From Gene to Protein Chapter 17 - Campbell What do genes code for? proteins All the traits of the body How does DNA code for cells & bodies? how are.

From Gene to Protein

Chapter 17 - Campbell

What do genes code for?

proteins All the traitsof the body

How does DNA code for cells & bodies? how are cells and bodies made from the

instructions in DNA

DNA

The “Central Dogma”• Flow of genetic information in a cell

– How do we move information from DNA to proteins?

transcriptiontranslation

replication

proteinRNADNA trait

RNA• Monomers = nucleotides• Phosphate• Ribose sugar • Nitrogen Bases

– uracil instead of thymine• U bonds with A• C bonds with G

• single stranded

RNADNAtranscription

Compare DNA and RNADNA RNA

Shape Double helix

2 strands

Single strand

Sugar Deoxyribose Ribose

Bases A, T, C, and G A, U, C and G

Location Only in the nucleus

Allowed to travel from nucleus to

cytoplasm

Types of RNA• Ribosomal RNA (rRNA)

– Major component of ribosomes• Transfer RNA (tRNA)

– Folded upon itself – Carries the amino acids to the mRNA

• Messenger RNA (mRNA)– Sequence of nucleotides that determines the primary sequence

of the polypeptide– Made in the nucleus from the DNA: transcription

• snRNA (small-nuclear “snurps”)– Forms the “spliceosomes” which are used to cut out introns

from pre-mRNA• siRNA (small-interfering)

– targets specific mRNA and prohibits it from being expressed

mRNA

Protein Synthesis: From gene to protein

DNAtranscription

nucleus cytoplasm

aa

aa

aaaa

a

aaa

a

a

aa

proteintranslation

ribosome

trait

Which gene is read on the DNA?• Promoter region

– binding site before beginning of gene – Generally referred to as a TATA box because it is a

repeating sequence of T and A– binding site for RNA polymerase

& transcription factors

• Enhancer region– binding site far

upstream of gene• Speeds up process

Transcription Factors– transcription factors bind to promoter region of DNA

• proteins • can be activated by hormones (cell signaling)• turn on or off transcription

– triggers the binding of RNA polymerase to DNA

Transcription: DNA to mRNA• Takes place in the nucleus• A section of DNA is unzipped• RNA polymerase lays down

nucleotides 5’ to 3’ direction.

• The mRNA then leaves the nucleus through the nuclear pores and enters the cytoplasm

Coding strand = this is the protein needed or “sense strand”

Template strand = this is the “anti-sense strand”

Eukaryotic genes have untranscribed regions!

• mRNA must be modified before it leaves the nucleus– exons = the real gene

• expressed / coding DNA– introns = non-coded section

• in-between sequence

• Spliceosomes cut out introns with ribozyme

eukaryotic DNA

exon = coding (expressed) sequence

intron = noncoding (inbetween) sequence

intronscome out!

Alternative splicing• Same piece of DNA can be read many different • Not all the exons may make it to the final product• Intron presence can determine which exons stay or go• Increases efficiency and flexibility of cell• snRNA’s have big role in alternative splicing

Starting to gethard to

define a gene!

A A AA

A3' poly-A tail

mRNA

5'

5' cap

3'

G PPP

50-250 A’s

Final mRNA processing…• Need to protect mRNA on its trip from nucleus

to cytoplasm (enzymes in cytoplasm attack mRNA)

• protect the ends of the molecule• add 5 GTP cap• add poly-A tail

– longer tail, mRNA lasts longer

AAAAAAAAGTP

20-30b

3'

promoter transcriptionstop

transcriptionstart

introns

The Transcriptional unit

transcriptional unit (gene)TAC ACT

DNA

DNATATA5'RNA

polymerase

pre-mRNA

5' 3'

translationstart

translationstop

mature mRNA

5' 3'

exonsenhancer

1000+b

Genetic Code• Genetic code is based on sets of 3 nucleotides …called

CODONS!– Read from the mRNA– 64 different possible combinations exist

• Only 20 amino acids commonly exist in the human body– Some codons code for the same amino acids

(degenerate or redundant)• Sequence of codons determines the sequence of the

polypeptide• Code is “almost” universal…same for all organisms

(evolutionary heritage)

The Code• You don’t need to

memorize the codons (except for AUG)

Start codon AUG methionine

Stop codons UGA, UAA, UAG

AUGCGUGUAAAUGCAUGCGCC

mRNA codes for proteins in triplets

TACGCACATTTACGTACGCGGDNA

mRNA

MetArgValAsnAlaCysAlaprotein

?

codon

How is the code “translated?”

Process of reading mRNA and creating a protein chain from the

code.

Ribosomes: Site of Protein Synthesis

• Facilitate coupling of tRNA anticodon to mRNA codon

• Structure– ribosomal RNA (rRNA) & proteins– 2 subunits

• large• small

E P A

Ribosomes: 3 binding sites

Met

5' UUA C

A G

• A site (aminoacyl-tRNA site) – holds tRNA carrying next amino acid to be

added to chain

• P site (peptidyl-tRNA site) – holds tRNA carrying growing polypeptide chain

• E site (exit site)– Empty tRNA

leaves ribosome from exit site

Transfer RNA• Found in cytoplasm• Carries amino acids to ribosome• Contains an “anticodon” of nitrogen bases• Anticodons use complementary bond with codons• Less tRNA’s than codons, so one tRNA may bind

with more than one codon.• Supports the degenerate code • “Wobble” hypothesis: anticodon with U in third

position can bind to A or G

Translation: mRNA to Protein• In the cytoplasm ribosomes attach to the mRNA

– Ribosome covers 3 codons at a time• Initiation - The tRNA carrying an amino acid comes into

P-site and bonds by base pairing its anti-codon with the mRNA start codon (what is the start codon?)

• Elongation – The second tRNA then comes into A-site and bonds to codon of mRNA– The two amino acids joined with peptide bond

• Termination – ribosome continues reading mRNA until a STOP codon is reached (doesn’t code for anything)

McGraw Hill Animations

Building a polypeptide• Initiation

– mRNA, ribosome subunits, initiator tRNA come together

• Elongation– adding amino acids based on

codons• Termination

– STOP codon = Release factor123

Leu

Leu Leu Leu

tRNA

Met MetMet Met

PE AmRNA5' 5' 5' 5'

3' 3' 3'3'

U UA AAACC

CAU UG G

GUU

A AAAC

CC

AU UG GGU

UA

AAAC

CC

AU UG GGU U

A AACCA U UG G

G AC

ValSer

AlaTrp

releasefactor

AA A

CCU UGG 3'

Good Overview animation

Can you tell the story?

DNA

pre-mRNA

ribosome

tRNA

aminoacids

polypeptide

mature mRNA

5' GTP cap

poly-A taillarge ribosomal subunit

small ribosomal subunit E P A

5'

3'

RNA polymerase

exon introntRNA

Prokaryote vs. Eukaryote Differences

• Prokaryotes– DNA in cytoplasm– circular chromosome– naked DNA– no introns– No splicing– Promoter &

terminator sequence– Smaller ribosomes

• Eukaryotes– DNA in nucleus– linear chromosomes– DNA wound on

histone proteins– introns and exons– TATA box promoter– Transcription factors

present

• Transcription & translation are simultaneous in bacteria – Both occur in

cytoplasm– no mRNA

editing – ribosomes

read mRNA as it is being transcribed

Protein Synthesis in Prokaryotes