RNA (Ribonucleic acid)

1
RNA (Ribonucleic acid) Structure Similar to
that of DNA except 1- it is single stranded
polyunucleotide chain. 2- Sugar is ribose 3-
Uracil is instead of thymine There are 3 types
of RNA 1- Ribosomal RNA (rRNA) 2- Messenger RNA
(mRNA) 3- Transfer RNA (tRNA) RNA are copies
from DNA sequences formed by a process called
transcription. After transcription some
modifications occur to obtain the three types of
RNA.
2

• 1- Ribosomal RNA (rRNA)
• 80 of total RNA in the cells are rRNA.
• rRNA are found in combination with several
  proteins (about 82 proteins) to form ribosome
  which is the site of protein synthesis.
• In Eucaryotic ( mammals),there are 4 size types
  of rRNA (5S, 5.8S, 18Ss and 28S)
• representing 2/3 particle mass of the ribosome.

3

• 2- Messenger RNA (mRNA)
• comprised only 5 of total cellular RNA.
• Function It is the type of RNA that carry the
  gene of specific protein.
• it carries the genetic information (code of the
  protein) from DNA (in the nucleus) to ribosomes
  (in cystol) where it is used in protein
  biosynthesis.
• NB Three nucleotide bases on mRNA form a codon
  which is then translated into specific amino
  acid.

4
3- Transfer RNA (tRNA) tRNA represents 15 of
total RNA in the cell. Structure 1- amino acid
attachment site or amino acid acceptor at 3 end
which terminates with the triplet CCA. 2-
Anticodon loop or anticodon triplet Functions of
tRNA 1- transport amino acids to ribosome for
protein synthesis. Each tRNA carry only one
amino acid. The specific amino acid is attached
enzymatically to 3' end of tRNA. 2- Help to
ensure the insertion of (adding) the correct
amino acid in the polypeptide chain. This
function is due to anticodon triplet which
recognize the specified codon on mRNA and binds
to it by base pairing.
5
Structure of tRNA
Funtions of tRNA
6
(No Transcript)
7
(No Transcript)
8
Gene expression Gene expression is the process
by which information from a gene is used in the
synthesis of a functional gene product. These
products are often proteins. Gene expression in
two major steps 1-Transcription Synthesis of
mRNA from DNA. By this process, the gene carried
on DNA will transferred into RNA. 2-Translation
The gene on mRNA is translated into a functional
protein Transcription DNA ? RNA Translation
RNA ? protein
9

• Transcription
• The enzyme responsible for transcription is RNA
  polymerase II
• Steps in RNA synthesis (Watch Movie)
• Initiation
• The transcription is initiated by the binding of
  RNA polymerase to a specific region of DNA double
  helix. This site is called promoter site or
  promoter region. This region is recognized by
  sigma factor (subunit) of RNA polymerase.
• When RNA polymerase recognizes this region, it
  binds to it leading to a local unwinding
  (separation) of the promoter region into 2 single
  strands

10
a- DNA strand that is transcripted into mRNA and
called template strand or antisense strand. b-
The other strand is coding strand or sense strand
that contains gene to be translated (This strand
not transcripted, not used) Direction of
transcription RNA polymerase will read the
information sequence on DNA template from 3' ? 5'
direction, so RNA is synthesized antiparallel to
DNA template i.e. from 5' ? 3' direction. 2)
RNA elongation Once RNA polymerase recognizes
promoter region, it begins to synthesize a
transcript (copy) of DNA template. 3)
Termination Process of elongation of RNA
continues until reach what is called
termination region which is recognized by rho
factor (subunit in RNA polymerase) resulting in
release of the enzyme, and the synthesized RNA
11
(No Transcript)
12
Notes 1- The synthesized RNA will have the
sequence of the sense strand except for U instead
of T. 2- In prokaryotic (bacteria) all
types of RNA are synthesized by only one species
of RNA polymerase. 3- In Eukaryotic ( mammaians),
there are 3 classes of RNA polymerase a- RNA
polymerase I synthesizes the precursor of rRNA
named pre rRNA b- RNA polymerase II
synthesizes pre mRNA c- RNA polymerase III
synthesizes pre tRNA All these enzymes
synthesize what is called primary transcript or
immature RNAs (pre form) which by some
modifications occur after transcription, will
give the mature rRNA, mRNA and tRNA.
13
Question Compare between transcription in
prokaryotics (bacteria) and eukaryotics (human)?
14
Post-transcriptional modifications of mRNA OR
called mRNA processing After transcription, the
formed immature mRNA will undergo the following
modifications to be mature and functioning 1)
5'-capping 5'- end in the first nucleotide is
blocked by 7-methyl guanosine triphosphate (7
methyl-GTP). Role of cap a- help to stabilize
and protect mRNA from degradation by 5'
exonucleases in the cytoplasm b- Permit
initiation of translation (specifies, where
translation should begin).
15
(No Transcript)
16
2) Poly A tail chain of about 50-250 AMP is
attached to 3'- end is added after
transcription Role of poly A Help to stabilize
mRNA and facilitate its exit from the nucleus.
Also protect mRNA from degradation in cytoplasm
when move from nucleus (cytoplasm contains
3exonuclease that degrades mRNA).

17
mRNA ready for splicing (what is splicing? , see
next slides)
18
3- RNA splicing The gene carried now on primary
mRNA contains two types of regions a- coding
regions called (exons) which will be translated
into a functional protein. b- non coding regions
(not code for amino acids of the target protein).
These regions called introns. The gene or mRNA
to be translated, it must contain only coding
sequence ( i.e. exons), for this reason RNA
splicing is essential to produce a correct
protein by translation. RNA splicing (or called
gene splicing) is the process by which introns
are removed and exons are joined (spliced) to
produce a typical mRNA for translation. Splicing
is catalyzed by enzyme called spliceosome which
is a group of nucleoproteins. Splicing is a
complex process and involve certain mechanism.
Briefly, after a two-step enzymatic reaction, the
intron is removed and two neighboring exons are
joined together (movie for explanation only)
19
(No Transcript)
20
RNA splicing (I)
21
RNA splicing (II)
22
RNA splicing (III)
23
(No Transcript)
24
(No Transcript)