Download The Central Dogma of Molecular Biology - APBiology2010-2011

Fig. 16-16b6
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RNA primer
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The Central Dogma of
Molecular Biology
DNA RNA Protein Trait
RNA vs. DNA
Study the images of the two molecules and observe their
similarities & differences.
Basic Principles of Transcription and
Translation
• RNA is the intermediate between genes and
the proteins for which they code
• Transcription is the synthesis of RNA under
the direction of DNA
• Transcription produces messenger RNA
(mRNA)
• Translation is the synthesis of a polypeptide,
which occurs under the direction of mRNA
• Ribosomes are the sites of translation
The Central Dogma of Molecular
Biology
DNA RNA Protein Trait
Transferring genetic information into protein.
Transcription: Rewriting DNA into mRNA
•Enzymes add
nucleotides to
mRNA from 5’ to 3’
•Transcription is
initiated at a of
DNA that
promoter, a
sequence signals
the start of a gene
Enzymes Modify the “pre-mRNA”
• Enzymes attach a “cap” to the mRNA which
binds the mRNA to the ribosome
• Enzymes attach a tail of nucleotides to the
mRNA, this controls the “lifespan” of the mRNA
mRNA is Spliced
• Introns: Non-coding regions of DNA
• Exons: Coding regions of DNA
The Functional and Evolutionary
Importance of Introns
• Some genes can encode more than one
kind of polypeptide, depending on which
segments are treated as exons during
RNA splicing
• The number of different proteins an
organism can produce is much greater
than its number of genes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
The Genetic Code
• How are the instructions for assembling
amino acids into proteins encoded into
DNA?
• There are 20 amino acids, but there are
only four nucleotide bases in DNA
• How many bases correspond to an amino
acid?
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Codons: Triplets of Bases
• The flow of information from gene to protein
is based on a triplet code: a series of
nonoverlapping, three-nucleotide words
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• During transcription, one of the two DNA
strands called the template strand
provides a template for ordering the
sequence of nucleotides in an RNA
transcript
• During translation, the mRNA base
triplets, called codons, are read in the 5
to 3 direction
• Each codon specifies the amino acid to
be placed at the corresponding position
along a polypeptide
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• Codons along an mRNA molecule are
read by translation machinery in the 5 to
3 direction
• Each codon specifies the addition of one
of 20 amino acids
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 17-4
DNA
molecule
Gene 2
Gene 1
Gene 3
DNA
template
strand
TRANSCRIPTION
mRNA
Codon
TRANSLATION
Protein
Amino acid
The Genetic Code
Translation: The RNA directed
synthesis of a polypeptide
• Molecules of tRNA are not identical:
– Each carries a specific amino acid on one end
– Each has an anticodon on the other end; the
anticodon base-pairs with a complementary codon on
mRNA
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Translation
mRNA codons are
translated into a
polypeptide chain
Ribosomes
• Ribosomes facilitate specific coupling of
tRNA anticodons with mRNA codons in
protein synthesis
• The two ribosomal subunits (large and
small) are made of proteins and
ribosomal RNA (rRNA)
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• A ribosome has three
binding sites for tRNA:
– The P site holds the
tRNA that carries the
growing polypeptide
chain
– The A site holds the
tRNA that carries the
next amino acid to be
added to the chain
– The E site is the exit site,
where discharged tRNAs
leave the ribosome
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 17-18-4
Amino end
of polypeptide
E
3
mRNA
Ribosome ready for
next aminoacyl tRNA
P A
site site
5
GTP
GDP
E
E
P A
P A
GDP
GTP
E
P A
• During and after synthesis, a polypeptide
chain spontaneously coils and folds into
its three-dimensional shape
• Proteins may also require posttranslational modifications before doing
their job
• Some polypeptides are activated by
enzymes that cleave them
• Other polypeptides come together to form
the subunits of a protein
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 17-25
DNA
TRANSCRIPTION
3
RNA
polymerase
5 RNA
transcript
RNA PROCESSING
Exon
RNA transcript
(pre-mRNA)
Intron
Aminoacyl-tRNA
synthetase
NUCLEUS
Amino
acid
CYTOPLASM
AMINO ACID ACTIVATION
tRNA
mRNA
Growing
polypeptide
3
A
Activated
amino acid
P
E
Ribosomal
subunits
5
TRANSLATION
E
A
Codon
Ribosome
Anticodon