Chapter 3 Part 3

Question 1

Protein Synthesis

Answer 1

• DNA is master blueprint for protein synthesis. It directs the order of amino acids in a polypeptide
• The segment of DNA which holds the genetic code for a polypeptide is called a gene. The gene is made of of three sequential bases (A,T,G,C)
• Each triplet combination of nitrogen bases codes for a different amino acid.

Question 2

Messenger RNA (mRNA)

Answer 2

• Singe Stranded, code from DNA is transcribed (copied) onto RNA with complementary Base Pairs making messenger RNA.
• mRNA carries triplet code from DNA. Holds instructions for building a Polypeptide.

Question 3

Ribosomal RNA (rRNA)

Answer 3

• Structural component of ribosomes which is the site of protein synthesis.
• Helps translate mRNA into a polypeptide.

Question 4

Transfer RNA (tRNA)

Answer 4

• Carrier of a specific polypeptide. Have a region which is the inverse of the mRNA codon called the anticodon which allows tRNA to bind to mRNA.
• EX) the codon AUG would be transferred by the tRNA with the codon UAC.
• Process is called translation

Question 5

Two Steps in Protein Synthesis

Answer 5

• Transcription) DNA is coded into mRNA

- Translation) mRNA is coded to be turned into polypeptide

Question 6

Transcription Factors

Answer 6

• Help Transcription start
• Histones of DNA loosen so the fragment of DNA that will be transcribed is exposed.
• Promoter gene (start gene) is bound so it can be transcribed.
• RNA Polymerase, which synthesises mRNA, is bound to promoter region.

Question 7

Transcription (three phases)

Answer 7

1) Initiation) RNA polymerase separates DNA at promoter region.
2) Elongation) RNA polymerase adds complementary nucleotides to DNA BASED on the template stand (opposite of coding strand)
3) Termination) Transcription stops when the RNA polymerase hits a termination signal code.

Question 8

Processing of mRNA

Answer 8

• Occurs after transcription and before translation can begin.
• Newley formed RNA is called pre-mRNA before processing. During processing the Intron Genes are removed by a special protein called a Spliceosome. This process leaves only Exon genes.

Question 9

Genetic Code

Answer 9

• Every sequence of three amino acids codes for one mRNA called a codon. There are 64 codons which code for amino acids (3 are considered stop codons)
• There are only 20 possible amino acids, so some amino acids are formed by more than one codon.

Question 10

Start and Stop Codons.

Answer 10

Start) AUG

Stop) UAA, UAG, and UGA.

Question 11

Role of tRNA

Answer 11

• Anticodon of tRNA will only bind to it’s complementary mRNA codon.
• Ribosomes coordinate coupling of mRNA and tRNA. Ribosomes have three binding sites for tRNA and one for mRNA.
  1) Aminoacyl (A) site) incoming tRNA
  2) Peptidyl (P) site) tRNA links to growing polypeptide chain,
  3) Exit (E) site) for outgoing tRNA.

Question 12

Initiation (Translation)

Answer 12

• Small ribosomal unit binds to start codon on tRNA (UAG or methionine) and then to mRNA to be decoded.
• When tRNA binds to start codon a large ribosomal unit and small unit can bind forming a functional ribosome.
• At end of initiation Site P holds start codod while A and E are empty.

Question 13

Elongation (Translation)

Answer 13

3 steps

• 2a) Codon recognition) tRNa binds complementary codon in A site of ribosome.
  2b) Peptide bond formation) Ribosomes transfer amino acid to polypeptide chain in A site and transfer the left over tRNA to P site
  2c) Translocation) Ribosome shifts down the three bases of mRNA, displacing tRNA’s by one position. tRNA goes A>P>E

Question 14

Termination (Translation)

Answer 14

A stop codon enters the ribosome which stops mRNA translation.

Question 15

Role of Rough ER in Protein Synthesis.

Answer 15

• Once polypeptide chain attaches to ER it enters the ER
• Here it can be modified by adding sugar groups or by changing its shape. Polypeptide is then packaged and sent to golgi apparatus.