1.5.5 - Genetic Code & Protein Synthesis

Question 1

Define the term genetic code

Answer 1

The genetic code is a
linear
triplet
non-overlapping
degenerate
unambiguous
universal code for the production of polypeptides

Question 2

What is the triplet code for amino acids?

Answer 2

• amino acids are coded for by triplets of bases in the DNA
• the DNA is transcribed to produce codons in mRNA
• then translated to produce a sequence of amino acids

Question 3

What are exons and introns?

Answer 3

• exons are regions that contain the code for proteins
• between the exons are introns which are regions of non-coding DNA

Question 4

What is the difference between eukaryotic genes and prokaryotic genes in terms of exons and introns?

Answer 4

• eukaryotic genes are usually discontinuous with coding exons and non-coding introns
• prokaryotic genes are usually continuous genes, lacking non-coding sequences

Question 5

What is the 2 step process of protein synthesis

Answer 5

1) transcription
2) translation

Question 6

What does the process of transcription produce?

Answer 6

• mRNA

Question 7

Describe and explain the process of transcription

Answer 7

• DNA helicase breaks the H bonds between the bases in the helix -> unwinding the DNA -> exposing unpaired bases on the template strand
• RNA polymerase links to the template strand of the DNA, inserting mRNA nucleotides one at a time using complementary base pairing and forming bonds between them
• at the end of the gene there is a stop sequence where RNA polymerase leaves the DNA
• the molecule is called pre-mRNA because it has both introns and exons
• to form functional mRNA, introns are removed by enzymes

Question 8

What organelle is involved in translation?

Answer 8

Ribosomes

Question 9

Describe the structure of a ribosome

Answer 9

• 2 attachment sites for tRNA on the large subunit
• 1 attachment site for mRNA on the smaller subunit

Question 10

What is the function of tRNA

Answer 10

• each tRNA molecule carries a specific amino acid
• they bind to the ribosome by codon-anticodon interactions

Question 11

Describe the process of translation

Answer 11

• the ribosome binds to the start codon on the mRNA
• tRNA molecules bind to the ribosome through codon-anticodon interactions
• a peptide bond is formed between the 2 amino acids
• the ribosome moves along the mRNA one codon (triplet) at a time
• this continues until a stop codon is reached

Question 12

What is the further modification and combination of polypeptides after protein synthesis?

Answer 12

• can be further modified by adding carbohydrates, lipids or phosphate molecules to the polypeptide
• polypeptides can be combined eg. haemoglobin

Question 13

What does degenerate mean?

Answer 13

• each amino acid is coded for by more than one triplet

Question 14

What is meant by non overlapping in the definition of the genetic code

Answer 14

• first 3 bases are read as one codon
• each base is only read once
• bases do not overlap

Question 15

What is meant by universal in the definition of the genetic code

Answer 15

• because all species use the same 4 bases, AT GC, each base sequence codes fir the same amino acid in all species

Question 16

What is a gene?

Answer 16

• sequence of nucleotide bases that code for 1 polypeptide is a gene
• section of DNA

Question 17

Explain why it is necessary that the code is a triplet code and why the code is said it be degenerate.
[2]

Answer 17

• a triplet code can produce 64 different combinations which us enough for the 20 different amino acids
• as there is more than 20 codes, some amino acids are coded for by more than one amino acid codon

Question 18

Why can a codon not contain 2 bases instead of 3?
[2]

Answer 18

• 2 bases would only give 16 combinations
• not enough for the 20 amino acids

Question 19

What are the differences between mRNA and DNA?

Answer 19

• single stranded
• contains ribose sugar instead of deoxyribose
• contains uracil instead if thymine

Question 20

Describe mRNA

Answer 20

• used to transfer the DNA from the nucleus to the cytoplasm
• mRNA is complementary to the DNA code
• small enough to leave nuclear pore
• used to determine sequence of amino acids during protein synthesis
• easily broke. Down
• sequence of nucleotides in the mRNA is referred to as the genetic code

Question 21

What is the process by which introns are removed from pre mRNA

Answer 21

RNA splicing

Question 22

What is translation

Answer 22

Process by which cell makes proteins using the genetic information carried in mRNA

Question 23

Describe the process of translation (booklet)

Answer 23

• helicase unwinds double helix by breaking H bonds between bases
• RNA polymerase moves along the template strand and matches up complementary nucleotides
• AU TA GC CG
• as RNA nucleotides join together the pre-mRNA us formed
• DNA behind the RNA polymerase rejoins into a double helix
• when the RNA polymerase reaches a stop codon the chain is terminated and the pre-mRNA detaches

Question 24

Describe the process of transcription (booklet)

Answer 24

• mRNA attaches to ribosomes at start codon
• tRNA with complementary anticodon attaches to the mRNA forming a codon-anticodon complex
• this tRNA molecule is attached to an amino acid
• the ribosome moves along the mRNA bringing in second tRNA molecule
• this brings the 2 amino acids close enough for a peptide bond
• ENZYME and ATP are used to join the acids with a peptide bond
• first tRNA is released and can collect another amino acid
• one moves, another moves into the vacant position
• process repeated until stop codon

Question 25

What is it called when many ribosomes travel along the mRNA at the same time?

Answer 25

Polysome

Question 26

Why is ATP required for protein synthesis?

Answer 26

- energy needed to attach amino acid to tRNA - to join amino acids by a peptide bond

Question 27

What is the one gene one polypeptide hypothesis?

Answer 27

- it states that one gene codes for a single polypeptide - the quaternary protein haemoglobin has 4 different polypeptide chains therefore 4 genes are needed to code for haemoglobin - collagen has 3 alpha helicies (secondary protein structure) but they are identical so 1 gene is sufficient