Gene Expression Overview

Gene Expression Overview

• Gene expression is the process by which DNA directs the synthesis of proteins (or RNA in some cases).
• Both prokaryotes and eukaryotes alter gene expression in response to their environments.
• In multicellular eukaryotes, gene expression regulates development. This is because it controls differences in cell types.
• RNA molecules play important roles in gene expression.

Stages of Gene Expression

• Gene expression includes two main stages: transcription and translation.
• Transcription is the synthesis of an RNA copy of a DNA segment.
• Translation is the conversion of the RNA sequence into a polypeptide (protein) sequence.
• The transcription process has three main steps: initiation, elongation, and termination.

Archibald Garrod

• Archibald Garrod (1902) suggested genes dictate phenotypes through enzymes. This is because these enzymes catalyse specific chemical reactions in a cell.
• His suggested "one gene-one enzyme" hypothesis, is that the symptoms of an inherited disease reflect a person's inability to make a particular enzyme.

Basic Principles of Transcription

• DNA makes RNA in a process called transcription.
• DNA code is transcribed into a sequence of codons in messenger RNA (mRNA).
• The base pairing rules are followed: A with U, and C with G.
• Uracil replaces thymine in RNA as there is no thymine in RNA.

Messenger RNA (mRNA)

• Pre-mRNA molecules are broken down after transcription.
• mRNA is made from a DNA template during a process called transcription.
• mRNA carries the DNA code to ribosomes where the message is translated.
• mRNA sequence is broken down after conveying the message.
• For example, if the DNA sequence is AAA TAA CCG GAC, then the complementary mRNA codon sequence is UUU AUU GGC CUG.

Translation

• The process by which mRNA information is used to synthesize a polypeptide is called translation.
• Translation, where there is a change in language from nucleotides to amino acids. The cell translates the nucleotide sequence of mRNA into the amino acid sequence of a polypeptide sequence.
• The sites of translation are ribosomes, molecular complexes that facilitate the orderly linking of amino acids into the polypeptide chain.

Overview of Transcription in Bacteria and Eukaryotes

• In bacterial cells, mRNA produced by transcription is immediately translated without further processing.
• In eukaryotic cells, the nucleus provides a separate compartment for transcription. The original RNA transcript (pre-mRNA) is processed before leaving the nucleus as mRNA.

The Genetic Code

• A sequence of three DNA or RNA nucleotides corresponds to a specific amino acid or a stop signal during protein synthesis.
• mRNA nucleotide triplets (codons) are written in the 5' to 3' direction.

The Codon Table

• The three nucleotide bases of an mRNA codon are designated the first, second, and third bases, reading in the 5' towards 3' direction along the mRNA.
• AUG codon marks the start signal for ribosomes to begin translating mRNA.
• Three codons act as stop signals, marking the endpoints of translation.

Split Genes and RNA Splicing

• Some genes have introns (intervening sequences) that are removed and exons (expressed sequences/coding regions) pieced together to form final mRNA transcript.
• Introns are noncoding regions between coding regions in a gene.
• Exons are the expressed coding regions of a gene.
• RNA splicing removes introns and joins exons, forming functional RNA.

Transfer RNA (tRNA)

• tRNA is a clover-leaf-shaped molecule that carries specific amino acid molecules to the ribosome to help form a polypeptide, during the process of translation.
• It's responsible for transferring amino acids from the cytoplasm to the ribosome.

Ribosomal RNA (rRNA)

• rRNA is a structural molecule that forms part of a ribosome. Ribosomes are made of a large and small subunit
• each subunit is comprised of proteins and one or more rRNA.

Translation Initiation

• A small ribosomal subunit binds to mRNA.
• An initiator tRNA binds to the start codon (AUG).
• A large ribosomal subunit joins to form the translation initiation complex.

Translation Elongation

• Anticodons of incoming aminoacyl tRNAs base-pair with mRNA codons in the A site.
• Peptide bond formation between the amino acids in the A site and the polypeptide chain in the P site.
• The ribosome moves along the mRNA, bringing the next codon to the A site.

Translation Termination

• Release factors bind to stop codons (UAG, UAA, or UGA) in the A site.
• No amino acids are added.
• Peptide chain is released.
• Ribosome subunits dissociate.

mRNA End Alterations

• Modified guanine nucleotide is added to the 5' end (5' cap), and a poly-A tail is added to the 3' end.
• The 5' cap protects mRNA from degradation and facilitates ribosome binding.
• The poly-A tail protects mRNA from degradation and helps with export from the nucleus.

Transcription Summary

• RNA polymerase attaches to a gene's promoter region, unwinding the DNA.
• RNA polymerase adds RNA nucleotides complementary to the DNA template strand.
• Elongation happens until the DNA sequence ends—the terminator sequence is reached.
• RNA polymerase detaches and the new RNA separates from the DNA, allowing the DNA to reform into its double helix structure.