BIO 100: Topic 10 - Gene Expression

Gene expression

The process of making proteins using instructions in DNA

Main stages of gene expression

Transcription and translation

Transcription

Copying a gene's DNA sequence into RNA

Translation

Converting mRNA into a protein

Location of transcription

Nucleus

Location of translation

Cytoplasm

Gene

A section of DNA that codes for a specific protein

Purpose of gene expression

To make proteins that carry out cellular functions

DNA

Deoxyribonucleic acid; stores genetic instructions

RNA

Ribonucleic acid; helps carry out instructions from DNA

Main difference between DNA and RNA

DNA has deoxyribose, RNA has ribose

Second difference between DNA and RNA

DNA is double-stranded, RNA is single-stranded

Third difference between DNA and RNA

DNA uses thymine, RNA uses uracil

mRNA

Messenger RNA; carries genetic code from DNA to ribosome

tRNA

Transfer RNA; brings amino acids to ribosome

rRNA

Ribosomal RNA; structural part of the ribosome

Function of mRNA

Transmits genetic info from nucleus to ribosome

Function of tRNA

Carries amino acids and matches them to mRNA codons

Function of rRNA

Helps build ribosomes and catalyzes peptide bonds

RNA polymerase

Enzyme that makes RNA using DNA as a template

Role of RNA polymerase

Unwinds DNA and adds RNA nucleotides

Promoter

A DNA sequence where RNA polymerase binds to start transcription

Terminator

A DNA sequence that signals the end of transcription

Template strand

DNA strand used to build the RNA molecule

Coding strand

DNA strand that matches the mRNA (except T/U)

Direction of transcription

Builds RNA from 5' to 3' end

Primary transcript

Initial RNA made before processing

RNA processing

Modifying RNA before it leaves the nucleus

5' cap

A modified G nucleotide added to the 5' end of mRNA

Function of 5' cap

Protects mRNA and helps ribosome binding

Poly-A tail

A chain of adenines added to the 3' end of mRNA

Function of poly-A tail

Protects mRNA and helps export it from nucleus

Introns

Non-coding RNA segments removed before translation

Exons

Coding RNA segments that remain after splicing

Splicing

Removal of introns and joining of exons

Why introns are removed

They do not code for protein

Why exons are kept

They contain the actual protein-coding information

Codon

A sequence of three mRNA bases that codes for an amino acid

Number of possible codons

64 codons

Start codon

AUG; signals where translation begins

AUG codon

Codes for methionine

Stop codons

UAA, UAG, and UGA; signal the end of translation

Reading frame

The way mRNA bases are grouped into codons

Importance of reading frame

Shifting it changes all amino acids downstream

Ribosome

Structure that reads mRNA and builds protein

Small ribosomal subunit

Binds first to mRNA during translation

Large ribosomal subunit

Contains sites for tRNA binding and peptide bonding

A site in ribosome

Holds incoming tRNA carrying amino acid

P site in ribosome

Holds tRNA with growing peptide chain

E site in ribosome

Where tRNA exits the ribosome

Anticodon

Three tRNA bases that pair with a codon on mRNA

Aminoacyl-tRNA

tRNA linked to its correct amino acid

Peptide bond

Link between amino acids during protein synthesis

Elongation step

Where amino acids are added one by one

Termination step

Occurs when ribosome reaches a stop codon

Polypeptide

Chain of amino acids linked by peptide bonds

Protein folding

Process where polypeptide forms its final shape

Gene mutation

A change in the DNA sequence of a gene

Silent mutation

Changes a base but does not change the amino acid

Missense mutation

Changes one amino acid in the protein

Nonsense mutation

Changes a codon into a stop codon

Frameshift mutation

Insertion or deletion that shifts the reading frame

Insertion

Extra base is added to the DNA sequence

Deletion

A base is removed from the DNA sequence

Point mutation

A single base is changed

Effect of frameshift

Changes every codon after the mutation

Causes of mutations

Errors in replication, chemicals, radiation

Gene regulation

Controlling when and how genes are expressed

Why gene regulation matters

Saves energy and prevents harmful protein overproduction

Transcription factors

Proteins that help start or block transcription

Enhancer

DNA sequence that increases transcription when activated

Silencer

DNA sequence that decreases transcription when bound by repressors

Epigenetics

Heritable changes in gene expression without changing DNA sequence

DNA methylation

Adds methyl groups to DNA to silence gene expression

Histone modification

Alters DNA packing to affect gene expression

Alternative splicing

Combines exons in different ways to make multiple proteins

Why alternative splicing matters

Increases protein diversity from one gene

Post-translational modification

Changes made to protein after translation

Example of post-translational modification

Adding phosphate or cutting the protein