5: Protein synthesis

Question 1

Similarity between DNA and RNA

Answer 1

Both are Nucleic acids

Question 2

4 differences between RNA and DNA

Answer 2

RNA is made of ribose nucleotides instead of deoxyribose nucleotides
RNA has the base uracil instead of thymine
RNA is single stranded
RNA is shorter than DNA

Question 3

Three kinds of RNA

Answer 3

mRNA - carries the message that codes for a particular protein from the nucleus to the cytoplasm
tRNA - matches amino acids to their codon
rRNA- together with proteins forms ribosomes, which are the site of mRNA translation and protein synthesis

Question 4

Facts about mRNA

Answer 4

• single stranded
• has a short lifetime
• degraded soon after use

Question 5

Facts about tRNA

Answer 5

• it folds up complementary base pairing to form a looped clover-leaf structure
• the middle loop contains the anticodon, which is a triplet nucleotide sequence
• the amino acids are attached to their tRNA molecule by specific amino acyl tRNA synthase enzymes

Question 6

Facts about rRNA

Answer 6

• ribosomes consist of two subunits
• assembled in the nucleolus of the nucleus and exported into the cytoplasm
• ribosomes free in the cytoplasm make proteins for use in the cell, while those attached to the RER make proteins for export.

Question 7

Features of the genetic code x4

Answer 7

• the code is degenerate
• the code is universal
• the code is non overlapping
• three codons do not code for an amino acid as they are stop codons

Question 8

Process of transcription

Answer 8

• start of each gene on DNA is marked by a special sequence of bases called the promoter
• the RNA molecule is built up from the 4 ribose nucleotides in the nucleoplasm. The ribose nucleotides attach themselves to the bases on the DNA by complementary base pairing. Only one strand of RNA is made. The copied DNA strand is the template strand. The other strand is a complementary copy called the non-template strand.
• the new nucleotides are joined to each other by strong covalent phosphodiester bonds by the enzyme RNA polymerase.
• a winding enzyme rewinds the DNA.
• at the end of the gene, transcription stops.
• mRNA diffuses out of the nucleus through a nuclear pore into the cytoplasm where it attaches to ribosomes for translation.

Question 9

What is post-transcriptional modification?

Answer 9

When introns are removed before mRNA can be translated into protein.

Question 10

What is pre-mRNA?

Answer 10

The initial mRNA that is transcribed.

Contains introns and exams as pre-mRNA is an exact copy of the gene on the DNA.

Question 11

Process of splicing

Answer 11

Introns in mRNA are cut out and exons are joined together by enzymes, such as protein complexes called snurps.
Results in shorter mature RNA containing only exons.
The introns are broken down.

Question 12

What is meant by the genetic code being:

• non overlapping
• degenerate
• universal

Answer 12

• each triplet is read in sequence, separate from the triplet before it and after it , base triplets done share their bases
• some amino acids are coded for by more than one triplet
• the same specific bases triplets code for the same amino acids in all living things

Question 13

Describe the process of translation

Answer 13

• mRNA attaches to a ribosome
• tRNA molecules carry amino acids to the ribosome
• tRNA has anticodon complementary to codon, attaches by specific base pairing
• a second tRNA attaches next to it
• the two amino acids are joined by a peptide bond
• the first tRNA moves away, a third tRNA binds
• the process continues, producing a polypeptide chain
• until theres a stop codon in the mRNA
• the polypeptide chain moves away and translation is complete

Question 14

Compare DNA, mRNA and tRNA in terms of:

• size
• where it’s found
• quantity over time
• chemical stability

Answer 14

• DNA largest, then mRNA, then tRNA
• DNA found in the nucleus, tRNA and mRNA found throughout the cell
• DNA quantity is constant for body cells, quantity of mRNA and tRNA varies with type of cell and level of metabolic activity
• DNA is chemically very stable, mRNA is unstable and easily broken down, tRNA is more stable than mRNA but less stable than DNA

Question 15

Define a mutation

Answer 15

Any change in the quantity of structure of the DNA

Question 16

Name and describe the 3 types of substitution mutation

Answer 16

• nonsense: mutation results in the formation of a stop codon, production of polypeptide would be stopped before completion, the final protein would be significantly different
• mis-sense: mutation results in a different amino acid being coded for, H bonds changed, different tertiary structure
• silent: the substituted base still codes for the same amino acid, due to degenerate nature of the genetic code

Question 17

Why do deletion mutations do more damage than substitution ones?

Answer 17

One deleted nucleotide causes a frame shift mutation, all triplets downstream of mutation will be different. Leads to different amino acids being coded for. Different tertiary structure, non functional

Question 18

Name some mutagenic agents

Answer 18

UV light
Xrays
Tabacco tar
Benzene 
Mustard gas

Question 19

What is the difference between acquired mutations and hereditary mutations?

Answer 19

acquired: occur in individual cells after fertilisation
hereditary: if a gamete containing a mutation is fertilised, the mutation will be present in the new foetus formed

Question 20

Describe the role of Tumour Supressor Genes

Describe the effect of mutation in these genes

Answer 20

Normally they slow cell division by producing proteins to stop cells dividing or cause them to cell destruct (apoptosis)
If a mutation occurs, the protein isn’t produced. The cells divide uncontrollably resulting in a tumour

Question 21

Describe the role of Proto-oncogenes

Describe the effect of mutation in these genes

Answer 21

Normally they stimulate cell divide, growth factors attach to a receptor the on cell membrane and switch on the genes necessary for gene replication
If a mutation occurs, the receptor protein can be permanently activated, even in the absence of GF’s
The oncogene may come a a growth factor that is produced in excessive amount, again stimulating excessive cell division