Chapter 17From Gene to Protein

Gene Expression

•DNA leads to specific traits by synthesizing proteins

•Gene expression – the process by which DNA directs protein synthesis

•Two stages:▫Transcription▫Translation

One Gene – One Polypeptide

•The fundamental relationship between proteins and genes

•The one gene – one polypeptide hypothesis states that each gene dictates production of a specific polypeptide

•Many proteins are composed of multiple polypeptides

Transcription and Translation• RNA (ribonucleic acid) is the intermediate

between genes and proteins

• Transcription – synthesizes RNA under the instruction of DNA▫ mRNA (messenger RNA) is produced

• Translation – synthesis of a polypeptide▫ Occurs at ribosomes under the direction of mRNA

Prokaryotes and Eukaryotes• Transcription and translation occur in both

prokaryotic and eukaryotic cells

In prokaryotes:• mRNA produced by transcription is immediately translated• Bacteria can simultaneously transcribe and translate the

same gene.

In eukaryotes:• Nuclear envelope separates transcription and translation• mRNA goes through RNA processing

TRANSCRIPTION

TRANSLATION

DNA

mRNARibosome

Polypeptide

Nuclearenvelope

TRANSCRIPTION

RNA PROCESSINGPre-mRNA

DNA

mRNA

TRANSLATION Ribosome

Polypeptide

ProkaryoteBacterial Cell

Eukaryotic Cell

Triplet Code• Triplet code – the genetic instructions for a

polypeptide chain are written in the DNA as a series of non-overlapping, three-nucleotide “words”

• Each triplet codes for an amino acid

Triplet Code

Transcription:•The template strand of DNA gives the

sequence of nucleotides in an RNA transcript

Translation:•mRNA triplets (codons) are read in the 5’

to 3’ direction•1 codon = 1 amino acid

DNAmolecule

Gene 1

Gene 2

Gene 3

DNAtemplatestrand

TRANSCRIPTION

TRANSLATION

mRNA

Protein

Codon

Amino acid sequence

Codons and Amino Acids

•Multiple codons can specify one amino acid

•One codon cannot specify multiple amino acids

•One start codon: AUG (specifies methianine)

•Three stop codons: UAA, UAG and UGA

Reading Frame

•Codons must be reading in the correct reading frame, which means:▫5’ to 3’ direction▫Codons must be read as groups of three▫Codons do not overlap

Genetic Code

•The genetic code is nearly universal•It is shared by simple bacteria to complex

animals

•Genes can be transplanted from one species to another

Plant with firefly gene

Pig with jellyfish gene

Transcription

•The enzyme RNA polymerase unzips the two DNA stands

•RNA synthesis base-pairing is similar to DNA synthesis, except uracil substitutes thymine

Transcription

•Promoter – the DNA sequence where RNA polymerase attaches

•Terminator (in bacteria) – sequence that signals the end of transcription

•Transcription unit - The transcribed DNA section

Transcription

Three Stages• Initiation – RNA polymerase binds to DNA•Elongation – RNA polymerase attaches

RNA nucleotides, creating mRNA•Termination – transcription stops

Transcription

•Promoters – signal beginning of RNA synthesis

•Transcription factors (in eukaryotes) – proteins that mediate the binding of RNA polymerase and initiate transcription

•Transcription initiation complex – the complex of transcription factors and RNA polymerase bound to a promoter

TATA Box

•In eukaryotes, a promoter commonly includes a nucleotide sequence containing TATA

•The TATA box is located about 25 nucleotides upstream from the start point

•Transcription factors bind to the DNA first, followed by RNA polymerase and more transcription factors

RNA Modification• In eukaryotes, mRNA is modified after transcription (still

in nucleus)• Both ends are altered

▫ 3’ end gets a poly-A tail▫ 5’ end gets a guanine cap

• These modifications▫ Facilitate mRNA export▫ Protects from hydrolytic enzymes▫ Help ribosomes attach to 5’ end

RNA Splicing

•Introns – noncoding stretches of mRNA •Exons – regions that are coding, they are

expressed•RNA splicing – removes introns and

connects exons▫Carried out by spliceosomes▫snRNPs (small nuclear ribonucleoproteins)

recognize splice sites

Ribozymes• Ribozymes – RNA molecules that function as

enzymes and can splice RNA• Properties that allow RNA to function as an

enzyme:▫RNA can form a three dimensional structure

because it can pair with itself▫Some bases in RNA contain functional groups▫RNA may hydrogen bond with other nucleic

acid molecules

Alternative RNA Splicing

•The segments that are treated as exons in RNA splicing determine the polypeptide

•Variations are called alternative RNA splicing

•The number of proteins an organism can produce is greater than its number of genes

Translation (outside nucleus)

•tRNA (transfer RNA) – translates mRNA message into protein

•Each tRNA molecule carries an amino acid at one end

•There is an anticodon at the other end, which base pairs with mRNA codons

tRNA

Ribosomes

•Ribosomes – facilitate coupling of tRNA anticodons with mRNA codons

•The large and small ribosomal subunits are made of proteins and rRNA (ribosomal RNA)

RibosomesThree bonding sites for tRNA• P site – holds the tRNA that carries the growing

polypeptide chain• A site – holds the tRNA with the next amino acid to be

added to the chain• E site – Exit site, tRNA leaves ribosome

Three Stages of Translation

•Initiation – brings together tRNA, mRNA, small and large subunit

•Elongation – amino acids are added with peptide bonds

•Termination – a stop codon cause the polypeptide to be released

Termination

•When a stop codon is detected, the A site accepts a release factor protein

•A water molecule is added instead of an amino acid

•The polypeptide is released and the translation assembly comes apart

Polyribosomes

•Polyribosome – multiple ribosomes translating a single mRNA simultaneously

•Polyribosomes produce polypeptides quickly

Secretion

•Polypeptides destined for the endomembrane system or secretion are marked with a signal peptide

•A signal recognition particle (SRP) recognizes the signal as it emerges from the ribosome

Point Mutations• Point mutations – occur in just one base pair of

a gene▫Can lead to abnormal protein production

Substitutions• Base-pair substitution – the replacement of one

nucleotide and its partner with another• Missense mutations – change one amino acid to another• Nonsene mutations – a codon for an amino acid is

changed into a stop codon• Silent mutations – no effect on the amino acid produced

by a codon

Insertions and Deletions

•Insertion – addition of nucleotide pairs in a gene

•Deletion – loss of nucleotide pairs in a gene

•Can alter the reading frame, causing a frameshift mutation

•These have disastrous effects on protein more often than substitutions

Mutagens

•Mutagens – physical or chemical environmental agents that cause mutation

•Spontaneous mutation can occur during DNA replication, recombination, or repair

TRANSCRIPTION

RNA PROCESSING

DNA

RNAtranscript

3

5RNApolymerase

Poly-

A

Poly-A

RNA transcript(pre-mRNA)

Intron

Exon

NUCLEUS

Aminoacyl-tRNAsynthetase

AMINO ACID ACTIVATIONAminoacid

tRNACYTOPLASM

Poly-A

Growingpolypeptide

3

Activatedamino acid

mRNA

TRANSLATION

Cap

Ribosomalsubunits

Cap

5

E

PA

AAnticodon

Ribosome

Codon

E

Questions

1. Compare and contrast transcription and translation in eukaryotes and prokaryotes.

2. Finish the chart:

1. Eukaryotes Prokaryotes

Separate transcription and translation

mRNA proceeds directly to translation

mRNA modification occurs No mRNA modification

Nuclear envelope divides transcription and translation

processes

A gene can be transcribed and translated simultaneously

2.

Carries amino acid information from DNA to ribosomes

Translates mRNA codons into amino acids in protein synthesis

Ribosomal RNA (rRNA)