D1.2 - Protein Synthesis

What is the process of transcription?

1. RNA polymerase unwinds DNA and separates it into two single strands, exposing the bases. only about one turn of the DNA double helix in unwound and split open at any time during the transcription of a gene

2. RNA polymerase pairs up free RNA nucleotides to DNA nucleotides on the template strand, by complementary base pairing, with the bases of the nucleotides linked by hydrogen bonds

3. RNA polymerase links the RNA nucleotides together with covalent bonds between the pentose sugar of one nucleotide and the phosphate of the next, to form a continuous strand of RNA nucleotides

4. The assembled RNA strand separates from the template strand

5. The DNA strand pair up again and wind back into a double helix

What is the change in complementary base pairing in transcription?

Uracil not thymine

What do the sense strand and RNA transcript have in common in transcription?

Have the same base sequence with uracil replacing thymine as they both have sequence complementary to that of the template strand

What does the stability of DNA ensure for the base sequence?

That the base sequence of a gene rarely changes as the structure of DNA promotes continuity rather than change.

What are the sequence of bases in a gene?

A store of information

When is a gene expressed?

When the information it holds is used and an observable characteristic is generated within a cell or organism

What is the first stage of gene expression?

Transcription - it can be switched on or off which controls gene expression

What is translation?

The synthesis of a polypeptide, with its amino acid sequence determined by the base sequence of an RNA molecule.

• RNA molecule is transcribed from a protein - coding gene and is called messenger RNA (mRNA)

• Each base in the polypeptide is coded for by one codon on the mRNA.

Where is the site of translation?

Ribosomes

What is the role of RNA is translation?

1. Messenger RNA binds to a site on the small subunit of the ribosome. the mRNA contains a series of codons consisting of three bases, each of which codes for one amino acid

2. Transfer RNA molecules are present around the ribosome in large numbers. Each tRNA has a special triplet of bases called an anticodon and carries the amino acid corresponding to this anticodon

3. There are 3 binding sites for tRNA molecules on the large subunit of the ribosome but inly two ever bind at once. A tRNA can only bind if it has the anticodon that is complementary to the codon on the mRNA. The bases on the codon and anticodon link together by forming hydrogen bonds, following the same rules of complementary base pairing.

Which bond is used to link amino acids?

Peptide bond made between the amino acid held bu the A site of tRNA and for the other amino acid the P site of tRNA.

What are the 3 bases at one end of transfer RNA (tRNA) called?

The anticodon

What is a triplet in the genetic code?

A condon consisting of 3 bases which codes for 1 amino acids - 64 different codons

What is degeneracy in the genetic code?

A code in which more than one symbol is used to represent the same thing

What is universality in the genetic code?

All 64 codons of the genetic code have the same meaning in the cells of all organisms, which shows strong evidence that all life on earth has evolved from the same original cell

How can protein structure be changed?

By a mutation as small as a singe base change

What is a point mutation?

A single base change in the gene coding for a protein. This usually chnaged one amino acid in the polypeptide.

What is a same- sense mutation?

Changing a codon into another codon for the same amino acid

What is sickle cell anaemia?

It is caused by a point nutation in the gene coding for haemoglobin. A single nucleotide change results in valine replacing glutamic acid, leading to misshaped red blood cell.

What is a silent mutation?

No change in the amino acid due to genetic code degeneracy

What is a mis-sense mutation?

A different amino acid is added, potentially altering protein function

What is a nonsense mutation?

Converts a codon to a stop codon

What is a start codon in the genetic code?

AUG (Met)

What is a stop codon in the genetic code?

UAG