2.6, 2.7, 2.8 Protein Synthesis - Transcription and Translation

Question 1

What is a gene?

Answer 1

a sequence of bases on a DNA molecule that codes for a sequence of amino acids in a polypeptide chain

Question 2

What does the order of mononucleotide bases in a gene determine?

Answer 2

• the order/sequence of amino acids of a protein (the primary structure)
• different sequences of bases code for different amino acids, so the sequence of bases in a section of DNA is a template that determines what protein is made during protein synthesis

Question 3

Describe the process of transcription:

Answer 3

1. the enzyme RNA polymerase attaches to the DNA at the beginning of a gene
2. the hydrogen bonds between the two DNA strands break, the strands are separated, and the DNA molecule unwinds
3. one of the strands of DNA is the template strand (the strand the mRNA is formed from) and the other strand is the coding strand (the mRNA is an exact copy)
4. RNA nucleotides align themselves alongside the complementary bases on the template strand. phosphodiester bonds form, forming a mRNA molecule
5. because of complementary base pairing, the order of bases on the mRNA is exactly the same as the order of bases on the DNA coding strand - but T is replaced by U
6. complementary base pairing also means that the mRNA strand is a complementary copy of the DNA template strand
7. only the section of DNA that codes for the protein being made is transcribed
8. the mRNA molecule then leaves the nucleus through a nuclear pore
9. the hydrogen bonds between the unwound strands of DNA reform once the RNA polymerase has passed by, winding back up into a double helix

Question 4

Why does DNA need to undergo transcription?

Answer 4

DNA is too large to move out of the nucleus, so sections are copied as mRNA

Question 5

Describe the process of translation:

Answer 5

1. mRNA attaches to a ribosome, and tRNA carries amino acids to the ribosome
2. the start codon on the mRNA is AUG. the anticodon on the tRNA complementary to the start codon is UAC. the tRNA (carrying an amino acid) with the anticodon attaches itself to the mRNA start codon by complementary base pairing (hydrogen bonds form)
3. the next codon on the mRNA is facing the second site in the ribosome. this attracts the next tRNA - amino acid complex with the complementary anticodon, which binds to the mRNA
4. the ribosome holds the mRNA, the tRNA - amino acid complex, and the associated enzyme while peptide bonds form between the two amino acids
5. once the peptide bond has formed, the ribosome moves along the mRNA to reveal the next codons at the binding site, and the tRNA returns to the cytoplasm to pick up new amino acids
6. this process repeats as tRNA continues to transfer amino acids to the growing polypeptide chain
7. the process stops when the stop codons on the mRNA are reached. the stop codons are UAA, UAG, UGA. they are stop codons because no tRNA with complementary anticodons exist so no amino acids are transferred
8. the polypeptide chain then detaches from the ribosome

Question 6

What is RNA polymerase?

Answer 6

an enzyme that attaches to DNA to start transcription

Question 7

What is mRNA and what does it do?

Answer 7

• it is made in the nucleus during transcription
• it carries the genetic code from the DNA in the nucleus to the cytoplasm

Question 8

What is tRNA and what does it do?

Answer 8

• found in the cytoplasm
• has an amino acid binding site at one end and an anticodon on the other end (the sequence of three bases complementary to a codon)
• each amino acid has its own tRNA that carries it to the ribosome during translation

Question 9

What are ribosomes and what is their role in protein synthesis?

Answer 9

• ribosomes are involved in protein synthesis to make proteins
• they have 2 subunits - one smaller and one larger
• they have 2 RNA binding sites on the larger subunit
• mRNA binds to the smaller subunit, with the 2 mRNA codons facing the 2 binding sites in the larger subunit

Question 10

What are stop and start codons and what is their role?

Answer 10

• start codon: it is the first codon on mRNA that undergoes translation - AUG
• stop codons: the last codons on the mRNA, where the process of translation is terminated - UAA, UAG, UGA. they are stop codons because no tRNA with complementary anticodons exist so no amino acids are transferred

Question 11

What are the sense and antisense strands?

Answer 11

• sense: coding strand (mRNA is a copy)
• antisense: template strand (mRNA is transcribed from to make a copy of the coding strand)

Question 12

What is the difference between codons, anticodons, and triplet codes?

Answer 12

• triplet code: the three base sequence on DNA template strand (complementary to a codon on mRNA)
• codon: three base sequence on mRNA
• anticodon: three base sequence on tRNA that is complementary to a codon on mRNA

Question 13

What is the nature of the genetic code?

Answer 13

• triplet code: sequence of base triplets in DNA or mRNA - each triplet codes for a specific amino acid
• non-overlapping: the triplet code is read separate from the triplet before and after it. each triplet code is adjacent and each adjacent base triplet codes for an amino acid (base triplets do not share bases)
• degenerate: several base triplets can code for each amino acid (e.g. tyrosine can be coded for by UAU or UAC)

Question 14

Why is the genetic code in triplets and degenerate?

Answer 14

• there are 20 amino acids
• and there are 4 bases
• because the genetic code is in triplets, there are 64 (4^3) possible combinations of triplets, which is enough to code for 20 amino acids
• the code is degenerate because there are 64 combinations of triplets but only 20 amino acids, so amino acids are coded for by more than one base triplet
• if the genetic code was in duplets, there would only be 16 (4^2) possible combinations of duplets, which is not enough to code for 20 amino acids