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