CFE Higher BiologyDNA and

the Genome

Transcription

Transcription is the synthesis of mRNA from a section of DNA.

Transcription of a gene starts from a region of DNA known as the promoter.

CFE Higher BiologyDNA and

the Genome

Promoter: Start of a gene

Terminator: End of a gene

CFE Higher BiologyDNA and

the Genome

RNA polymerase

This enzyme is responsible for transcription.RNA polymerase binds at the promoter and unwinds the DNA.

CFE Higher BiologyDNA and

the Genome

RNA polymerase adds nucleotides onto the 3’ end of the growing mRNA molecule.Due to the base-pairing rules the mRNA produced will be complementary to the DNA.The molecule elongates until it reaches the terminator sequence.The molecule produced is called the primary transcript.

CFE Higher BiologyDNA and

the Genome

CFE Higher BiologyDNA and

the Genome

Modification of the primary transcript

Not all the regions in a eukaryotic gene are required to produce the final protein.These non-coding regions are called introns.The coding regions are called exons.

CFE Higher BiologyDNA and

the Genome

Primary transcript

Primary transcript

Mature transcript

CFE Higher BiologyDNA and

the Genome

RNA splicingAfter the mRNA has been transcribed the introns are removed.The remaining exons are spliced together to form a continuous sequence.This is called the mature transcript.The mature transcript then leaves the nucleus to travel to the cytoplasm.

CFE Higher BiologyDNA and

the Genome

Translation

CFE Higher BiologyDNA and

the Genome

Genetic code

Translation is the synthesis of protein following the code with in the mature mRNA transcript.The mRNA is made of sequences of three nucleotides (a triplet of bases) called codons.Each codon is code for one amino acid.

CFE Higher BiologyDNA and

the Genome

CFE Higher BiologyDNA and

the Genome

tRNA

A further type of RNA is found in the cell’s cytoplasm.This is called tRNA (transfer RNA) and is made of a single chain of nucleotides.It is folded into a 3D structure, held together by hydrogen bonds.

CFE Higher BiologyDNA and

the Genome

CFE Higher BiologyDNA and

the Genome

Each tRNA has an attachment site for a specific amino acid and a triplet of bases known as an anticodon.

Many different types of tRNA are present in cell, one or more for each type of amino acid.

CFE Higher BiologyDNA and

the Genome

The tRNA picks up its appropriate amino acid and takes it to the ribosome to be matched with the mRNA.

CFE Higher BiologyDNA and

the Genome

CFE Higher BiologyDNA and

the Genome

Ribosomes

CFE Higher BiologyDNA and

the Genome

Ribosomes are small, roughly spherical structures found in all cells.

They contain the enzymes essential for protein synthesis.

The ribosome’s function is to bring the tRNA molecules bearing amino acids in contact with the mRNA.

CFE Higher BiologyDNA and

the Genome

Site P – holds the tRNA carrying the growing polypeptide chain.

Site A – holds the tRNA carrying the next amino acid to be joined to the chain.

Site E – releases the empty tRNA once it has dropped off its amino acid.

CFE Higher BiologyDNA and

the Genome

The translation process

1. The ribosome binds to the 5’ end of the mRNA so that the start codon (AUG) is in site P.2. Next a tRNA carrying the amino acid methionine becomes attached to site P.

CFE Higher BiologyDNA and

the Genome

3. The mRNA codon at site A bonds complementary anticodon on the appropriate tRNA bearing the correct amino acid.

4. A peptide bond then forms between these two amino acids.

5. The ribosome then moves along one codon.

6. The tRNA from Site P is move to Site E and released.

7. Steps 3-6 then repeat until it reaches a stop codon.