chapter 17 from gene to protein

Chapter 17From Gene to

Protein

17.1 – Genes specify proteins via transcription & translation

Gene Expression DNA directs the synthesis of proteins (or RNA) Includes translation & transcription Proteins are the links between genotype and

phenotype

One gene-one polypeptide hypothesis Each gene codes for a polypeptide Can be a protein or part of a protein

Messenger RNA (mRNA) Produced during transcription Carries the genetic message of DNA to the protein

making machinery of the cell (ribosome)

In Eukaryotes Transcription results in pre-mRNA, which undergoes

RNA processing to yield the final mRNA

In Prokaryotes Transcription directly makes mRNA Transcription & Translation occur at the same time

Transcription Synthesis of RNA using DNA as a template Occurs in the nucleus Only one strand of DNA is transcribed (called

template strand) The mRNA produced is a complementary

strand The mRNA base triplets are called codons

Written in the 5’ to 3’ direction

The genetic code is redundant More than one codon codes for the 20 Amino

Acids Read based on a consistent reading frame

Groups of 3 must be read in the correct groupings in order for translation to be successful

All 64 codons were deciphered by the mid-1960s

Of the 64 triplets, 61 code for amino acids; 3 triplets are “stop” signals to end translation

Translation Production of a polypeptide chain using

mRNA Occurs at the ribosomes The instructions for the PP chain are written

as a triplet code

The genetic code is nearly universal, shared by the simplest bacteria to the most complex animals

Genes can be transcribed and translated after being transplanted from one species to another

17.2 – Transcription is the DNA-directed synthesis of RNA

RNA polymerase Enzyme that separates the two DNA strands Connects the RNA nucleotides as they base-

pair Can add RNA nucleotides only to the 3’ end so

it elongates in the 5’ to 3’ direction Uracil replaces thymine

Promoter DNA sequence that RNA polymerase attaches

Terminator DNA sequence that signals the end of

transcription

Transcription unit Entire stretch of DNA that is transcribed into

RNA May code for a polypeptide or an RNA such

tRNA or rRNA

3 stages of transcription

1) Initiation 2) Elongation 3) Termination

1) Initiation In bacteria

RNA polymerase recognizes & binds to the promoter

In Eukaryotes RNA polymerase II cannot bind to the

promoter without supporting help from proteins known as transcription factors

Transcription Factors Assist the binding of RNA polymerase to the

promoter, & the initiation of transcription

Transcription initiation complex The whole complex of RNA polymerase II &

transcription factors

A promoter called a TATA box is crucial in forming the initiation complex in eukaryotes

2) Elongation RNA polymerase moves along the DNA

(untwists the double helix) 10 to 20 bases at a time

RNA nucleotides are continually added to the 3’ end of the growing chain 40 nucleotides per second

As the complex moves down the DNA strand, the double helix re-forms with the new RNA molecule straggling away from the DNA template

3) Termination RNA transcript is released & the polymerase

detaches upon transcribing a terminator sequence in the DNA

17.3 – Eukaryotic cells modify RNA after transcription

Modifications to RNA after transcription: Adding a 5’ cap & a poly-A tail Facilitate the export of mRNA from the nucleus Help protect mRNA from degradation by

enzymes Facilitate the attachment of the mRNA to the

ribosome

RNA splicing (in Eukaryotic Cells) Large portions of the newly made RNA strand

are removed – called INTRONS The ones left behind are called EXONS & are

spliced together by a spliceosome

Special RNA called small nuclear RNA (snRNA) aid the spliceosomes Play a role by catalyzing the excision of the

introns & joining the exons When RNA is an enzyme it is called a

RIBOZYME

17.4 – Translation is the RNA-directed synthesis of a

polypeptide Utilizes mRNA, tRNA, & rRNA tRNA

Transfers AA from a pool of AA in the cytoplasm to a ribosome

The ribosome accepts the AA & adds it into a growing PP chain

Each tRNA is specific for an AA One one end, it binds to the AA & the other

end has a triplet called an anticodon which allows it to pair with a codon on an mRNA

Codon – mRNA triplet (there are 64) mRNa is read codon by codon & one AA is

added to the chain for each codon read The rules for base-pairing between the third

base of a codon & the corresponding tRNA anticodon are not as strict as DNA & mRNA so it is called a wobble

rRNA complexes with proteins Forms the 2 subunits that form ribosomes Ribosomes have 3 binding sites for tRNA P-site – holds the tRNA that carries the

growing PP chain A-site – holds the tRNA that carries the AA that

will be added next E-site – exit site for tRNA

3 stages of Translation

1) Initiation 2) Elongation 3) Termination

Initiation A) a small ribosomal subunit binds to mRNA in a

way that the first codon of the mRNA strand (AUG) is placed in the proper position

B) tRNA with the anticodon UAC (carries the AA methionine), hydrogen bonds to the first codon (proteins called initiation factors aid)

C) Large subunit of ribosome attaches Allows the tRNA with methionine to attach to the

P-site The A-site will now be available for the next tRNA

with the 2nd AA

2) Elongation A) Codon Recognition

The codon in the A-site is matched by the incoming tRNA anticodon

B) Peptide bond formation The incoming AA in the A-site forms a peptide bond

with the existing chain of AA held in the P-site Catalyzed by an rRNA (ribozyme)

C) Translocation Occurs when tRNA in the A-site is moved to the P-site

& the tRNA in the P-site is moved to the E-site A-site is now clean and is ready for another AA

3) Termination A stop codon in the mRNA is reached &

translation stops A protein called release factor binds to the stop

codon & the PP is freed from the ribosome PP’s then will fold to assume their specific shape May be modified further to make them functional The destination of the protein is determined by

the sequence of about 20 AA’s at the leading end of the PP chain (signal peptide)

17.5 – Point mutations can affect structure & function

Point mutation are alterations of just one base pair – 2 basic types:

1) Base-pair substitutions The replacement of one nucleotide & its

complementary base pair in the DNA with another pair of nucleotides

Missense – enable the codon to still code for an AA although it may not be the correct one

Nonsense – change a regular AA codon into a stop codon

2) Insertions & deletions Additions & loses of nucleotide pairs in genes If they interfere with the codon groupings they

can cause a frameshift mutation Causes the mRNA to be read incorrectly

Mutagens Substances or forces that interact with DNA in

ways that cause mutations X-rays & chemicals