Transcript ppt 2015 edit

RNA & Gene Expression
•Gene:
A segment of DNA that specifies the
amino acid sequence of a
polypeptide
•DNA does not directly control protein
synthesis, instead its information is transcribed
into RNA
•The “Central Dogma”:
1
RNA properties
• RNA (ribonucleic acid)
2
Types of RNA
• Three Classes of RNA
– Messenger RNA (mRNA)
• Takes a message from DNA to the ribosomes
• strand
– Ribosomal RNA (rRNA)
• Makes up ribosomes (along with proteins)
• globular
– Transfer RNA (tRNA)
• Transfers amino acids to ribosomes
• Hairpin shape
3
Gene Expression
• Gene Expression Requires Two Steps:
• Transcription
– Is the synthesis of RNA under the direction of DNA
– Produces messenger RNA (mRNA)
• Translation
– Is the actual synthesis of a polypeptide, which occurs
under the direction of mRNA
– Occurs on ribosomes
http://highered.mcgrawhill.com/sites/dl/free/0072835125
/126997/animation1.html
4
Ribonucleic Acid
• Why would the cell want to have an intermediate
between DNA and the proteins it encodes?
– The DNA can then stay pristine and protected,
away from the caustic chemistry of the
cytoplasm.
– Gene information can be amplified by having
many copies of an RNA made from one copy of
DNA.
– Regulation of gene expression can be effected
by having specific controls at each element of
the pathway between DNA and proteins.
– The more elements there are in the pathway,
the more opportunities there are to control it in
different circumstances.
5
24.2 Gene Expression
• Transcription
– During transcription, a segment of the DNA serves as a template
for the production of an RNA molecule
– Messenger RNA (mRNA)
• RNA polymerase (enzyme) binds to a promoter (“start”
sequence)
• DNA helix is opened so complementary base pairing can
occur
• RNA polymerase joins new RNA nucleotides in a sequence
complementary to that on the DNA, in a 5’ to 3’ direction
6
Transcription of DNA to form mRNA
7
Messenger RNA
• mRNA - of the 64 possible 3-base combinations:
– 61 code for the twenty different amino acids
– 3 code for "stop"; i.e. chain termination
• Specific nucleotide sequences call for “start” of
transcription (usually AUG = methionine) = PROMOTOR
sequence
• “stop” of mRNA synthesis = TERMINATION sequence
(UAA, UGA, UAG)
• Finished mRNA strands are ~500-10,000 nucleotides
long
8
• During transcription
– The gene determines the sequence of bases along
the length of an mRNA molecule
Gene 2
DNA
molecule
Gene 1
Gene 3
DNA strand 3
(template)
A C C A A A C C G A G T
5
TRANSCRIPTION
mRNA
U G G U U U G G C U C A
5
3
Codon
TRANSLATION
Protein
Figure 17.4
Trp
Amino acid
Phe
Gly
Ser
9
24.2 Processing of mRNA
• After Transcription
• Primary “Pre-”mRNA must be modified into
mature mRNA
– Introns are intragene segments (often, junk)
– Exons are the portion of a gene that is expressed
• Intron sequences are removed, and a poly-A tail is
added
– Ribozyme splices exon segments together
– http://highered.mcgrawhill.com/sites/dl/free/0072835125/126997/animation20.html
10
mRNA Processing
pre-RNA must be modified before translation
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The Functional and Evolutionary Importance of Introns
• The presence of introns
– Allows for alternative RNA splicing
– Animations of RNA processing:
http://highered.mcgrawhill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/00
72437316/120077/bio25.swf::Processing%20of%20Gene%20Infor
mation%20-%20Prokaryotes%20versus%20Eukaryotes
– http://highered.mcgrawhill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/00
72437316/120077/bio30.swf::How%20Spliceosomes%20Process%
20RNA
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Transposons
Transposons
• “jumping genes”
• Can move to new locations and disrupt gene
sequences
13
24.2 Gene Expression
• Translation
– The Genetic Code
• Triplet code: each 3-nucleotide unit of a mRNA molecule is
called a codon
• There are 64 different mRNA codons
– 61 code for particular amino acids
» Redundant code; some amino acids have numerous code
words
» Provides some protection against mutations
– 3 are stop codons signal polypeptide termination
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Messenger RNA Codons
15
Overview of Gene Expression
Protein synthesis
http://highered.mcgrawhill.com/olcweb/cgi/plugi
npop.cgi?it=swf::535::53
5::/sites/dl/free/0072437
316/120077/micro06.sw
f::Protein%20Synthesis
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24.2 Gene Expression
• Transfer RNA
– tRNA transports amino acids to the
ribosomes (creates polypeptide chain)
– Single stranded nucleic acid that
correlates a specific nucleotide sequence
with a specific amino acid
– Amino acid binds to one end, the
opposite end has an anticodon
– the order of mRNA codons determines
the order in which tRNA brings in amino
acids
Protein synthesis
http://highered.mcgrawhill.com/olcweb/cgi/pluginp
op.cgi?it=swf::535::535::/si
tes/dl/free/0072437316/12
0077/micro06.swf::Protein
%20Synthesis
17
Transfer RNA: Amino Acid Carrier
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rRNA
– Ribosomal RNA is the most abundant type of RNA in
cells
– Ribosomes: comprised of subunits 2/3 RNA, 1/3
protein
• Two populations of
ribosomes are evident
in cells, Free and
bound
19
“Bound” vs “Free” Ribosomes
•
Free ribosomes are located in the cytoplasm of the cell. They are
not attached to any structure, but they may group together with other
ribosomes to form polysomes (polyribosomes). In the cytoplasm,
ribosomes are free floating. They can move all around the cell.
• Bound ribosomes are located on the surface of the endoplasmic
reticulum. The endoplasmic reticulum that contains ribosomes is
described as the rough endoplasmic reticulum because of the
bumpy surface. Bound ribosomes can not move to other areas of
the cell. They are attached to the cytosolic side of the endoplasmic
reticulum.
• Free ribosomes produce proteins for the cell, while bound ribosomes
produce proteins that are transported out of the cell.
20
Polyribosomes
• A number of ribosomes can translate a single
mRNA molecule simultaneously
– Forming a polyribosome
Completed
polypeptide
Growing
polypeptides
Incoming
ribosomal
subunits
Start of
mRNA
(5 end)
End of
mRNA
(3 end)
(a) An mRNA molecule is generally translated simultaneously
by several ribosomes in clusters called polyribosomes.
Ribosomes
mRNA
0.1 µm
Figure 17.20a, b
(b) This micrograph shows a large polyribosome in a prokaryotic
cell (TEM).
21
• The ribosome has three binding sites for
tRNA
– The P site
– The A site
– The E site
P site (Peptidyl-tRNA
binding site)
A site (AminoacyltRNA binding site)
E site
(Exit site)
Large
subunit
E
mRNA
binding site
Figure 17.16b
P
A
Small
subunit
(b) Schematic model showing binding sites. A ribosome has an mRNA
binding site and three tRNA binding sites, known as the A, P, and E sites.
This schematic ribosome will appear in later diagrams.
22
• Concept 17.4: Translation is the RNAdirected synthesis of a polypeptide: a closer
look
Quicktime movie:
http://carbon.cudenver.edu/~bstith/transla.MOV
Narrated animation: http://highered.mcgrawhill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/
120077/micro06.swf::Protein%20Synthesis
Interactive practice:
http://learn.genetics.utah.edu/content/begin/dna/transcribe/
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24.2 Gene Expression
• Ribosome and Ribosomal RNA
– Ribosome has a binding site for mRNA and
for 2 tRNAs
– Facilitates complementary base pairing
– Ribosome moves along mRNA and new
tRNAs come in and line up in order
– This brings amino acids in line in a specific
order to form a polypeptide
– Several ribosomes may move along the
same mRNA
• Multiple copies of a polypeptide may be made
• The entire complex is called a polyribosome
24
Translation (Building a polypeptide)
requires Three Steps:
– Initiation (requires energy)
– Elongation (requires energy)
– Termination
Amino end
Growing polypeptide
Next amino acid
to be added to
polypeptide chain
tRNA
3
mRNA
5
Codons
(c) Schematic model with mRNA and tRNA. A tRNA fits into a binding site when its
anticodon base-pairs with an mRNA codon. The P site holds the tRNA attached to
the growing polypeptide. The A site holds the tRNA carrying the next amino acid to
be added to the polypeptide chain. Discharged tRNA leaves via the E site.
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Summary of Gene Expression
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24.2 Gene Expression
• Genes and Gene Mutations
– A gene mutation is a change in the sequence of
bases within a gene.
– Gene mutations can lead to malfunctioning proteins in
cells.
27
24.2 Gene Expression
• Genes and Gene Mutations
– Causes of Mutations
• Errors in replication
– Rare
– DNA polymerase “proofreads” new strands and errors
are cleaved out
• Mutagens
– Environmental influences
– Radiation, UV light, chemicals
– Rate is still fairly low because DNA repair enzymes
monitor and repair DNA
28
Types of Gene Mutations
Point Mutations
– The substitution of one nucleotide for another
• Missense mutations
– a point mutation in which a single nucleotide is changed, resulting in a
codon that codes for a different amino acid
– Missense mutations are responsible for about 75% of the mutations in
the p53 gene. Mutations of this gene are responsible for about 30-50%
of cancers in humans
• Silent mutations
• Nonsense mutations
– mutations that change an amino acid to a stop codon
29
Types of Gene Mutations
• Frameshift Mutations
– One or more nucleotides
are inserted or deleted
– Results in a polypeptide
that codes for the wrong
sequence of amino acids
– Codons must be read in
the correct reading frame
for the specified
polypeptide to be
produced
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