chapter 17 from gene to protein. 17.1 – genes specify proteins via transcription & translation...
TRANSCRIPT
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