Gene Structure & Expression

Define Genes

Regions of DNA that may be translated into a polypeptide or functional RNA molecule

What is the term when Information is stored as 3 letter codes?

Triplet in DNA and Codon in RNA (codes for amino acids, which make proteins)

What are the characteristics of Codons?

Universal and Degenerate

What do they mean when Codons are ‘Universal’?

Almost every cell follows the same rules (translate codons to the same amino acids)

What do they mean when Codons are ‘Degenerate’?

More than one codon can code for the same amino acid

What is found before the start codon?

The Promoter Region

What does the The Promoter Region do?

It is where transcription begins and when the RNA polymerase attaches and it Identifies the DNA being transcribed

What is the purpose of RNA Polymerase?

Transcribes DNA to RNA

Define Exons

DNA regions that are the coding segments. Make up mRNA to form the protein

Define Introns

DNA regions that are non-coding segments. “Spliced” out during transcription

What are the 3 stages of gene expression?

1. Transcription

2. RNA processing

3. Translation

Why are Gene Expression ‘Highly regulated’?

So that proteins are produced only when required and to also conserves energy

Describe the Initiation stage in Transcription

Describe the Initiation stage in Transcription

• Proteins combine with promoter region

• TATA box recognised by RNA polymerase

• RNA polymerase attaches to promoter, unzipping the DNA to expose the bases

Describe the Elongation stage in Transcription

• RNA polymerase moves along the DNA, producing a strand of pre-mRNA

• Complementary base pairing adds nucleotides from 5’ to 3’

Describe the Termination stage in Transcription

• The RNA polymerase reaches the end sequence: poly-A tail

• RNA polymerase detaches

What is involved in RNA Processing?

An addition of methyl cap (added to the 5’ end of the pre-mRNA) and poly-A tail (Added at the 3’ end)

What is the purpose of the methyl cap and Poly-A Tail being added to the pre-mRNA?

To increases stability, prevents degradation and promotes translation

Define Splicing

When Introns are removed / cut out, and exons are spliced together

What is the purpose of Splicing?

To turn the pre-mRNA into mature mRNA

Define Spliceosome

Enzyme that removes introns and combines exons

What is the process of Translation and where?

Process of turning mRNA into a polypeptide (protein)

Occurs in the cytoplasm on ribosomes

Involves mRNA and tRNA

What happens in the Initiation in Translation

• Ribosomal subunit attaches to 5’ end of mRNA strand

• Moves down the strand until it reaches the START (AUG) codon

• tRNA molecule with complementary anticodon (UAC) binds to start codon

What happens in the Elongation in Translation

• tRNA carries amino acids

• Specific amino acid for codon is bonded via peptide bond

• Ribosome moves along mRNA, allowing more tRNAs to add amino acids

What happens in the Termination in Translation

• STOP codon reached, mRNA released from ribosome

• Polypeptide released into cytoplasm / rough ER

• Further modifications to the polypeptide chain can be made to make it a functioning protein (e.g. chains joining together)