3.4.2 DNA and protein synthesis

3.4.2 DNA and protein synthesis 

The genome is the complete set of genes in a cell and the proteome is the full range of proteins that a cell is able to produce. 

We need to be able to recall and talk about the structure of mRNA and tRNA.

RNA overall:

• pentose sugar (ribose)
• A, U, C, G
• a phosphate group
• a polymer made up of repeating mono nucleotide sub-units
• single stranded

mRNA:

• small enough to leave the nucleus through clear pores to enter the cytoplasm
• it contains coded information to determine the sequence of amino acids in the proteins which are synthesised
• a codon is a sequence of three bases on mRNA that codes for a single amino acid
• long strand arranged in a single helix (1,000s of nucleotides)
• base sequence determined by the base sequence of a length of DNA (transcription)
• associated with ribosomes in the cytoplasm
• acts as a template for protein synthesis

tRNA:

• relatively small (around 80 nucleotides)
• single stranded chain folded into a clover-leaf shape with one end of the chain extending beyond the other
• an amino acid can easily bind to this extending part (there are many different types of tRNA, each is specific to a different amino acid)
• on the other end is an anticodon (sequence of three bases) this binds with the codon during protein synthesis (the codon and anticodon are complimentary).

Transcription

This is the process of making pre-mRNA from DNA (the DNA acts as a template):

• an enzyme acts on a specific region of the DNA causing the two strands to separate exposing the nucleotide bases in that region
• the nucleotide bases on one of the two DNA strands (this is the template strand) pair with complimentary free nucleotides by complimentary base pairing
• RNA polymerase moves along the strand joining the nucleotides together forming a pre-mRNA molecule
• the DNA strand rejoins behind the pre-mRNA strand
• when the RNA polymerase reaches a particular sequence of bases (a stop triplet code) it detaches and the production of pre-mRNA is complete

Splicing

In prokaryotes transcription results directly in the production of mRNA from DNA. In eukaryotic cells transcription results in pre-mRNA which must be spliced to form mRNA. The DNA of a gene of eukaryotic cells is made up of sections known as exons (coding) and introns (non-coding). Introns prevent polypeptide synthesis, so we must remove these and join the exons together. This process is known as splicing. The mRNA molecule now leaves the nucleus via a nuclear pore.

Translation

• A ribosome becomes attached to the start codon at one end of the mRNA molecule
• the tRNA molecule with the complementary anticodon moves to the ribosome and pairs up with the codon on the mRNA. This tRNA carries a specific amino acid
• A tRNA molecule with a complementary anticodon binds to the next codon
• the ribosome moves along the mRNA bringing together the two tRNA molecules
• the two amino acids are joined by a peptide bond using ATP which is hydrolysed to provide the energy required
• the ribosome moves onto the next codon and the first tRNA is released from its amino acid (simultaneously)
• this tRNA is now free to collect another amino acid
• this continues until the ribosome reaches a stop codon
• here the ribosome, mRNA, and last tRNA molecule all separate and the polypeptide chain is complete