IB BIO TOPIC 3.5 : TRANSCRIPTION & TRANSLATION

Question 1

Where does Transcription and Translation happen ?

Answer 1

Transcription happens in the nucleus of the cell and Translation in the cytoplasm at the Ribosome.

Question 2

What is transcription ?

Answer 2

In protein synthesis, it’s the copying of a part of the DNA.

Question 3

What is translation ?

Answer 3

In protein synthesis, it’s the production (cooking) of the copied DNA.

Question 4

Compare DNA and RNA Structures

Answer 4

DNA RNA

sugar deoxyribose ribose

purines adenine, guanine same

pyrimidines thymine, cytosine uracil, cytosine

strands double single (usually)

Question 5

Outline DNA transcription in terms of formation of complementary RNA strand by RNA polymerase:

1. Initiation

Answer 5

RNA polymerase is an enzyme complex which:

unwinds and unzips DNA double strand
attaches to promoter region of gene, which marks the beginning point for transcription

Question 6

Outline DNA transcription in terms of formation of complementary RNA strand by RNA polymerase:

2. Elongation

Answer 6

RNA polymerase:

uses DNA anti-sense strand as a template
synthesizes a complementary RNA strand using base pairing rules

         A = U             T = A          G ≡ C          C ≡ G

Question 7

Outline DNA transcription in terms of formation of complementary RNA strand by RNA polymerase:

3. Termination

Answer 7

RNA polymerase :

reaches termination region of the gene, which marks the end of the coding sequence
terminates transcription by releasing both DNA and RNA

Question 8

Describe the Genetic Code in terms of codons composed of triplets of bases :

Answer 8

triplet code = 3 nucleotide bases code for one amino acid
codon = a group of 3 nucleotide bases is called a codon
there are 64 different codons (4 x 4 x4 = 64)

Question 9

Explain the process of translation, leading to peptide linkage formation :

Initiation :

Answer 9

• mRNA binds to the small subunit of the ribosome
• tRNA anticodon binds to mRNA codon by complementary base pairing
• large ribosomal subunit binds, completing ribosomal structure, and producing two ribosomal binds sites: P site & A site

Question 10

Explain the process of translation, leading to peptide linkage formation

Elongation : part 1

Answer 10

• tRNA with anticodon complementary to second mRNA codon binds to ribosomal A site, with appropriate amino acid attached to tRNA
• enzymes in ribosome catalyze formation of peptide bond between 1st, P site, and 2nd, A site, amino acids
• P site tRNA, now separated from amino acid, exits ribosome
• ribosome moves one codon (3 nucleotides) along the mRNA, thus shifting previous A-site tRNA to P-site, and opening A-site

Question 11

Explain the process of translation, leading to peptide linkage formation

Elongation : part 2

Answer 11

• tRNA with anticodon complementary to A-site mRNA codon binds to ribosomal A-site, with appropriate amino acid attached to tRNA terminal
• enzymes in ribosome catalyze formation of peptide bond between 2nd and 3rd amino acids
• P site tRNA, now separated from its amino acid, exits ribosome
• ribosome moves one codon (3 nucleotides) along the mRNA, thus shifting previous A-site tRNA to P-site, and opening A-site
• repetition of process until stop codon is reached

Question 12

Explain the process of translation, leading to peptide linkage formation :

Termination : Part 1

Answer 12

• when ribosomal A-site reaches a stop codon, no tRNA has a complementary anticodon
• release factor protein binds to ribosomal A-site stop codon
• polypeptide and mRNA are released
• large and small ribosomal subunits separate
• polysomes: several to many ribosomes translating the same mRNA into protein; each moving in the 5’ to 3’ direction

Question 13

Explain the process of translation, leading to peptide linkage formation :

Termination : Part 2

Answer 13

• start codon: the mRNA triplet codon AUG is universally the start codon used to mark the beginning of the coding sequence of a gene; thus, the tRNA with the anticodon UAC and carrying the amino acid methionine is always the first tRNA to enter the P-site during translation
• stop codon: there are three stop codons in the genetic code; none of these have a corresponding tRNA; instead, when a ribosome encounters a stop codon, a release factor binds to the stop codon, which terminates translation and allows the separation of all of its components

Question 14

Define degenerate :

Answer 14

two or more codons can code for the same amino acid

Question 15

Define universal :

Answer 15

all living organisms use the same genetic code, indicating common ancestry; even viruses use the same code

Question 16

Explain the relationship between one gene and one polypeptide and its significance

Answer 16

• when the relationship between genes and proteins was first discovered it was initially thought that the relationship was one-to-one: one gene coding for one polypeptide

Question 17

What is a gene ?

Answer 17

• information coded in DNA nucleotide sequences
• transcribed into mRNA
• mRNA translated into a sequence of amino acids joined by peptide bonds to produce a polypeptide

Question 18

What is a polypeptide ?

Answer 18

• polymer of amino acids joined by peptide bonds
• each polypeptide’s function dependent on its precise sequence of amino acids

Question 19

What are the exeptions for the genes ?

Answer 19

• some genes produce more than one polypeptide
• there are only about 21,000 human genes, but over 120,000 human proteins
• therefore, many genes produce more than one protein
• this is possible because of post-transcriptional modification, combining exons in various combinations
• example: lymphocyte production of antibodies:
• millions of different antibody proteins are produced from just a few genes
• different lymphocytes splice together parts of these genes in different ways

Question 20

What are some other exeptions ?

Answer 20

• some genes do not code for protein
• some genes code for tRNA
• not translated into protein
• transports amino acids to ribosomes
• some genes code for rRNA
• not translated into protein
• a component of ribosome structure and function
• some DNA sequences act as regulators of gene expression
• regulatory DNA is transcribed into regulatory RNA
• which then binds to other DNA sequences
• determining whether those genes are transcribed or not