Download Chapter 17: RNA

RNA
Ribonucleic Acid
A. RNA has ribonucleotides which
have the sugar ribose and the
nitrogenous bases A, C, G, and
Uracil. RNA is single stranded.
B. The 4 bases act like letters of the
alphabet to communicate genetic
information
C. Transcription- the process of
copying the genetic message on the
DNA into a messenger RNA,
mRNA.
a. The sequence of nucleotides in
a gene carries the information for
the primary structure of a protein.
ie, the linear sequence of a.a. on
the protein
b. In transcription, a gene on the
DNA strand provides a template
for the synthesis of a
complimentary RNA strand.
D. Translation- the process of
synthesizing protein using the code
(base sequences) on the mRNA.
This process requires ribosomes,
rRNA, and tRNA, transfer RNA.
E. Transcription occurs in the nucleus
and translation in the cytoplasm
DNA  RNA  Protein
F. Triplet Code- Nucleotide triplets
(3 consecutive bases) specify each
of the 20 amino acids. Therefore,
There are 43 = 64 possible triplets
G. 1. During transcription, one DNA
strand, the template strand,
provides a template for ordering the
sequence of nucleotides of one
gene, in an mRNA transcript.
2. blocks of three nucleotides base
sequences on the mRNA are the
codons
3. Then during translation, the
codons, are decoded into a sequence
of a.a. on a protein.
4. Because codons are base triplets,
the number of nucleotides making
up genetic messages must be three
times the number of a.a. making up
the protein product.
5. a. the genetic code is
redundant (more than one
triplet code for a particular a.a)
b. the genetic code is not
ambiguous (one triplet does not
code for more than one a.a.)
c. codons synonymous for the
same a.a. usually differ in the
third base position, only.
d. the AUG codon indicates the
start of translation
e. UAA, UAG, and UGA
codons indicate the termination
of translation
H. The genetic code is universal for all
species  it must have evolved early.
I. Steps of Transcription: 3 stages:
Initiation, Elongation, Termination
1.Initiation
Specific sequences of the DNA template
strand determine where gene
transcription begins and ends.
a. RNA polymerase attaches and
initiates transcription of a gene at
the promotor region on the DNA
template strand.
(1) transcription factors
recognize the promotor
region’s TATA box & bind
(2) then RNA polymerase binds
to the transcription factors
to create a transcription
initiation complex
2.Elongation- RNA polymerase moves
along the DNA, it untwists the double
helix about 10-20 base at a time.
(1) RNA polymerase adds
nucleotides to the 3 end of the
growing RNA strand, elongating
the transcript 5 3.
(2) As RNA polymerase leaves
behind the point of synthesis, the
DNA double helix re-forms.
3. Termination – the transcription of
one of the 3 stop codons: UAA, UAG,
UGA, signals the end of transcription.
(1) RNA Polymerase continues until
the end of the terminator
sequence: AAUAAA
(2) RNA polymerase detaches and the
(3) ‘pre mRNA’ is released
J. The pre-mRNA is modified after
transcription
1. 5 end of the pre-mRNA gets a
guanine 5 cap. This serves as the
attachment signal or ribosomes.
2. at the 3 end 50-250 adenine
nucleotides are added- the poly-A-tail,
this helps to export mRNA from the
nucleus
3. the mRNA has nontranslated leader
and trailer segments, which fringe the
coding segment.
4. Introns are excised. Introns are non
coding regions that lie between coding
regions
5. Exons are spliced together. Exons are
coding regions that are translated into
a.a. sequences.
6. Spliceosome- has snRNP small
nuclear ribonucloeproteins that act as
ribozymes (RNA enzymes) that help
remove the introns and join the exons to
create mRNA
K. Translation- Protein Synthesis
1. Materials needed:
a. mRNA
b. (1) tRNA – transfer RNA
transfers aa to the ribosome’s
growing polypeptide.
(2) A tRNA has an anticodon
that is complimentary to the
codon on the mRNA.
(3) The anticodon base-pairs
with a complimentary codon on
the mRNA and its a.a. is added
to the growing polypeptide.
(4) The codon sequence on the
mRNA determines the sequence
in which tRNAs come are placed
(5) Wobble: the anticodons of
tRNA recognize more than one
codon, because it base pairs with
only the first two bases of the
mRNA, and wobbles for the
third base
c. ribosomes(1) each ribosome is made up of
two subunits, composed of
proteins and rRNA
(2) each ribosome has a binding
site for mRNA and three
binding sites for tRNA
(3) RNA acts as a catalyst for
for peptide bond formation
2. Translation has three stages:
a. Initiation- joins the mRNA,
tRNA, and ribosome. Energy is
provided by GTP
b. Elongation- three steps to add
amino acids
(1) codon recognition
hydrogen bonding between
the mRNA codon under the
A-site with the
corresponding anticodon
on the tRNA
(2) peptide bond formation,
the new a.a. on the A-site is
bonded to the a.a. of the
polypeptide on the P-site,
this is catalyzed by rRNA.
(3) Translocation-the ribosome
moves the tRNA with the
attached polypeptide from
the A site to the P site. The
mRNA moves along with it.
Therefore mRNA is read5’
to 3’ codon by codon.
c. Termination- occurs when one
of the 3 stop codons reaches the
A site.
(1) the polypeptide is freed
(2) the translation complex
disassembles
3. Polyribosomes- many ribosomes
translate a single mRNA simultaneously