Download Replication, Transcription, Translation

Replication,
Transcription, and
Translation
Information Flow from DNA to Protein
The Central Dogma of Molecular Biology
Replication is the copying of DNA in the course of cell division.
Transcription is the synthesis of RNA from DNA.
Translation is the synthesis of polypeptides through the combined efforts of rRNA, mRNA,
and tRNA.
Approximately 2% of the nucleotides in DNA result in the formation of polypeptides. These
regions in the DNA are called coding regions. The remaining 98% of the nucleotides do not
appear to have any function and are called junk DNA.
Basic Model of
Replication
Semiconservative Replication
Replication proceeds simultaneously at thousands of replication bubbles along each DNA
strand. Replication of an entire DNA strand requires approximately 10 minutes.
A single parent DNA duplex molecule is transformed into two daughter DNA duplexes, each
containing one strand of the parent DNA and one newly synthesized DNA strand.
This form of DNA replication is termed semiconservative.
Replication
at the
Molecular
Level
Nucleoside triphosphates
carrying each of the four bases
move into place by forming
hydrogen bonds with the bases
exposed on the DNA template
strand.
DNA polymerase
catalyzes bond formation
between the 5’ phosphate
group of the arriving
nucleoside triphosphate and
the 3’ —OH at the end of
the growing polynucleotide
strand.
Replication
Human cell division is extremely complex and takes a little less than one day.
Approximately one third of this time is involved in replicating DNA.
Replication of DNA is carried out with an error level of less than 1 wrong nucleotide
per 10 billion. This extreme accuracy preserves the genetic integrity from generation
to generation.
DNA replication involves:
A replication bubble,
with two replication forks.
Several Enzymes, including:
Unwinding Enzyme
DNA polymerase for continuos replication
DNA ligase for discontinuous replication
Exo- and Endonucleases - to correct errors
Replication
Replication: The Big Picture
The template strand can only be
read in the 3’ to 5’ direction, and
the new DNA strand can grow
only in the 5’ to 3’ direction.
Only the leading strand, is able
to grow continuously; the DNA
polymerase traveling in the 3’ to
5’ direction, is moving in the
same direction as the
replication fork.
On the other strand, the DNA
polymerase is moving in the
opposite direction as the
replication fork.
The lagging strand is replicated
in short segments called
Okazaki fragments. These short
DNA segments are joined
together by DNA ligase.
leading strand
lagging strand
Transcription
Transcription is the synthesis of rRNA, tRNA, and mRNA, using the nucleotide
sequence information from DNA.
The RNA is synthesized as a complementary copy of one of the two DNA single
strands (template strand), in a process similar to DNA synthesis.
The template strand in a transcription bubble is read by RNA polymerase,
starting at an initiation site and ending at a termination site.
The error rate of RNA polymerase is less than 1 base in 10,000, much higher than
for DNA polymerase.
Transcription: The Big Picture
Transcription begins when RNA
polymerase recognizes a control
segment in DNA that precedes the
nucleotides to be transcribed.
Transcription: The Big Picture
RNA polymerase moves down
the DNA segment, adding
complementary nucleotides to
the growing RNA strand.
Transcription ends when the RNA
polymerase reaches a termination
sequence that signals the end of
the sequence to be copied.
Introns and Exons
After its initial synthesis, an RNA molecule is called primary transcript
RNA (ptRNA).
ptRNA is subsequently modified by posttranscriptional processing to
form m-, r-, and t-RNA’s. Some posttranscription processing occurs in the
nucleus and some in the cytoplasm.
An important part of posttranscriptional processing is the deletion of noncoding RNA segments (introns), and splicing together the coding segments
(exons).
Repressors and Inducers
Only a fraction of the DNA in the coding regions of any one cell is actually
expressed (~2%).
Repressor proteins turn off DNA synthesis coding for proteins not needed in a
particular cell type.
Inducer proteins turn on DNA synthesis for required proteins.
Note: The binding of some inducer and repressor proteins to DNA is
influenced through alteration of their three - dimensional structure by
interactions with hormone molecules.
Translation
Translation is the process of polypeptide
synthesis, mediated by rRNA, mRNA, and tRNA.
Translation occurs in the cytoplasm at
structures called ribosomes (which contain
rRNA).
mRNA binds to a ribosome and provides the
genetic message which specifies a particular
polypeptide sequence.
tRNA molecules carrying attached α-amino
acids supply the amino acids to be polymerized
into the peptide.
Translation: The Big Picture
Ribosomes consist of
two subunits, one large
and one small, and are
composed of rRNA
(60%) and protein
(mostly enzymes
required for protein
synthesis).
tRNA Structure
Translation: tRNA synthesis
Each amino acid is attached to a tRNA molecule by an enzyme
molecule called an aminoacyl-tRNA synthetase. This enzyme has
recognition sites for both the amino acid and the corresponding tRNA.
Translation: tRNA synthesis
Translation at the Molecular Level
Translation at the Molecular Level
Translation at the Molecular Level
Translation at the Molecular Level
Translation at the Molecular Level
Translation at the Molecular Level
Translation at the Molecular Level
Translation at Multiple Sites
Translation at Multiple Sites
The Genetic Code
The sequence in an mRNA is a coded sentence that
spells out the order in which amino acid residues
should be joined to form a protein.
Codon A sequence of three ribonucleotides that codes
for a specific amino acid or stops translation.
Genetic code The sequence of nucleotides, coded in
triplets (codons) in mRNA, that determines the
sequence of amino acids in protein synthesis.
The Genetic Code
Of the 64 possible three-base combinations in RNA, 61
code for specific amino acids and 3 code for chain
termination (the stop codons).
The “meaning” of each codon is the genetic code, and is
universal to all but a few living organisms.
Most amino acids are specified by more than one codon.
Codons are always written in the 5’ to 3’ direction.
The Genetic Code
DNA informational strand:
5’ ATG CCA GTA GGC CAC TTG TCA 3’
DNA template strand:
3’ TAC GGT CAT CCG GTG AAC AGT 5’
mRNA:
5’ AUG CCA GUA GGC CAC UUG UCA 3’
Protein:
Met
Pro
Val
Gly
His
Leu
Ser
Expansion of the Central Dogma
DNA-Directed-DNA-Polymerase
RNA-DirectedDNA-Polymerase
DNA-Directed-RNA-Polymerase
RNA-Directed-RNA-Polymerase