Higher Biology | Key Area 1.3: Gene Expression

Gene expression

The process by which specific genes are switched on to produce a required protein, involving the transcription and translation of DNA sequences. Only a fraction of these genes are expressed since cells are specialised.

Ribonucleic acid (RNA)

- A type of nucleic acid involved in the stages of gene expression.

- RNA is a single-stranded molecule; DNA is a double-stranded molecule.

- RNA is comprised of nucleotides of ribose sugar, phosphate group and one of four nitrogenous bases.

- RNA uses the base uracil instead of thymine, which still pairs with adenine.

There are three types of RNA involved in transcription and translation. Name them:

- messengerRNA (mRNA)

- transferRNA (tRNA)

- ribosomalRNA (rRNA)

messengerRNA (mRNA)

- A linear molecule which carries a copy of the DNA code (gene) to be expressed from the nucleus to the ribosome, which is then translated into proteins.

- Contains codons (each triplet of bases) which will individually code for a specific animo acid.

- Synthesised in the nucleus by RNA polymerase. Consists of usually 300 nucleotides and longer, and lasts for minutes-days.

transferRNA (tRNA)

- A molecule which carries its specific amino acid to the ribosome and is involved with the translation of mRNA into a polypeptide at a ribosome. Folds due to complementary base paring, and has hydrogen bonding.

- Each tRNA molecule has an anticodon (exposed triplet of bases to bond with a codon via complementary base pairing) at one end, and an attachment site for a specific amino acid at the other.

ribosomalRNA (rRNA)

A molecule that with protein forms the ribosome.

Transcription

The process of transcribing the base sequence of DNA into messengerRNA in the nucleus to express a gene.

Explain the process of transcription:

- RNA polymerase moves along DNA (5’→3’), unwinding the double helix at a specific gene, and breaks the hydrogen bond between bases.

- As RNA polymerase breaks bonds, it reads one of the DNA strands, and adds complementary RNA nucleotide bases to the exposed 3’ end of DNA (forming hydrogen bonds) to synthesise a primary mRNA transcript.

- The primary mRNA transcript is processed and spliced to become a mature mRNA transcript, then it is ready to leave the nucleus and go to the ribosome.

RNA polymerase

An enzyme which works in a 5’→3’ direction and can only add RNA nucleotides to the 3’ end of the growing strand. Synthesises the primary transcript of mRNA.

What is the difference between primary and mature mRNA transcripts?

- Primary transcripts still have introns (non-coding regions of a gene for a protein).

- Mature transcripts are genes (DNA sequences) that have omitted introns and retained exons.

RNA splicing

- The process of copying and editing out the non-coding regions of a gene for a protein (when it is ready to be expressed) before they can be turned into protein.

- Introns are removed and exons are joined together (order of exons remains unchanged) to create a mature mRNA transcript.

- Different exons result in different mature transcripts and so produce different proteins.

Alternative RNA splicing

When different proteins can be expressed from the same primary transcript (one gene), depending on which exons are retained, ie. different mature transcripts can be produced from the same primary transcript.

Translation

The process of messengerRNA being translated into proteins by ribosomes in the cytoplasm.

Explain the process of translation:

- The mRNA molecule leaves the nucleus and travels through the cytoplasm, where a ribosome (with 2 tRNA molecules enclosed) attaches onto the mRNA.

- Translation begins at a start codon (first codon of an mRNA molecule) and ends at a stop codon (last codon of an mRNA molecule).

- Each tRNA molecule carries its specific amino acid to the ribosome to each exposed codon, where their anticodons bond to the correct codons by complementary base pairing, translating the genetic code into amino acids.

- Peptide bonds form between adjacent amino acids, allowing tRNA molecules to leave the ribosome as the polypeptide is formed.

- Polypeptide synthesis continues until the stop codon is reached, signalling the end of translation.

How can polypeptide chains be modified after translation?

- Cutting and combining polypeptide chains.

- Adding phosphate or carbohydrate groups to the protein.

How do polypeptides form proteins?

Polypeptide chains fold to form the 3-D shape of a protein. These shapes are held together by hydrogen bonding, disulfide bonds, and other interactions between amino acids.

Protein

Molecules made of many amino acids, and are specialised for a specific function. A protein’s function is dependent on its structure, and thus have a wide variety of structures to support these functions.

What determines a gene’s phenotype (physical way in which a gene expresses its appearance?

- The proteins produced as a result of gene expression.

- Environmental factors.

Give some examples of specialised proteins:

- Hormones

- Antibodies and white blood cells

- Receptors

- Enzymes

- Structural support