D1.2 - protein synthesis processes (2g)

Question 1

when are proteins made?

Answer 1

a protein is made when a gene is expressed

Question 2

what is the central process for protein synthesis?

Answer 2

DNA – transcription –> mRNA – translation –> protein

Question 3

what does it mean when a gene is being expressed?

Answer 3

• genes are said to be expressed when they are being used to make a protein
• not all genes in a cell are expressed in a given time
• this is what causes genetically identical cells in an organism to gain specialized structures and functions during differentiation

Question 4

why is transcription required for the expression of genes?

Answer 4

transcription, being the first stage of gene expression, is a key stage at which expression of a gene can be:
- switched off (not used to make a protein)
- switched on (used to make a protein)

Question 5

what is transcription?

Answer 5

transcription is the synthesis of mRNA using a DNA template

Question 6

what determines the sequence of mRNA?

Answer 6

the sequence of mRNA is determined by the order of base sequence in the gene being transcribed

Question 7

what is the process of transcription?

Answer 7

1. in the nucleus, DNA is unwound and the strands are separated, through breaking of the hydrogen bonds, by RNA polymerase
2. new RNA nucleotides are attached to template strand of DNA by RNA polymerase, using complementary base pairing
   - as nucleotides line up along the DNA template, they are linked together using phosphodiester bonds into a strand of mRNA
3. when the terminator sequence is reached, the whole gene has been used to make the complementary mRNA sequence and the mRNA molecule detaches from DNA template strand
   - the mRNA molecule will then move (out of the nucleus, if in a eukaryote) to a ribosome in the cytoplasm
4. the DNA rewinds

Question 8

what is the role hydrogen bonding and complementary base pairing in transcription?

Answer 8

new RNA nucleotides bind to the template strand of DNA via hydrogen bonding between the DNA and RNA bases

Question 9

what are the complementary base pairings of the four bases between DNA and RNA?

Answer 9

Adenine on the DNA strand with Uracil on the new RNA nucleotide
Thymine on the DNA strand with Adenine on the new RNA nucleotide
Cytosine on the DNA strand with Guanine on the new RNA nucleotide
Guanine on the DNA strand with Cytosine on the new RNA nucleotide

Question 10

what is the importance of the stability of DNA templates for protein synthesis?

Answer 10

• single DNA strands can be used as a template for transcribing a base sequence, without the DNA base sequence changing
• in somatic cells that do not divide, such sequences must be conserved throughout the life of a cell, with transcription occurring frequently (in contrast to DNA replication which does not occur)

Question 11

what is translation?

Answer 11

translation is the synthesis of polypeptides from information encoded in an mRNA and it occurs at ribosomes in the cytoplasm

Question 12

what is the process of translation?

Answer 12

1. initiation
   - the small ribosomal subunit binds to the start of the mRNA sequence, at the ‘start’ codon
   - a tRNA molecule transfers a specific amino acid to the large ribosomal subunit, based on the complementary base pairing between the anti-codon and codon on the respective molecules
2. elongation: stepwise movement of the ribosome along mRNA and linkage of amino acids by peptide bonding to the growing polypeptide chain
   - another tRNA molecule binds to the next mRNA codon, using complementary base pairing to the tRNA’s anti-codon, and transfers that corresponding amino acid
   - the ribosome joins the two amino acids by carrying out a condensation reaction to form a peptide bond
   - the ribosome moves down the mRNA molecule to the next codon, releasing the first tRNA molecule, moving the second into the position of the first tRNA and allowing a third tRNA to bring the next amino acid
   - the elongation process continues in a cycle
3. termination: when a stop codon is reached the ribosome, tRNA, and mRNA molecules dissociate, releasing the polypeptide

Question 13

what are codons?

Answer 13

groups of three mRNA bases, called codons, code for each amino acid in the polypeptide

Question 14

what does mRNA translate into?

Answer 14

the base sequence of mRNA is translated into the amino acid sequence of a polypeptide

Question 15

what determines the sequence of amino acids in a polypeptide?

Answer 15

the amino acid sequence of polypeptides is determined by the sequence of mRNA codons, according to the genetic code of 3 base triplets in DNA

Question 16

why does translation depend on complementary base pairing?

Answer 16

• tRNA contains an anticodon, which codes for the specific amino acid it carries
• complementary base pairing between the anticodon and codon therefore brings a specific amino acid to the ribosome, to be joined onto the polypeptide chain

Question 17

what are the roles of codons in translation?

Answer 17

codon - a sequence of three bases on an mRNA molecule that codes for one amino acid (or a stop signal to terminate translation)

Question 18

what are the roles of anti-codons in translation?

Answer 18

anti-codon - a sequence of three bases on a tRNA molecule that binds to the corresponding codon through complementary base pairing

Question 19

what are the roles of mRNA in translation?

Answer 19

mRNA - messenger RNA that carries the information from a gene within the DNA, to a ribosome in the cytoplasm, where it binds to the small subunit and determines the amino acid sequence of the protein that is synthesised due to the sequence of bases on the molecule

Question 20

what are the roles of ribosomes in translation?

Answer 20

ribosomes - large ‘molecular machines’ containing both protein and catalytic rRNA (ribosomal RNA) which carry out translation to synthesis polypeptides
- it also binds mRNA via its small subunit and two tRNA molecules via the large subunit

Question 21

what are the roles of tRNA in translation?

Answer 21

tRNA - carries a specific amino acid to a ribosome, based on the sequence of bases in each tRNA’s anticodon
- two tRNA molecules can bind simultaneously to the large subunit of the ribosome, holding the amino acids in place while the ribosome catalyses the condensation reaction to form a peptide bond