Chapter 17
From Gene to Protein
Gene expression: from gene to protein
I. Genes specify proteins via transcription and translation
A. Evidence from the Study of Metabolic Defects
Archibald Garrod (1909) He was a physician who studied inborn errors in
metabolism. He suggested that genes dictate phenotypes through enzymes
that catalyze specific chemical reactions in cells.
B. Nutritional Mutants in Neurospora.
George Beadle and Edward Tatum using a bread mold (Neurospora
crassa) stated the One Gene-One enzyme hypothesis. Noble Prize in 1958
C. The Products of Gene Expression: A Developing Story. Currently we state one
gene-one polypeptde.
D. Basic Principles of Transcription and Translation Fig. 17.3 Page 337
Getting from DNA to protein requires two major stages:
1. Transcription - the synthesis of RNA under the direction DNA.
2. Translation - synthesis of polypeptide, which occurs under the direction
of mRNA (messenger RNA). The site of translation is ribosomes. They
facilitate the orderly linking of amino acids into polypeptide chains.
Primary Transcript is the initial RNA transcript from any gene including
those coding RNA not translated into proteins.
Since Prokaryotes lack nuclei, DNA is not separated from the proteinsynthesizing machinery. Hence transcription and translation follow in rapid
succession.
Eukaryotes have nuclear envelopes that segregate transcription in the nucleus
from translation in the cytoplasm.
Cells are governed by a molecular chain of command: DNA to RNA to protein.
E. The Genetic Code
1. Codons: triplets of bases
Thus there are 64 triplets called a codon in mRNA to code for 20
amino acids.
F. Cracking the Genetic Code
1. In 1961 Marshall Nirenberg synthesized an artificial mRNA.
2. Fig. 17.5 page 339 shows the dictionary of the genetic code. Know
how to use it. AUG has a dual function coding for methionine and also
functioning as a “start” signal.
G. Evolution of the Genetic Code.
This genetic code is shared nearly universally among all living organism.
II. Transcription is the DNA-directed synthesis of RNA: a closer look. Fig. 17.7 page
340
Transcription occurs in three key steps: a) binding and initiation; b) elongation;
and c) termination.
A. RNA polymerase binding and initiation of transcription. Fig. 17.8 p. 341
1. The RNA polymerase binds at regions called promoters.
2. TATA box is where a transcription factor binds enabling RNA
polymerase to recognize the promoter region.
B. Elongation of the RNA strand by RNA polymerase II.
1. First function: untwists and opens a short segment of DNA.
2. Links incoming RNA nucleotides so that RNA grows one
nucleotide at a time in the 5' to 3' direction.
C. Termination of Transcription
In bacteria, transcription proceeds through a terminator sequence in the
DNA. The transcribed terminator (an RNA sequence) functions as the
termination signal, causing the polymerase to detach from the DNA and
release the transcript, which is available for immediate use as mRNA.
In eukaryotes the pre-mRNA is cleaved from the growing RNA chain
while RNA polymerase II continues to transcribe the DNA. Transcription
is terminated when the polymerase eventually falls off the DNA. The
mRNA is further processed before leaving the nucleus.
III. Eukaryotic cells modify RNA after transcription.
A. Alteration of mRNA Ends Fig. 17.10 Page 343
At the 5' end it is given a cap and at the 3' end a poly-A tail.
B. Split Genes and RNA Splicing
The noncoding segments of nucleic acid that lie between coding regions
are called intervening sequences, or introns for short. The other regions
are called exons (those regions eventually expressed). The introns are cut
out from the molecule and the exons joined together. This is the process
of RNA splicing.
C. Ribozymes: RNA molecules that function as catalyst. Page 344
D. The functional and Evolutionary Importance of Introns: Allows a gene to
encode more than one kind of polypeptide. Alternative RNA Splicing Page
344
IV. Translation is the RNA-directed synthesis of a polypeptide: a closer look
A. Structure and function of transfer RNA (tRNA) Fig. 17.15 Page 346
1. A clover leaf shape about 80 nucleotides long containing an
anticodon (3 nucleotides complementary to the 3 on mRNA) and then
twist and folds into an L shape.
2. The 3’ end of tRNA links with the appropriate amino acid.
3. Function: to carry one amino acid to the site of protein synthesis
catalyzed by Aminoacyl-tRNA synthetase Fig. 17.16 Page 347
B. Ribosome Fig. 17.17 Page 347
1. Two subunits separated when not synthesizing protein.
2. 60% ribosomal RNA and 40%protein
3. Constructed in the nucleolus for eukaryotes
4. A site is the aminoacyl-tRNA-binding site and the P site is the
Peptidyl-tRNA binding site. The E site is the Exit site.
C. Building a polypeptide.
1. Initiation - in eukaryotes the tRNA carries methionine codon of AUG
hence anticodon of UAC Fig. 17.18 Page 348
2. Elongation Fig. 17.19 Page 350
a. Codon recognition occurring at the A site
b. Peptide bond formation occurring at the P site. The enzyme
involved is peptidyl transferase.
c. Translocation - the tRNA in the A site is moved still attached to
mRNA to the P site
3. Termination - occurs when a termination codon is reached.
Stop codons are UAA, UAG and UGA. Fig. 17.20 Page 351They
bring with them a release factor.
E. Polyribosomes (polysomes) clusters of ribosomes, which simultaneously
translate mRNA. Found in both prokaryotic and eukaryotic cells. They enable
a cell to make many copies of a polypeptide very quickly. Fig. 17.22 Page 353
F. Polypeptide to Functional Protein
1. Genes determine primary structure.
2. Primary structure determines how a polypeptide chain will
spontaneously coil.
3. Some proteins undergo post-translational modification
G. Targeting Polypeptides to Specific Locations Fig. 17.21 Page 352
If destined for the endomembrane system or for secretion, the polypeptide
is marked by a signal peptide. This is recognized by a signal-recognition
particle (SRP)
Finally Fig. 17.24 Page 343 Shows a summary of Transcription and Translations
processes.
V Mutations of one or a few nucleotides can affect protein structure and function
Mutations: are changes in the genetic material of a cell.
Point mutations: are chemical changes in just one base pair of a gene.
A. Types of point mutations Fig. 17.25
1. Substitutions - base pair substitution
shift
2. Insertions and deletions have disastrous effects. These cause frame
mutations.
B. Mutagens - physical or chemical agent that interact with DNA to cause
mutations X-rays, ultraviolet (UV) light and chemicals
Hence: What is a gene? It is a region of DNA that can be expressed to produce a final
functional product that is either a polypeptide or an RNA molecule.