from dna to protein genes determine traits & functions genes are sections of dna on...
TRANSCRIPT
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CHAPTER 14From DNA to Protein
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GENES “EXPRESSED” Genes determine traits & functions Genes are sections of DNA on
chromosomes DNA “code” must be converted to be
used
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PROTEIN SYNTHESIS
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RNA VS. DNA Ribonucleic
acid (RNA)Single strandUracil instead
of thymineA—U, U—A
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TYPES OF RNA Messenger RNA (mRNA)
Carry genetic code from nucleus to cytoplasm
Ribosomal RNA (rRNA)“Read” genetic code of mRNA to build
polypeptide
Transfer RNA (tRNA)Bring amino acids (AAs) to rRNA
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SUMMARY OF PROTEIN SYNTHESIS
RNA Polymerase unravels part of the DNA strand
The polymerase creates a template based on a section of DNA. This is the mRNA.
mRNA leaves the nucleus
mRNA binds to ribosome
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SUMARY (CONT.) tRNA carries an amino acid (AA), matches
to a section of the mRNA in the ribosome
Another tRNA carries the next AA, the two AAs bond
tRNA leaves
Subsequent AAs continue to bond, forming a peptide chain
At the end of the sequence, the peptide breaks off into the cytoplasm, the ribosome releases the mRNA, and the mRNA breaks apart to be recycled.
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TRANSCRIPTION “The process by which an mRNA
template, carrying the sequence of the protein, is produced for the translation step from the genome.”
Copying something to be read by someone else in another place.
In this case…copying the gene to be used as an instruction manual to make protein in another part of the cell.
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TRANSCRIPTION
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STEP BY STEP! Initiation
RNA polymerase binds to a specific part of the DNA strand. This is the gene.
Polymerase unravels this part of the strand, separating the base pairs.
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ElongationPolymerase begins to
slide along the DNARibonucleotides
match to opposite base, then bond
This creates a chain, which is the mRNA
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Termination Introns (unwanted
parts) are “cut out” and removed
Exons (coding parts) are fused together
A “cap” and “tail” are put on the ends of the mRNA for protection
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RNA PROCESSING
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The bases in the introns are recycled. There are several hypothesis about the origin of introns. It has been suggested that some may have a function in the cell, before or
after excision. The RNA comes off the DNA template which reforms a double-stranded DNA molecule
The introns are removed. A typical human gene has 6 introns, but there may be hundreds; a few have none. A typical human intron is around 1 kb but it can be
hundreds of kb. On average, introns are 10-20 times larger than exons.
The exons are joined together, in sequence. The removal of introns and the joining of the exons occur concurrently and the process is known as “splicing”. It actually occurs
while the RNA is still being synthesized.
A poly A tail is added. The tail is usually around 150 Adenine bases; a few bases at the end of the RNA molecule are “trimmed”, before poly-adenylation. The tail is also
required for the initiation of translation and stability. Capping, splicing and poly-adenylation are known as “RNA processing”.
A cap is added to the RNA: The cap is necessary for the RNA to bind to the ribosome for translation to begin, and probably aids stability. “Capping” actually occurs
immediately after RNA synthesis begins.
Using the DNA template strand, RNA polymerase manufactures an RNA molecule (pre-mRNA) using the complementary base pair rules. In RNA, Uracil is complementary to Adenine in DNA. The DNA strand that acts as the template strand varies from gene to
gene.
In the region of the gene, the DNA unwinds and the 2 strands come apart. RNA polymerase, an enzyme, is the key molecule for the manufacture of RNA from DNA
A gene: a sequence of double-stranded DNA. On average, a human gene is about 14kb (14,000 base pairs) but the size is highly variable. The dystrophin gene,
encoding a muscle protein, has 2.4Mb (2.4 million base pairs). Typically, there is 75kb between human genes.
Note the complementary base pair rules; A-T and G-CTranscription: making RNA from a DNA template.
While an average human mRNA molecule has about 2,600 bases, the size is highly variable.
TRANSCRIPTION
C G C A U A A G C G A C U A G
C C G C A U A A G C G A C U A
G G C U U C A C C C G C U A
A G A U U A U G
C U U C A C C C G C U A A G
C G C A U A A G C G A C U A G G C U U C A C C C G C U A A G A U
C G C A T A A G C G A C T A G G C T T C A C C C G C T A A G A T
C G A C U A G G A C C C G C U A A G
U C A C C C G C U A A G
C G C A U A U G
C G C A U A A G C G A C U A G G C U U C A C C C G C U A A G A U
DNA
G C G T A T T C G C T G A T C C G A A G T G G G C G A T T C T AC G C A U A A G C G A C U A G G C U U C A C C C G C U A A G A U
A U A U G C G A C U A G G A C C C G C U A A G
C U U C A C C C G C U A A G
A A A A A A A A A A A AmRN
A
C G C A T A A G C G A C T A G G C T T C A C C C G C T A A G A TG C G T A T T C G C T G A T C C G A A G T G G G C G A T T C T A
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TRANSLATION “Protein translation involves the transfer of
information from the mRNA into a peptide, composed of amino acids. This process is mediated by the ribosome, with the adaptation of the RNA sequence into amino acids mediated by transfer RNA.”
Taking something in a language you can’t use, and making it into a language that you can use.
In this case…taking the “language” of the DNA & mRNA, which the cell can’t directly use, and converting it into the “language” of proteins, which the cell can use.
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STEP BY STEP! (AGAIN) Ribosomes (two
subunits) are in cytoplasm/Rough ER
tRNA attaches to amino acids in cytoplasm
Ribosome binds to the beginning of the mRNA chain
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InitiationStarts with a 3-base
section of the template, a “codon”, specifically the “start codon”
tRNA with matching “anticodon” attaches
Another tRNA matches to the next codon
Amino acids bond
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ElongationRibosome moves
down the mRNA one codon at a time
tRNA continues to match codon to anticodon, bringing more AAs
AAs bond, forming a lengthening chain
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Termination rRNA reaches “stop
codon”, which signals stop of process
rRNA detaches, polypetide can now move into rest of cell
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G A
U
C C G
U A C
met
G C U
arg
A A G
phe
thr
G C G
arg
C A U
val
U G C
thr
gly le
u
StopC G C A U A U G C G A C U A G G C U U C A C C C G C U A A G
mRNA
Start
TRANSLATION
Polypeptide
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BRING IT ALL TOGETHER…. Transcription—in nucleus
Initiation—RNA polymerase unravels DNA
Elongation—mRNA created by bonding nucleotides
Termination—Introns removed, mRNA finalized
Translation—in cytoplasm Initiation—rRNA binds to mRNA,
tRNA bring AAs matched by codon
Elongation—rRNA moves along mRNA, AAs bond into polypeptide
Termination—ends at stop codon
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GENETIC CODE
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SUMMARY OF PROTEIN SYNTHESIS
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MUTATIONS Radiation Chemicals Spontaneous (“mistakes”)
DNA pattern is altered This also alters RNA copied from DNA Pattern of RNA determines protein
structure
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MUTATIONS Base substition
Mistake where an incorrect base is inserted on DNA
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MUTATIONS Frameshift
Base is added to or deleted from DNASince codon is read in “threes”, shifts how
codons are counted
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MUTATIONS
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MUTATIONS Altered DNA leads
to altered products of translation
These products have affects on function or appearance of phenotype