From DNA to Proteins

Chapter 15

Functions of DNA

Heredity: passing on traits from parents to offspringReplication

Coding for our traits by containing the information to make proteinsProtein Synthesis

Transcription Translation

Genes

Genes are units of DNA that code to make a single polypeptide (protein)

Found within specific location on the chromosomes (loci)

Humans have >30,000 genes How do we make a protein from the

information in a gene?

Same two steps produce all proteins:1) Transcription:

DNA (Gene) is transcribed to form messenger RNA (mRNA)

Occurs in the nucleus

2) Translation: mRNA is translated to form polypeptide

chains, which fold to form proteins Occurs in ribosomes which are in the

cytoplasm

Steps of Protein synthesis

Transcription and Translation

RNA vs. DNADNA RNA

Number of strands

Two One

Nucleotides A T G C A U G C

Sugar Deoxyribose Ribose

Location Nucleus only Nucleus and Cytoplasm

Three Classes of RNAs

Messenger RNA (mRNA)Carries protein-building instruction

Ribosomal RNA (rRNA)Major component of ribosomes

Transfer RNA (tRNA)Delivers amino acids to ribosomes

A Nucleotide Subunit of RNA

phosphate group

sugar (ribose)

uracil (base)

Figure 14.2Page 228

Transcription

DNA RNA Occurs in the nucleus Requires the enzyme RNA Polymerase Consists of 3 steps:

Initiation Elongation Termination

RNA Polymerases

No primers needed to start complementary copy

RNA is made in the 5´→ 3´ directionDNA template strand is 3´→ 5´

Steps of Transcription: Initiation

RNA Polymerase binds to PromoterPromoter: A base sequence in the DNA that

signals the start of a gene DNA is unwound

i.e. hydrogen bonds are broken

Transcription: Initiation

Steps of Transctription: Elongation

RNA ploymerase adds complementary RNA nucleotides to one strand of DNA – Template strand

Forms Pre-mRNA

Transcription: Elongation

Steps of Transcription: Termination

When mRNA synthesis is complete, RNA Polymerase falls off of DNA, RNA is released from DNA, and DNA rewinds

Transcription: Termination

Transcription vs. DNA Replication

Like DNA replicationNucleotides added in 5’ to 3’ direction

Unlike DNA replicationOnly small stretch is templateRNA polymerase catalyzes nucleotide

additionProduct is a single strand of RNA

Production of mRNAs in Eukaryotes Eukaryotic protein-coding genes are

transcribed into precursor-mRNAs that are modified in the nucleus

Introns are removed during pre-mRNA processing to produce the translatable mRNA

Introns contribute to protein variability

Messenger RNA

ProkaryotesCoding region flanked by 5´ and 3´

untranslated regions

EukaryotesCoding region flanked by 5´ and 3´

untranslated regions (as in prokaryotes)Additional noncoding elements

Eukaryotic Pre-mRNA Precursor-mRNA (pre-mRNA)

Must be processed in nucleus to produce translatable mRNA

5´ capReversed guanine-containing nucleotideSite where ribosome attaches to mRNA

Poly(A) tail50 to 250 adenine nucleotides added to 3´ endProtects mRNA from RNA-digesting enzymes

Eukaryotic Pre-mRNA

IntronsNon-protein-coding sequences in the pre-mRNAMust be removed before translation

ExonsAmino acid coding sequences in pre-mRNAJoined together sequentially in final mRNA

RNA Processing

mRNA Splicing

Introns in pre-mRNAs removed Spliceosome

Pre-mRNASmall ribonucleoprotein particles (snRNP)

Small nuclear RNA (snRNA) + several proteins Bind to introns Loop introns out of the pre-mRNA, Clip the intron at each exon boundary Join adjacent exons together

mRNA Splicing

Why are Introns Present?

Alternative splicingDifferent versions of mRNA can be produced

Exon shufflingGenerates new proteins

Alternative Splicing

Exons joined in different combinations to produce different mRNAs from the same gene

Different mRNA versions translated into different proteins with different functions

More information can be stored in the DNA

Alternative mRNA Splicing α-tropomyosin in smooth and striated muscle

The next step: Translation

“Translating” from nucleic acid (DNA/RNA) “language” (nucleotides) to protein “language” (amino acids)

Occurs in the ribosome within the cytoplasm Requires tRNA – transfer RNA How does the mRNA (and DNA) code for

proteins?

The Genetic Code

Genetic Code

Information4 nucleotide bases in DNA or RNA sequences

DNA: A,T,G,C RNA: A,U,G,C

20 different amino acids in polypeptides

CodeOne-letter words: only 4 combinationsTwo-letter words: only 16 combinationsThree-letter words: 64 combinations

Genetic Code

DNAThree-letter code: triplet

RNAThree-letter code: codon

Genetic Code

Features of the Genetic Code

Sense codons61 codons specify amino acidsMost amino acids specified by several codons

(degeneracy or redundancy)Ex: CCU, CCC, CCA, CCG all specify proline

Start codon or initiator codonFirst amino acid recognized during translationSpecifies amino acid methionine

Features of the Genetic Code

Stop codons or termination codonsEnd of a polypeptide-encoding mRNA sequenceUAA, UAG, UGA

CommalessNucleic acid codes are sequentialNo commas or spaces between codonsStart codon AUG establishes the reading frame

The Genetic Code

Genetic Code is Universal

Same codons specify the same amino acids in all living organisms and virusesOnly a few minor exceptions

Genetic code was established very early in the evolution of life and has remained unchanged

Translation Overview

Translation

Purpose To “translate” from nucleic acid “language”

to protein “language” RNAprotein

What is needed for translation? mRNA transcript (processed) tRNAs Ribosomes

tRNAs

Transfer RNAs (tRNA) Bring specific amino acids to ribosomeCloverleaf shape

Bottom end of tRNA contains anticodon sequence that pairs with codon in mRNAs

tRNA Structure

Ribosomes Made of ribosomal RNA (rRNA) and proteins

Two subunits: large and small

Translation Stages Initiation

Ribosome assembled with mRNA molecule and initiator methionine-tRNA

ElongationAmino acids linked to tRNAs added one at a time to

growing polypeptide chain Termination

New polypeptide released from ribosomeRibosomal subunits separate from mRNA

Initiation Initiator tRNA (Met-tRNA) binds to small subunit

Initiation Complex binds to 5´ cap of mRNA, scans

along mRNA to find AUG start codon

Initiation Large ribosomal subunit binds to complete

initiation

Elongation

tRNA matching the next codon enters A site carrying its amino acid

A peptide bond forms between the first and second amino acids, which breaks the bond between the first amino acid and its tRNA

Ribosome moves along mRNA to next codon Empty tRNA moves from P site to E site, then released Newly formed peptidyl-tRNA moves from A site to P

site A site empty again

Elongation

Termination

Begins when A site reaches stop codon Release factor (RF) or termination factor binds to A site Polypeptide chain released from P site Remaining parts of complex separated

Termination

What Happens to the New Polypeptides?

Some just enter the cytoplasm

Many enter the endoplasmic reticulum and move through the cytomembrane system where they are modified

Gene ExpressionSummary:

Transcription

Translation

mRNA rRNA tRNA

Mature mRNA transcripts

ribosomal subunits

mature tRNA

Gene Mutations

Changes in genetic material

Base-pair mutations change DNA triplet Results in change in mRNA codonMay lead to changes in the amino acid

sequence of the encoded polypeptide

Gene Mutation Types

Missense mutation Nonsense mutation Silent mutation Frameshift mutation

Missense Mutation Changes one sense codon to one that

specifies a different amino acid

Sickle-Cell Anemia

Caused by a single missense mutation

Nonsense Mutation Changes a sense codon to a stop codon

Silent Mutation Changes one sense codon to another

sense codon that specifies the same amino acid

Frameshift Mutation Base-pair insertion or deletion alters the

reading frame after the point of the mutation

Mutation Rates

Each gene has a characteristic mutation rate

Average rate for eukaryotes is between 10-4 and 10-6 per gene per generation

Only mutations that arise in germ cells can be passed on to next generation

Mutagens

Ionizing radiation (X rays)

Nonionizing radiation (UV)

Natural and synthetic chemicals