Protein synthesis

Question 1

what are the three stages in protein synthesis ?

Answer 1

Stage 1: transcription
Stage 2: activation of amino acids
Stage 3: translation

Question 2

transcription

Answer 2

the process by which the DNA nucleotide base sequence of a gene is copied into the RNA nucleotide base sequence in a molecule of messenger RNA - controlled by 3 groups of enzymes

Question 3

Stage 1: transcription

Answer 3

• part of a DNA molecule unwinds and the hydrogen bonds between the complementary base pairs break
• this exposes the gene to be transcribed
• a complementary copy of the code from the gene is made by building a mRNA
• free activated RNA nucleotides pair up with their complementary bases on one strand of the unzipped DNA molecule
• the sugar phosphate groups of these RNA nucleotides are then bonded together by the enzyme RNA polymerase to form the sugar phosphate backbone of the mRNA molecule
• when the gene has been transcribed the hydrogen bonds between the mRNA and DNA strands break and the double stranded DNA molecule reforms
• the mRNA molecule then leaves the nucleus

Question 4

where does transcription in protein synthesis occur ?

Answer 4

in the nucleus of the cell

Question 5

Stage 2: activation of amino acids

Answer 5

• the amino acids are activated for protein synthesis by combining with short lengths of a different sort of RNA - transfer RNA
• all molecules of tRNA have the shape of a clover leaf but there is a different tRNA of each of the 20 amino acids
• at one end of each tRNA molecule is a site where a particular amino acid can be joined
• at the other end there is a sequence of three bases called an anticodon

Question 6

anticodon

Answer 6

complementary to the codon of mRNA that codes for the specific amino acif

Question 7

translation

Answer 7

process by which the sequence of nucleotides present in mRNA is read by the ribosomes and is translated into the sequence of amino acids in a polypeptide chain

Question 8

where does translation in protein synthesis occur ?

Answer 8

cytoplasm of the cell

Question 9

Stage 3: translation

Answer 9

protein chain is assembled one amino at a time
- tiny organelles called ribosomes move to the mRNA and move along it reading the codons from start codon - held by hydrogen bonds
- the amino acids of neighbouring amino acids - tRNA are joined by peptide bonds
- this frees the first tRNA which moves back into the cytoplasm for reuse
- ribose moves on to the next mRNA codon until stop codon occurs

Question 10

stop codon

Answer 10

acts a signal for translation to stop and at this point the amino acid chain coded for by the mRNA molecule is complete

Question 11

where can non coding DNA be found ?

Answer 11

• between genes as non coding multiple repeats
• within genes, as introns

Question 12

during transcription, eukaryotic cells transcribe the whole gene to produce what ?

Answer 12

pre - mRNA molecules, which contains the introns and exons of a certain gene

Question 13

Before the pre- mRNA exits the nucleus splicing occurs :

Answer 13

• non coding sections are removed
• the coding sections are joined together
• mRNA molecule carries only the coding sequences of the gene
• mRNA contains only exons and exits the nucleus before joining a ribosome for translation