2. Transcription
• It is the synthesis of RNA using DNA as a template.
• The base sequence of RNA id the same as one of the
DNA, but uracil takes the place of thymine.
• Types of RNA:
• mRNA – messenger RNA
• tRNA – transfer RNA
• rRNA – ribosomal RNA
• ―One gene – one polypeptide‖ concept: one
polypeptide is synthesized using one type of mRNA
that is obtained by transcribing one gene.
3. Remember
DNA RNA
Deoxyribonucleic acid Ribonucleic acid
Deoxyribose sugar Ribose sugar
C2 no oxygen C2 bonds to one OH group and one H
Two polynucleotide chains One polynucleotide chain
Bases are: ATGC Bases are: AUGC
Uracil replaces Thymine
Because RNA is single-stranded, transcription occurs along
one strand only.
4. Formation of an RNA strand
• The RNA that is produced has a sequence that is
complimentary to the DNA that is used as a
template for transcription.
• The enzyme RNA polymerase binds to a site
called the promoter (specific base sequences)
• The region of DNA that is separated is the region
of the gene to be transcribed.
• Free nucleotides are in the nucleus space and
they pair up with DNA bases following the
sequence.
5. Formation of
RNA
• mRNA is a single polynucleotide chain but the
base thymine is replaced by Uracil.
• RNA polymerase forms covalent bonds between
nucleotides (sugars and phosphates).
• After the mRNA is complete, this "transcribed"
message, detaches from the DNA and leaves the
nucleus and directs the making of proteins in the
cytoplasm, while the DNA remains in the
nucleus.
• The DNA helix reforms.
6. Translation
• The base sequence of mRNA molecule is used as a
guide for assembling the sequence of amino acids
that will be a polypeptide a.k.a protein.
• The process of protein production using mRNA is
called translation.
• Much as English can be translated into Russian,
genetic translation converts nucleic acid language
into amino acid language.
• The flow of information from gene to protein is
based on codons. A codon is a three-base "word"
that codes for one amino acid. Several codons form a
"sentence" that translates into a polypeptide.
7. example
• Consider the following RNA sequence:
• UCGCACGGU
• This sequence would be read three bases at a
time as: UCG-CAC-GGU
• The codons represent the different amino acids:
• UCG-CAC-GGU
• Serine-Histidine-Glycine
8. The genetic code
• The
―translation
dictionary‖
is called the
genetic
code.
9. • Because there are 4 different bases, there are 64
possible three-base codons.
• The codon AUG can either specify methionine or
serve as the initiation ―start‖ codon.
• There are 3 ―stop‖ codons that don’t code for any
amino acid.
• The genetic code is universal in that it operates
the same way in nearly all living things on Earth.
10. Exercise your DNA
• 1. A certain gene has the following sequence:
• GACAAGTCCACAATC
• 2. From left to right, write the mRNA sequence
transcribed from this gene.
• 3. Using the genetic code, read the mRNA codons
from left to right, and write the amino acid sequence
of the polypeptide translated from the mRNA.
• 4. Repeat step 3 from right to left.
• Why did steps 3 and 4 produce different
polypeptides?
11. Translation
• It takes place in the ribosomes:
• An mRNA binds to the small subunit
of the ribosome.
• Following each codon, tRNA brings
the proper amino acid into the
ribosome
• tRNA has 3 unpaired bases called
anti-codon that are complimentary to
one mRNA codon.
12. "P" site, holds the tRNA
carrying the growing
polypeptide chain.
"A" site, holds a tRNA
carrying the next amino
acid to be added to the
chain.
13. • The start codon AUG dictates where translation
will begin.
This lengthening process continues until the ribosome
reaches a stop codon—UAA, UAG, or UGA. It releases the
newly formed polypeptide and the mRNA molecule.
14. Introns and exons
• Noncoding nucleotides:
interrupt nucleotide
sequences that actually code
for amino acids.
• Introns,inernalnoncoding
regions
• Exons, the coding regions
that will be translated.
• RNA splicing: intronsare
removed and the exons are
joined together
