Chapter 17 - Transcription and Translation

Gene expression

The process by which information in DNA is used to produce a functional product, usually a protein

DNA vs RNA

DNA contains deoxyribose and thymine and is double-stranded, while RNA contains ribose and uracil and is single-stranded

mRNA (messenger RNA)

Carries genetic instructions from DNA to the ribosome

tRNA (transfer RNA)

Brings amino acids to the ribosome during translation

rRNA (ribosomal RNA)

Forms the core of ribosome structure and catalyzes protein synthesis.

Transcription

The process of copying a gene’s DNA sequence into mRNA.

Translation

The process of converting mRNA sequence into a chain of amino acids (protein).

Central dogma

DNA → RNA → Protein

Where transcription occurs

In the nucleus (eukaryotes)

Where translation occurs

At ribosomes in the cytoplasm

RNA polymerase

Enzyme that synthesizes RNA from a DNA template

Promoter

DNA sequence where RNA polymerase binds to start transcription

Terminator

DNA sequence that signals the end of transcription

Nontemplate (coding) strand

DNA strand with the same sequence as mRNA (except T → U)

Template strand

DNA strand used by RNA polymerase to build mRNA

mRNA processing

Modification of pre-mRNA including 5’ cap, poly-A tail, and splicing

5’ cap

Modified guanine that’s added to the beginning of an mRNA for protection and ribosome binding

Poly-A tail

Chain of adenines added to the end of mRNA for stability

Introns

Noncoding regions removed from pre-mRNA

Exons

Coding regions that remain in mature mRNA

RNA splicing

Removal of introns and joining of exons

Alternative splicing

Different combinations of exons produce multiple proteins from one gene

Genetic code

Set of rules by which mRNA codons are translated into amino acids

Codon

Three-nucleotide sequence on mRNA that codes for an amino acid

Start codon

AUG, codes for methionine and signals start of translation

Stop codons

UAA, UAG, UGA signal the end of translation

Reading frame

Grouping of nucleotides into codons during translation

Ribosome

Structure that reads mRNA and assembles amino acids into a protein

A site

Ribosome site where incoming tRNA binds

P site

Holds tRNA with growing polypeptide chain

E site

Exit site where empty tRNA leaves

Anticodon

Three-nucleotide sequence on tRNA complementary to an mRNA codon

Steps of translation

Initiation, elongation, termination

Initiation (translation)

Ribosome assembles at start codon with initiator tRNA

Elongation (translation)

Amino acids are added to the growing chain

Termination (translation)

Ribosome reaches stop codon and releases protein

Mutation

A change in the DNA sequence

Point mutation

A change in a single nucleotide

Substitution mutation

One nucleotide is replaced by another

Silent mutation

Nucleotide change does not change the amino acid

Missense mutation

Nucleotide switch changes one amino acid

Nonsense mutation

Nucleotide change creates a stop codon prematurely

Insertion mutation

Addition of one or more nucleotides

Deletion mutation

Removal of one or more nucleotides

Frameshift mutation

Insertion or deletion that shifts the reading frame

Effect of frameshift mutations

Alters all downstream amino acids and often produces nonfunctional proteins

Gene regulation

Control of when, where, and how much a gene is expressed

Why gene regulation is important

Allows cells to specialize and respond to environmental changes