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Chapter 15: Genes and How They Work
I. Proteins Are Tools of Heredity
A. Genes Code for Particular
B. Cells Use RNA to Make
1. Polypeptides Assembled on
in
2. Ribosomes composed of
3. RNA similar in structure to DNA
C. Cells Contain Three Classes of RNA
1.
a. With proteins, make up the ribosomes
b. Site of
2.
a.Transport amino acid molecules to
b. Position amino acid along growing polypeptide chain
c. Smaller in size than rRNA, 40 different kinds
3.
a.Long strand of RNA copied from
b.Passes from nucleus to
c. Conveys information from
II. An Overview of Gene Expression
A. Basic Apparatus of Gene Expression Shared by All Organisms
1. Transcription
a. Production of mRNA copy of
b.Initiated by
c. Binds to
at beginning (
) of DNA strand
d.mRNA complementary to
assembled
1)
2)
e. New RNA strand contains
not thymine
f. At stop signal polymerase disengages, mRNA is released
g. mRNA made is primary RNA transcript of DNA information
2. Translation
a.Synthesis of polypeptide by
b. mRNA directs choice of
c. Nucleotide sequence translated into
d. Initiated by rRNA molecule of ribosome that binds to mRNA
e. Ribosome moves along mRNA chain in
nucleotide groups
f. Disengages at stop signal,
3. DNA -------------- protein (
)
B. How Genes Encode Information
1. Crick Determined Nature of Genetic Code
a. Blocks of information corresponding to
b. Group of nucleotides called a
c. Postulated code was
nucleotides long
d. Two nucleotide block would code for only 16 amino acids
e.
known amino acids
f. Three nucleotide block would code for 64 amino acids
2. Questioned Whether Code Was Simple or Punctuated
a. In simple code, each nucleotide is part of a codon
b. Punctuated code has
nucleotide between codons
3. Experimental process involved altering reading frame
a. Change in
nucleotides restored reading frame
b. Change of less than three caused
reading
c. Concluded code was simple
, not punctuated
4. Determination of words of code
a. Added artificial RNA to cell-free RNA and protein
b. Poly-U resulted in synthesis of polyphenylalanine
c. Concluded UUU coded for phenylalanine
d. Repeated for all other triplets
e.
codons possible for only 20 amino acids
f. Some amino acids coded by
C. The Genetic Code
1. Deviations From the "Universal Genetic Code"
a. Most of the code is similar
1) indicates a common
heritage
2) genes from one organism can be
into another
3) produces common products using human genes in bacteria
ex.
b. Examples of differences in stop signals
1) UGA and AUA not read as
2)
read as stop
3) examples
a)Mammalian mitochondria
b)Chloroplasts
c)Some single-celled ciliates
D. Transcription in Prokaryotes
1. RNA Polymerase
a. only
strand is transcribed (-)
b. coding strand of DNA is not transcribed (+)
c. nucleotides are added to the
end, no
is needed
2. Promoter
a.
polymerase binding site on the
strand
b. common bacteria promoters
1) TTGACA – 35 sequence
2) TATAAT – 10 sequence
c. some are
,some are
3. Initiation
a. bound to the promoter
, which unwinds the
b. unwinds two turns of the helix
4. Elongation
a. begins with
b. region with RNA polymerase, DNA, and the RNA transcript forms the
c. the transcription bubble moves down the bacterial DNA at a constant
d. after the bubble passes, the DNA is
e. RNA polymerase, unlike DNA polymerase, does not
1) more mistakes, but they are not
2) the gene is transcribed
5. Termination
a. the stop sequence
pairs are followed by
b. forms a hairpin turn followed by four
c.
is the weakest bond, it can’t hold the DNA-RNA together
E. Transcription in Eukaryotes
1. Multiple RNA polymerases ( ) promoters
a. RNA polymerase I – makes
1) species specific
b. RNA polymerase II – makes
1) complex due to
2) the core promoter is the “
“ box
3) second core promoter is the “
“ box
c. RNA polymerase III – makes
1) found
in a gene
2. Initiation
a. RNA poly II and transcription factors form an
b. more complex than
3. Post transcriptional modification
a. 5’ cap is
linkage
b. the “G” in GTP is
to protect the 5’ cap
c. 3’ poly A tail is added by poly A polymerase to prevent
F. Translation
1. Examine process of translation in prokaryotes
a. Initial portion of mRNA binds to rRNA in ribosome
b. Single mRNA codon exposed at
c. tRNA with complementary
binds to mRNA
d.Each tRNA specific for an
e.Amino acid added to growing string of
1)Activating
specify amino acid to be
2)Binds amino acid to tRNA
3))One aminoacyl-tRNA synthetase enzyme for each
f. Recognizes nucleotide-sequence information
g. Recognizes protein-sequence information
h. Code word is
nucleotides long
i. Each recognizes different identities and numbers of
j. Special, non-amino acid associated codons
1)Nonsense codons are stop signals:
2)
is the start signal
3) AUG also codes for
2. Inititation
a. protein synthesis begins with the formation of an
binds to the small ribosomal subunit
b. initiation factors position the tRNAfmet at the site (peptide bonds form)
1) two other sites form nearby
a) A site – where
bind
b) E site – where empty
exit
c. the beginning of each mRNA is marked by a
sequence
complementary to one of the
d. Prokaryotes often include
genes within a single mRNA transcript
(
)
e. Each eukaryotic gene is transcribed on a
mRNA
1) in eurkaryotes, the initiating gene is methionine not
2) eukaryotic initiation complex is more
3. Elongation
a. proteins called elongation factors assist in binding
to the exposed
mRNA at the
site
b. two amino acids undergo a chemical reaction which releases methionine
from its
and forms a
4. Translocation
a. the ribosomes moves nucleotides along the mRNA in the
b. relocates the initial
to the
site and ejects it from the ribosome
c. repositions the growing polypeptide chain to the
site and exposes
the next codon on the mRNA at the
site
5. Termination
a. Continues until a chain terminating
codon is exposed which is
not recognized by the tRNA, but recognized by a
III. Eukaryotic Gene Transcripts Are Spliced
A. Introns –
sequences of the gene that interrupt
sequences (exons)
1. 1- 1.5% of the human genome are
that encode proteins
2. 24% is noncoding
B. RNA splicing – the primary transcript (with the introns) is capped and a poly A tail
Added and cut and reconnected (
)
1. occurs in the
before export of mRNA to cytoplasm
2. intron/exon junctions are recognized by snRNPs – “
”
3. “snurps’ cluster and for a
which removes the introns
a) a loop (lariat) is formed and
b) exon shuffling – intron – exon arrangements represent the shuffling
of the
C. Alternative splicing – a single primary
is spliced into different mRNAs
by the Inclusion of different sets of
a) explains how 30,000 human genes encode 120,000 different translated
IV. Differences between Prokaryotic and Eukaryotic Gene Expression
A. Most eukaryotic genes possess
, most prokaryotic genes lack
B. Individual prokaryotic mRNA molecules often contain transcripts of
genes
Therefore coordinating their functions. Eukaryotes rarely contain the transcript of
More than
gene
C. Eukaryotic mRNA must be fully
and pass across the nuclear membrane
before
It is
. Prokaryotes, without a nucleus often begin translation before
is complete.
D. In prokaryotes, translation begins with
, In eukaryotes, mRNA is modified by
Adding a cap which starts translation by binding mRNA at
to a ribosomal subunit
E. Eukaryotic mRNA is
it is translated)
F. Eukaryotic ribosomes are a little
(to prevent mRNA destruction before