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Structure and Function of
RNA
RNA- ribonucleic acid
 RNA is a single strand
 RNA is made up of smaller subunits called
nucleotides
 Nucleotides consist of:
Sugar-ribose
phosphate group
nitrogen base-4 types
Adenine-A
Uracil- U
Guanine- G
Cytosine- C
Adenine(A) pairs with Uracil(U)
Cytosine(C) pairs with Guanine(G)
and
There are 3 kinds of RNA
1. messenger(m) RNA- carries nucleotide
sequence from nucleus to ribosome
2. transfer(t) RNA-picks up amino acids
in cytoplasm and carries them to
ribosome
3. ribosomal(r) RNA- found in ribosome,
joins mRNA and tRNA
Transcription
 The process that makes messenger RNA
from DNA.
 Occurs in the nucleus.
 DNA unzips into 2 separate strands.
 Free floating RNA nucleotides in the
nucleus pair up with these DNA nucleotides
Cytosine(C) pairs with Guanine(G)
Uracil(U) pairs with Adenine(A)
 After all the pairing is done, a single
strand of RNA has been produced.
 It is complimentary to one side of the
DNA molecule.
 Genetic code from DNA is transferred
to mRNA.
 A codon is a set of 3 nitrogen bases on
mRNA.
Section 12-3
Figure 12–14 Transcription
Adenine (DNA and RNA)
Cystosine (DNA and RNA)
Guanine(DNA and RNA)
Thymine (DNA only)
Uracil (RNA only)
RNA
polymerase
DNA
RNA
Go to
Section:
Translation
 Conversion of mRNA into amino acid
sequence that makes up a protein.
 The mRNA leaves the nucleus and enters
the cytoplasm and attaches to a ribosome.
 The mRNA is read by ribosomes 3
nucleotides (codon- A G C) at a time.
tRNA (anticodon) brings the appropriate
amino acids to the ribosome.
 They are assembled in a long chain called a
protein.
Section 12-3
Figure 12–18 Translation
Nucleus
Messenger RNA
Messenger RNA is transcribed in the nucleus.
Phenylalanine
tRNA
The mRNA then enters the cytoplasm and
attaches to a ribosome. Translation begins at
AUG, the start codon. Each transfer RNA has
an anticodon whose bases are complementary
to a codon on the mRNA strand. The ribosome
positions the start codon to attract its
anticodon, which is part of the tRNA that binds
methionine. The ribosome also binds the next
codon and its anticodon.
Ribosome
Go to
Section:
mRNA
Transfer RNA
Methionine
mRNA
Lysine
Start codon
Figure 12–18 Translation
(continued)
Section 12-3
The Polypeptide “Assembly Line”
The ribosome joins the two amino acids—
methionine and phenylalanine—and breaks
the bond between methionine and its tRNA.
The tRNA floats away, allowing the ribosome
to bind to another tRNA. The ribosome moves
along the mRNA, binding new tRNA molecules
and amino acids.
Lysine
Growing polypeptide chain
Ribosome
tRNA
tRNA
mRNA
Completing the Polypeptide
mRNA
Ribosome
Go to
Section:
Translation direction
The process continues until the ribosome reaches
one of the three stop codons. The result is a
growing polypeptide chain.
Practice
DNA code
mRNA code
A
U
U
A
C
G
C
G
G
C
G
C
T
A
A
U
A
U
U
A
G
C
A
U
Section 12-3
Go to
Section:
DNA
mRNA
tRNA
CAT
GUC
AAU
AUG
ACG
TAT
GAU
Amino Acid