA are needed in addition to DNA

Question 8

Is mRNA longer or shorter tRNA?

Answer 8

Longer it is a few thousand nucleotides long, whereas tRNA molecules are about 75 nucleotides long

Question 9

What is the structure of tRNA?

Answer 9

• The tRNA strand folds back on itself and hydrogen bonds form between complementary base pairs. The overall shape is called clover leaf
• at one end of the molecule is an amino acid binding site
• three bases are exposed = anticodon

Question 10

How many types of tRNA are there?

Answer 10

61, each with a different sequence of bases in the anticodon. Each type of tRNA carries only one specific type of amino acid

Question 11

What is the role of anticodons?

Answer 11

Anticodon on tRNA molecule binds to a complementary codon on a mRNA molecule during the process of translation

Question 12

Fill in summary table

Answer 12

Question 13

Describe the process of transcription

Answer 13

Takes place in the nucleus to make pre - mRNA from DNA

• one gene unwinds due to hydrogen bonds between complementary bases being broken
• complementary free RNA nucleotides bind to the exposed bases on the template strand (only one strand acts as a template) e.g A-U, T-A, C-G
• sugar phosphate backbone between the free RNA nucleotides is joined using RNA polymerase
• both the intron and exon sections of the gene are transcribed to make pre-mRNA
• transcription stops at a stop triplet such as ATC. This marks the end of a gene and causes the disengagement of the RNA polymerase
• a gene is transcribed repeatedly to make many pre-mRNA at any one time
• most genes are transcribed to make pre-mRNA however a few genes are transcribed to make ribosomal RNA and transfer RNA

Question 14

Describe the process of splicing mRNA

Answer 14

Occurs in the nucleus, pre-mRNA is made into mRNA

• in eukaryotic nuclei the pre-mRNA is then spliced, the non coding sections (transcribed from the introns) are cut out and the coding sections (transcribed from the exons) are edited together to produce mRNA which is a copy of the exons only
• prokaryotic DNA and mitochondrial and chloroplast DNA does not have introns so the mRNA does not need splicing

• the mRNA then leaves the nucleus through a nuclear pore and binds to a ribosome. Ribosomes contain ribosomal RNA

Question 15

Describe translation

Answer 15

Occurs in the cytoplasm - makes polypeptides

1) the ribosome binds to the first two codons on the mRNA. A tRNA carrying a methionine (START) amino acid comes to the ribosome. The anticodon on the tRNA binds to the complementary codon on the mRNA
2) A second tRNA binds to the next codon, the tRNA is carrying a specific amino acid
3) The two amino acids undergo a condensation reaction and a peptide bonds form between them
4) the first tRNA then leaves the ribosome and picks up another amino acid. The ribosome moves along the mRNA to cover the next codon. The anticodon of another tRNA, again carrying a specific amino acid, binds to the next codon. The amni acid binds to the growing polypeptide chain
5) when the ribosome reaches a stop codon it disengages from the mRNA. There are no tRNA with anticodons complementary to stop codons

ATP is required to provide the energy for the bond formation between the amino acid and the tRNA molecule, allowing the tRNA to carry an amino acid to the ribosome during translation