Download DNA Transcription and Protein synthesis

DNA
 Is a molecule that encodes the genetic instructions used in the
development and functioning of all known living organisms and
many viruses.
 Genetic information is encoded as a sequence of nucleotides
(guanine, adenine, thymine, and cytosine) recorded using the
letters G, A, T, and C. Most DNA molecules are double-stranded
helices, consisting of two long polymers of simple units called
nucleotides, molecules with backbones made of alternating sugars
(deoxyribose) and phosphate groups (related to phosphoric acid),
with the nucleobases (G, A, T, C) attached to the sugars.
DNA
 DNA is well-suited for biological information storage, since
the DNA backbone is resistant to cleavage and the doublestranded structure provides the molecule with a built-in
duplicate of the encoded information.
 The amounts of A = T, G = C, and purines = pyrimidines
[Chargaff’s Rule].
 DNA is a double-stranded helix with antiparallel strands
[Watson and Crick].
 Nucleotides in each
phosphodiester bonds
strand
are
linked
by
5’-3’
 Bases on opposite strands are linked by hydrogen bonding: A
with T, and G with C
DNA
DNA
DNA Transcription
Transcription
 Process by which a DNA sequence is copied to produce a
complementary RNA.
 In other words, it is the transfer of genetic information
from DNA into RNA.
 Like replication, but making RNA.
 Beginning of the process that ultimately leads to the
translation of the genetic code (via mRNA) into a protein.
DNA
DNA Transcription
DNA
 The enzyme used in transcription is “RNA polymerase”. There
are several forms of RNA polymerase. In eukaryotes, most
genes are transcribed
 RNA polymerase types :
 RNA polymerase I : it synthesizes the precursor of the 28s
,18s, and 5.8s r-RNA in the nucleolus
 RNA polymerase II : it synthesizes the precursor of m-RNA
in addition to srRNA
 RNA polymerase III : it produces the small RNA including
tRNA 5s ribosomal RNA and some snRNA
DNA Transcription
 Unlike replication, transcription does not need to
build on a primer. Instead, transcription starts at a
region of DNA called a “promoter”. For proteincoding genes, the promoter is located a few bases 5’
to (upstream from) the first base that is
transcribed into RNA.
 Promoter sequences are very similar to each other,
but not identical. If many promoters are compared,
a “consensus sequence” can be derived.
All
promoters would be similar to this consensus
sequence, but not necessarily identical.
DNA
Process of Transcription
DNA Transcription
The process of transcribing a typical gene in an eukaryotic cell
is divided into three phases : initiation , elongation and
termination .
Initiation
Eukaryotic RNA polymerase does not directly recognize the core
promoter sequences. Instead, a collection of proteins called
transcription factors mediate the binding of RNA polymerase
and the initiation of transcription. Only after certain
transcription factors are attached to the promoter does the
RNA polymerase bind to it. The completed assembly of
transcription factors and RNA polymerase bind to the promoter,
forming a transcription initiation complex. Transcription in the
archaea domain is similar to transcription in eukaryotes.
DNA
DNA Transcription
DNA
Initiation
DNA Transcription
Elongation
One strand of the DNA, the template strand (or noncoding
strand), is used as a template for RNA synthesis. As
transcription proceeds, RNA polymerase traverses the
template strand and uses base pairing complementarity
with the DNA template to create an RNA copy. Although
RNA polymerase traverses the template strand from 3' →
5', the coding (non-template) strand and newly formed RNA
can also be used as reference points, so transcription can
be described as occurring 5' → 3'. This produces an RNA
molecule from 5' → 3', an exact copy of the coding strand
(except that thymines are replaced with uracils, and the
nucleotides are composed of a ribose (5-carbon) sugar
where DNA has deoxyribose (one less oxygen atom) in its
sugar-phosphate backbone).[citation needed]
DNA
DNA Transcription
Elongation
 Unlike DNA replication, mRNA transcription can involve
multiple RNA polymerases on a single DNA template and
multiple rounds of transcription (amplification of
particular mRNA), so many mRNA molecules can be rapidly
produced from a single copy of a gene.[citation needed]
 Elongation also involves a proofreading mechanism that
can replace incorrectly incorporated bases. In eukaryotes,
this may correspond with short pauses during
transcription that allow appropriate RNA editing factors
to bind. These pauses may be intrinsic to the RNA
polymerase or due to chromatin structure
DNA
DNA Transcription
Termination
Eukaryotic protein genes contain a poly-A signal located
downstream of the last exon. This signal is used to add a
series of adenylate residues during RNA processing.
Transcription often terminates at 0.5 - 2 kb downstream of
the poly-A signal, but the mechanism is unclear.
DNA
DNA Transcription
The role of regulatory transcription factors
 In eukaryotes, the association between DNA and histones
prevents access of the polymerase and general transcription
factors to the promoter. Histone acetylation catalyzed by
HATs can relieve the binding between DNA and histones.
 Although a subunit of TFIID (TAF250 in human) has the HAT
activity, participation of other HATs can make transcription
more efficient. The following rules apply to most (but not all)
cases:
1. Binding of activators to the enhancer element recruits
HATs to relieve association between histones and DNA,
thereby enhancing transcription.
2. Binding of repressors to the silencer element recruits
histone deacetylases (denoted by HDs or HDACs) to
tighten association between histones and DNA.
DNA
Eukaryotic mRNA posttranscriptional
modification
DNA Transcription
 The mRNA molecule synthesized in eukaryotic nuclei by RNA
polymerase II is a collection of the precursor molecules of
mRNA called as heterogeneous nuclear RNA (hnRNA) . The
primary transcription are extensively modified in the nucleus
after transcription . these modification usually include :
 1_5 > capping : this process is the first of the processing
reaction for hnrna the cap is a 7-methylguanosine attached (
backward) to the 5’ –terminl end of the mRNA , forming an
unusual 5’_5’ triphosphate linkage . the creation of the
guanosine triphosphate part of the cap requires the nuclear
enzyme guanylyltransferase . methylation of this terminal
guanine occurs in the cytosol , and is catalyzed by guanine -7methyltransferase .the presence of this 7-methyl guanosine
triphosphate cap is very essential in starting the mRNA
translation later on ( i.e. protein synthesis )
DNA
Protein synthesis (Translation)
Translation is the RNA-directed synthesis of a polypeptide
Translation involves:
 Messenger RNA (mRNA): Carries information specifying
amino acid sequences of proteins from DNA lo ribosomes.
 Ribosomal RNA (rRNA): Plays catalytic (ribozyme) rotes
and structural roles in ribosomes.
 Transfer RNA (tRNA): Serves as adapter molecule in
protein synthesis; translates mRNA codons into amino
acids.
 Genetic coding – codons : Genetic information is encoded as
a sequence of nonoverlapping base triplets, or codons
DNA
 This is a molecule of messenger RNA.
 It was made in the nucleus by transcription from a DNA molecule.
 A ribosome on the rough endoplasmic reticulum attaches to the
mRNA molecule
DNA
 Another tRNA molecule comes into place, bringing a second amino
 Acid.
 Another tRNA molecule brings the next amino acid into place.
DNA
 A peptide bond joins the second and third amino acids to form a
Polyoeotide chain
 The process continues , the poly peptide chain get longer . This
continues until termination (stop) codon is reached, the poly
peptide is then completed
DNA
DNA
References:
World Wide Web ((Internet))
Student Contribution:
Zaid Thabit: Collection from references about DNA Transcription.
Zubaida Hamza: Collection from references about Translation.
Fajer Bashir: Made the PowerPoint presentation.