A.P. Biology
CHAPTER 17: FROM GENE TO PROTEIN
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The DNA inherited by an organism leads to specific traits by dictating the synthesis of proteins.
This process is called gene expression and includes two stages: transcription and translation.
Proteins are the link between genotype and phenotype
GENES SPECIFY PROTEINS VIA TRANSCRIPTION AND TRANSLATION
HISTORY:
 1909: Archibald Garrod first suggested that genes dictate phenotypes through enzymes
 1930’s: George Beadle and Edward Tatum formulated the one gene-one enzyme hypothesis based
on their data on bread mold (Neurospora crassa) in different growth mediums
 Today: With continued research, we now have revised Beadle & Tatum’s conclusion and have
created the one gene-one polypeptide hypothesis.
BASIC PRINCIPLES OF TRANSCRIPTION AND TRANSLATION:
 DNA→Transcription→RNA→Translation→Protein
 Ribonucleic Acid (RNA): a nucleic acid that uses information from DNA to synthesize proteins.
 RNA and DNA compared:
1. RNA is single stranded and DNA has two strands
2. RNA has a ribose sugar and DNA has deoxyribose
3. RNA has a base of Uracil (U) and DNA has the base Thymine
**There is no T in RNA, so A=U and C=G**
TRANSCRIPTION: The synthesis of RNA under the direction of DNA
 A gene’s unique nucleotide sequence is transcribed from the DNA template to a complementary
nucleotide sequence in messenger RNA (mRNA).
 The resulting mRNA carries this transcript of protein-building instructions to the cell’s proteinsynthesizing machinery.
TRANSLATION: The synthesis of a polypeptide, which occurs under the direction of mRNA
 During translation, the linear sequence of bases in mRNA is translated into the linear sequence of
amino acids in a polypeptide
 Translation occurs on ribosomes (in the cytoplasm of the cell). Ribosomes are complex particles
composed of ribosomal RNA (rRNA) and protein that facilitate the orderly linking of amino acids
into polypeptide chains.
PROKARYOTIC/EUKARYOTIC DIFFERENCES IN GENE TRANSFER
 Because bacteria lack nuclei, their DNA is not segregated from ribosomes and other proteinsynthesizing equipment.
THE GENETIC CODE
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The template strand of DNA provides the template for ordering the sequence of nucleotides in an
RNA transcript.
The RNA instructions are written in a series of three-nucleotide sequences called a triplet code.
Each mRNA triplet code (codon) “codes” for a specific amino acid. There are 20 different amino
acids and 64 codons.
The amino acids are linked together to form a protein
This process of protein synthesis occurs in the 5’ to 3’ direction along the mRNA.
TIDBITS:
 All 64 codons were deciphered by the mid 1960’s.
 The reading frame is the order in which the codons should be translated (in bases of three)
 The genetic code is nearly universal, shared by organisms from the simplest bacteria to the most
complex animals.
TRANSCRIPTION IS THE DNA-DIRECTED SYNTHESIS OF RNA
Transcription involves three main steps: Initiation, Elongation, and Termination
STEPS OF TRANSCRIPTION: Figure 17.7, page 332
1. RNA Polymerase (an enzyme that adds and links complementary RNA nucleotides) binds to
a gene’s promoter (a sequence of DNA that acts like a start signal, it typically extends
several dozen nucleotide pairs from the start point).
2. In Eukaryotes, a collection of proteins, called transcription factors assist the binding of
RNA polymerase and the initiation of transcription. The transcription factors and a DNA
sequence called TATA box initiate transcription (See Figure 17.8, page 333).
3. RNA polymerase unwinds the double helix, exposing DNA nucleotides.
4. RNA polymerase adds nucleotides to the 3’ end and links complementary RNA nucleotides
together. The stretch of DNA that is transcribed into an RNA molecule is called a
transcription unit.
5. RNA polymerase transcribes until it reaches a DNA sequence known as a
terminator…Details become “murky”, but transcription is terminated when the polymerase
eventually falls off the DNA.
EUKARYOTIC CELLS MODIFY RNA AFTER TRANSCRIPTION:
RNA transcripts in eukaryotes are modified, or processed, before leaving the nucleus to yield
Functional mRNA. Eukaryotic RNA transcripts can be processed in two ways: a) covalent
alteration of both the 3’ & 5’ ends and b) removal of intervening sequences.
Alteration of mRNA ends: Fig. 17.9, pg. 334
Primary transcript: General term for initial RNA transcribed from DNA
Pre-mRNA: Primary transcript that will be processed to functional mRNA
 The 5’ end is capped off with a modified form of guanine, which forms a 5’cap.
 At the 3’ end, an enzyme adds 50 to 250 adenine nucleotides, forming a poly-A tail.
The 5’ cap and poly-A tail facilitate the export of mRNA from the nucleus, protect the mRNA
from degradation by enzymes, and help ribosomes attach to the 5’ end of the mRNA.
Split genes and RNA splicing. Fig. 17.10, pg. 335
Introns: Noncoding sequences in DNA that intervene between coding sequences (exons).
They are initially transcribed, but not translated, because they are excised from the
transcript before mature RNA leaves the nucleus.
Exons: Coding sequences of a gene that are transcribed and expressed
RNA splicing: RNA processing that removes introns and joins exons from eukaryotic premRNA; produces mature mRNA that will move into the cytoplasm from the nucleus.
This is a “cut and paste” job.
Pre-mRNA splicing is carried out by small nuclear ribonucleoproteins (snRNPs). Several
snRNPs join with additional proteins to form a splicosome. The spliceosome interacts with
certain sites along an intron, releasing the intron and joining together the two exons that flanked
the intron (See Figure 17.11, page 335).
Ribozymes are RNA molecules that function as enzymes. In some organisms, RNA splicing
occurs when the ribozyme catalyzes its own excision.
Introns may play regulatory roles in the cell. Depending on which segments of are treated as
exons during RNA processing, some genes can encode more than one type of polypeptide
(alternative RNA splicing). Also, introns increase the probability of potentially beneficial
crossing over between the exons of alleles (creating more potentially useful proteins)
TRANSLATION IS THE RNA-DIRECTED SYNTHESIS OF A POLYPEPTIDE
(Fig. 17.18, pg. 341)
1. mRNA leaves the nucleus and enters the cytoplasm. There, each type of tRNA associates a
distinct mRNA codon with one of the 20 amino acids used to make proteins. The tRNA has
an amino acid on one end and an anticodon (nucleotide triplet in tRNA that base pairs with
a complementary nucleotide codon in mRNA). (Fig. 17. 14, pg 338 for tRNA structure)
2. Initiation: The tRNA reads the start code of mRNA (AUG) and orients itself in a region of a
ribosome called the P site
3. Elongation: Amino Acids are added one by one to the first amino acid. The A site of the
ribosome is ready to receive the next tRNA. The tRNA then binds to the codon by
hydrogen bonds.
4. The P and A sites are holding tRNA molecules (each with its own amino acid). The amino
acids are linked together by a peptide bond.
5. Translocation: The tRNA in the P site moves to the E site, the tRNA in the A site moves
over (translocates), and a new tRNA molecule binds to the codon in the A site
6. Termination: The amino acids continue to join together until a stop codon is reached (UAG,
UAA, or UGA). A protein is thus created.
Once the protein is made, it is often times not ready to be functional. The protein has to go through
post-translational modifications. Free and Bound ribosomes will direct the protein in the right
direction.
A signal recognition particle (SRP) is a protein-RNA complex that functions as an adapter that brings
the ribosome to a receptor protein in the ER membrane.
POINT MUTATIONS CAN AFFECT PROTEIN STRUCTURE AND FUNCTION:
Mutations: Heritable changes in the genetic material of a call (or virus). Fig 17.23 on pag 345
Point mutations: A mutation limited to about one or a few base pairs in a single gene
Types of point mutations:
1. Substitutions (Base-pair substitution) The replacement of one base pair with another;
occurs when a nucleotide and its partner in the complementary DNA strand are replaced
with another pair of nucleotides according to base-pairing rules
 Missense mutation: Base-pair substitution that alters an amino acid codon (sense
codon) to a new codon that codes for a different amino acid. These alterations make
sense, but not necessarily “the right sense”
 Nonsense mutation: Base-pair substitution that changes an amino acid codon to a
chain termination codon. Leads to nonfunctional proteins.
2. Insertions and Deletions: Usually have a greater negative effect on proteins then
substitutions. Whenever the number of nucleotides inserted or deleted is not or a multiple of
3, it will result in a frameshift mutation.
 Framesift mutation: A base-pair insertion or deletion that causes a shift in the
reading frame, so that codons beyond the mutation will be the wrong grouping
of triplets and will specify the wrong amino acids.
3. Mutagenesis: The creation of mutations. May be caused naturally or because of exposure to
mutagens.
 Mutagen: Physical or chemical agents that interact with genetic material to cause
mutations
a. Radiation is the most common physical mutation
b. The Ames test is one of he most widely used tests for measuring the
mutagenic strength of various chemicals, such as carcinogens.