Dna and rna_molecular_biology_part3_dent_april_2nd_2012

DNA/RNA…… part 3

By

Taghrid El Abaseri MD, PhD.

April 2nd 2012
SCU Dentistry School D1

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The Replication Fork

Only one strand of DNA is synthesized in a continuous manner in the
direction of overall DNA replication 5 to 3
The leading strand is the strand of DNA that is synthesized continuously in
the direction of movement of the replication fork.
Okazaki fragments are small pieces of newly synthesized DNA that are joined
to form an intact new DNA strand.
DNA ligase is an enzyme that seals breaks in DNA strands.
The lagging strand is the strand of DNA synthesized opposite to the direction
of movement of the replication fork, by ligation of Okazaki fragments.
Short fragments of RNA serve as primers for DNA replication.
Primase is an enzyme that synthesizes short fragments of RNA
complementary to the lagging strand template at the replication fork.
RNase H is an enzyme that degrades the RNA strand of RNA-DNA hybrids,
and 5 to 3 exonucleases.

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Synthesis of leading and lagging strands of DNA

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Initiation of Okazaki fragments with RNA primers

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Removal of RNA primers and joining of Okazaki fragments

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Expression of Genetic Information

Genes act by determining the structure of proteins, which are responsible for
directing cell metabolism through their activity as enzymes.

Proteins, in turn, are polymers of 20 amino acids, the sequence of which
determines their structure and function.

The first direct link between a genetic mutation and an alteration in the amino
acid sequence of a protein was made in 1957.
• Individuals with the inherited disease sickle-cell anemia have
hemoglobin molecules that differ from normal ones by a single amino
acid substitution.

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Colinearity of Genes and Proteins

The simplest hypothesis to account for the relationship between genes and
enzymes was that the order of nucleotides in DNA specified the order of
amino acids in a protein.

Another hypothesis predicted that different mutations within a single gene
could alter different amino acids in the encoded protein.

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Synthesis of RNA from DNA

Although the sequence of nucleotides in DNA
appeared to specify the order of amino
acids in proteins, it did not necessarily
follow that DNA itself directs protein
synthesis.
RNA appeared a likely candidate for such an
intermediate because the similarity of its
structure to that of DNA suggested that
RNA could be synthesized from a DNA
template.

RNA polymerase is the principal
enzyme responsible for RNA
synthesis.

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Eukaryotic RNA Polymerases and General Transcription Factors

eukaryotic cells contain multiple different RNA polymerases that transcribe
distinct classes of genes.

Transcription in eukaryotes takes place on chromatin rather than on free DNA.

Regulation of chromatin structure is an important factor in the transcriptional
activity of eukaryotic genes.

Eukaryotic cells contain three distinct nuclear RNA polymerases that
transcribe different classes of genes.
Although all three of the nuclear RNA polymerases recognize different
promoters and transcribe distinct classes of genes, they share several
features in common with each other

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Transcription in eukaryotes

A promoter is the DNA sequence to which RNA polymerase binds to initiate
transcription of a gene.

After transcription stops, the RNA is released from the polymerase, and the
enzyme dissociates from its DNA template.

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Transcription by RNA polymerase

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Processing of mRNA in Eukaryotes

Pre-mRNA is the primary transcript that is processed to form messenger RNA
in eukaryotic cells.

A 7-methylguanosine cap is what is added during the modification of the 5
end of a transcript.

A poly-A tail is a tract of about 200 adenine nucleotides added to the 3 ends
of eukaryotic mRNAs.

Polyadenylation is the process of adding a poly-A tail to a pre-mRNA.

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Processing of eukaryotic messenger RNAs

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The Role of Messenger RNA

The central dogma of molecular biology states that RNA molecules are
synthesized from DNA templates, and proteins are synthesized from RNA
templates.
• Transcription is the synthesis of an RNA molecule from a DNA
template.
• Translation is the synthesis of a polypeptide chain from an mRNA
template.

Messenger RNAs (mRNAs) are RNA molecules that serve as templates for
protein synthesis.

RNA polymerase is an enzyme that catalyzes the synthesis of RNA.

Ribosomal RNA (rRNA) is a component of ribosomes.

Transfer RNAs (tRNAs) serve as adaptor molecules that align amino acids
along the mRNA template.

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The Genetic Code

Genetic information is
transferred between
nucleic acids and
proteins.
The genetic code is the
correspondence that
takes place between
nucleotide triplets and
amino acids in
proteins.
Codons are the basic
units of the genetic
code.

Function of transfer RNA 16

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Translation of mRNA

Proteins are synthesized from mRNA templates by a process that has been
highly conserved throughout evolution.

Translation is carried out on ribosomes, with tRNAs serving as adaptors
between the mRNA template and the amino acids being incorporated into
protein.

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Structure of tRNAs

Transfer RNAs, or tRNAs, possess
unique identifying sequences
that allow the correct amino
acid to be attached and aligned
with the appropriate codon in
mRNA.
Transfer RNAs are approximately
70 to 80 nucleotides long and
have characteristic cloverleaf
structures.

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Transfer RNAs

Aminoacyl tRNA synthetases are a group of
enzymes that recognize a single amino acid, as
well as the correct tRNA (or tRNAs) to which that
amino acid should be attached.
After being attached to tRNA, an amino acid is
aligned on the mRNA template by complementary
base pairing between the mRNA codon and the
anticodon of the tRNA.

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Ribosome structure

Like tRNAs, rRNAs form characteristic secondary structures by
complementary base pairing.
The role of rRNA in the formation of peptide bonds extends the catalytic
activities of RNA beyond self-replication to direct involvement in protein
synthesis.

Ribosomes are the sites of protein
synthesis in both prokaryotic and
eukaryotic cells.

Ribosomal RNAs, or rRNAs, are the
ribosomal components of ribosomes.

The general structures of prokaryotic and
eukaryotic ribosomes are similar,
although they differ in some details.

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The Organization of mRNAs and the Initiation of Translation

The signals that identify initiation codons in eukaryotic cells.

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Overview of translation

Translation is generally divided into three stages:
initiation, elongation, and termination.

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Initiation of translation in eukaryotic cells

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Elongation stage of translation

After the initiation complex has
formed, translation proceeds by
elongation of the polypeptide
chain.
Elongation factors, which are
complexed to GTPs, escort the
aminoacyl tRNA to the ribosome.
The next step in elongation is
translocation, which requires
another elongation factor and is
coupled to GTP hydrolysis.

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Termination of translation

Release factors are proteins that
recognize stop codons and terminate
translation of mRNA.

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Dna and rna_molecular_biology_part3_dent_april_2nd_2012

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