Genuttrykk og transkripsjon

• Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product
• These products are often proteins, but can also be functional RNA molecules like tRNA or rRNA
• Gene expression is a tightly regulated process that allows a cell to respond to its changing environment
• Gene expression also allows cells to specialize, with different cell types expressing different sets of genes
• The central dogma of molecular biology describes the flow of genetic information within a biological system
• DNA is transcribed into RNA, which is then translated into protein
• Not all genes are protein-coding genes; some genes encode functional RNA molecules

Transcription

• Transcription is the process of synthesizing RNA from a DNA template
• It is carried out by an enzyme called RNA polymerase
• In eukaryotes, transcription occurs in the nucleus
• The basic steps of transcription are initiation, elongation, and termination

Initiation

• RNA polymerase binds to a specific DNA sequence called the promoter
• The promoter contains specific sequences that help RNA polymerase bind
• In eukaryotes, transcription factors mediate the binding of RNA polymerase to the promoter, thus initiating transcription

Elongation

• RNA polymerase moves along the DNA template, unwinding the DNA double helix
• RNA polymerase adds nucleotides to the 3' end of the growing RNA molecule
• The sequence of the RNA molecule is complementary to the DNA template strand

Termination

• Transcription continues until RNA polymerase reaches a terminator sequence
• In eukaryotes, the RNA molecule is released from the DNA template and undergoes processing, specifically a 5' cap and a 3' poly-A tail are added

RNA Processing in Eukaryotes

• Eukaryotic pre-mRNA molecules are processed in the nucleus to produce mature mRNA molecules
• RNA processing includes:
  • Addition of a 5' cap
  • Addition of a 3' poly-A tail
  • RNA splicing

5' Cap and 3' Poly-A Tail

• The 5' cap is a modified guanine nucleotide added to the 5' end of the pre-mRNA molecule
• The 5' cap protects the mRNA from degradation and helps it bind to ribosomes
• The 3' poly-A tail is a string of adenine nucleotides added to the 3' end of the pre-mRNA molecule
• The 3' poly-A tail also protects the mRNA from degradation and helps it bind to ribosomes

RNA Splicing

• RNA splicing is the process of removing introns from the pre-mRNA molecule and joining together the exons
• Introns are non-coding regions of the pre-mRNA molecule
• Exons are the coding regions of the pre-mRNA molecule
• RNA splicing is carried out by a complex called the spliceosome which is composed of proteins and small RNAs

Translation

• Translation is the process of synthesizing a polypeptide from an mRNA template
• Translation occurs on ribosomes
• In eukaryotes, ribosomes are found in the cytoplasm and the rough endoplasmic reticulum but translation can also occur in mitochondria and chloroplasts

Transfer RNA (tRNA)

• Transfer RNA (tRNA) molecules carry amino acids to the ribosome
• Each tRNA molecule has an anticodon that is complementary to a specific codon on the mRNA molecule
• Each tRNA molecule carries the amino acid specified by the codon to which it binds

Ribosomes

• Ribosomes are composed of two subunits: a large subunit and a small subunit
• Ribosomes have three binding sites for tRNA: the A site, the P site, and the E site
• The A site binds to the tRNA carrying the next amino acid to be added to the polypeptide chain
• The P site binds to the tRNA carrying the growing polypeptide chain
• The E site is the exit site, where tRNA molecules leave the ribosome

Translation - Initiation, Elongation, and Termination

• The basic steps of translation are initiation, elongation, and termination

Initiation

• The small ribosomal subunit binds to the mRNA molecule
• A tRNA molecule carrying methionine binds to the start codon (AUG) on the mRNA molecule
• The large ribosomal subunit binds to the small ribosomal subunit, forming the translation initiation complex

Elongation

• A tRNA molecule carrying the next amino acid binds to the A site on the ribosome
• A peptide bond forms between the amino acid in the A site and the growing polypeptide chain in the P site
• The ribosome moves down the mRNA molecule, shifting the tRNA in the A site to the P site and the tRNA in the P site to the E site
• The tRNA in the E site leaves the ribosome
• These steps are repeated until the ribosome reaches a stop codon

Termination

• When the ribosome reaches a stop codon (UAA, UAG, or UGA), a release factor binds to the A site
• The release factor causes the polypeptide chain to be released from the tRNA in the P site
• The ribosome dissociates into its large and small subunits

Protein Folding

• Protein folding is the physical process by which a polypeptide chain folds into its biologically active three-dimensional structure
• The amino acid sequence of a protein determines its three-dimensional structure
• Proteins fold spontaneously, but they are often assisted by chaperone proteins
• Chaperone proteins help proteins fold correctly and prevent them from aggregating

Eukaryotic vs. Prokaryotic Gene Expression

• Eukaryotic and prokaryotic gene expression differ in several ways
• In prokaryotes, transcription and translation occur in the cytoplasm
• In eukaryotes, transcription occurs in the nucleus and translation occurs in the cytoplasm
• Eukaryotic mRNA molecules are processed before translation, while prokaryotic mRNA molecules are not
• Eukaryotic genes contain introns, while prokaryotic genes do not
• Eukaryotic mRNA molecules have a 5' cap and a 3' poly-A tail, while prokaryotic mRNA molecules do not

Regulation of Gene Expression

• Gene expression is regulated at multiple levels
• Transcriptional control regulates the amount of RNA produced
• Post-transcriptional control regulates the processing and stability of RNA molecules
• Translational control regulates the amount of protein produced
• Post-translational control regulates the activity of proteins

Transcriptional Control

• Transcriptional control is the most common way to regulate gene expression
• Transcriptional control is mediated by transcription factors
• Transcription factors bind to specific DNA sequences and either activate or repress transcription
• Activators are transcription factors that increase the rate of transcription
• Repressors are transcription factors that decrease the rate of transcription

Post-Transcriptional Control

• Post-transcriptional control regulates the processing and stability of RNA molecules
• RNA processing includes:
  • Addition of a 5' cap
  • Addition of a 3' poly-A tail
  • RNA splicing
• RNA stability is affected by:
  • The length of the poly-A tail
  • The presence of specific sequences in the mRNA molecule
  • RNA-binding proteins

Translational Control

• Translational control regulates the amount of protein produced
• Translational control is mediated by:
  • The availability of ribosomes
  • The presence of specific sequences in the mRNA molecule
  • RNA-binding proteins
  • MicroRNAs (miRNAs)

Post-Translational Control

• Post-translational control regulates the activity of proteins
• Post-translational control is mediated by:
  • Chemical modifications of the protein (e.g., phosphorylation, glycosylation)
  • Protein folding
  • Protein degradation

Mutations

• A mutation is a change in the nucleotide sequence of DNA
• Mutations can be spontaneous or induced by mutagens
• Mutagens are physical or chemical agents that cause mutations

Types of Mutations

• Point mutations are changes in a single nucleotide base
  • Base-pair substitutions: replace one nucleotide base with another
  • Insertions: add one or more nucleotide bases to a DNA sequence
  • Deletions: remove one or more nucleotide bases from a DNA sequence
• Frameshift mutations are insertions or deletions that alter the reading frame of the mRNA
• Chromosomal mutations are changes in the structure or number of chromosomes
  • Deletions: remove a portion of a chromosome
  • Duplications: repeat a portion of a chromosome
  • Inversions: reverse the order of a portion of a chromosome
  • Translocations: move a portion of one chromosome to another chromosome

Effects of Mutations

• Mutations can have a variety of effects on gene expression and protein function with outcomes typically classified as harmful, neutral or beneficial
• Some mutations have no effect on gene expression or protein function (silent mutations) due to redundancy in the genetic code
• Other mutations can alter gene expression or protein function in a way that is harmful to the organism
• Rarely, mutations improve gene expression or protein function in a way that is beneficial to the organism

Causes of Mutations

• Mutations can be caused by a variety of factors, including:
  • Errors in DNA replication
  • Exposure to mutagens
  • Spontaneous mutations