Transcript Transcription & Translation

Transcription &
Translation
Chapter 17 (in brief)
Biology – Campbell • Reece
RNA vs. DNA

DNA…
– Four bases are
adenine,
guanine, cytosine
& thymine
– Sugar is
deoxyribose
– Is a doublestranded helix

RNA…
– Four bases are
adenine,
guanine, cytosine
& uracil
– Sugar is ribose
– Is a singlestranded helix
3 Types of RNA



Messenger RNA (mRNA) – carries the
instructions for making the protein
from the nucleus to the ribosomes
Ribosomal RNA (rRNA) – makes up a
ribosome
Transfer RNA (tRNA) – carries the
amino acids to the ribosomes
Transcription



The process of making RNA from DNA
The DNA serves as a template for
making the RNA
Similar base-pairing rules apply:
– Cytosine still pairs with Guanine
– But Adenine now pairs with Uracil
Transcription: The Process

RNA polymerase pries the two
strands of DNA apart and hooks
together the RNA nucleotides
– Moves 3’→5’ along the DNA template
strand


Promoter – the DNA sequence where
the RNA polymerase attaches to begin
transcription
Terminator – the sequence that
signals the end of transcription
Transcription: The
Process
3 steps to Transcription



Initiation – the RNA polymerase binds
to the promoter, DNA unwinds, RNA
synthesis begins
Elongation – the polymerase moves
along the DNA, elongating the RNA
Termination – the polymerase reaches
the terminator, transcription stops, the
RNA is released, polymerase detaches
Introns and Exons

Introns are noncoding regions of DNA or
RNA
– Introns are “in the way” or are intervening and
must be removed (RNA splicing)

Exons are the coded regions that are
expressed
Translation

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

The process of making the protein
from the instructions on the mRNA
Occurs in the ribosomes
Codon – a sequence of 3 bases that
codes for a specific amino acid
Anticodon – the complementary 3
base sequence on a tRNA molecule
Codons & Anticodons
Transcription: The Process


Initiation – the mRNA attaches to the
ribosome, the tRNA brings the first
amino acid
Elongation
– 1) codon recognition
– 2) peptide bond formation
– 3) translocation
Elongation
Transcription: The
Process

Termination – when a stop codon is
reached, the polypeptide detaches, the
mRNA leaves
– Stop codons – UGA, UUA, UAG
Substitution

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An incorrect base
is substituted for
a correct one
Can result in no
effect, missense
or nonsense
Insertion or Deletion


An extra nucleotide
is added or one is
missing
Results in
frameshifts that can
cause missense,
nonsense, an extra
amino acid, or a
missing amino acid