12-3 Notes
RNA and Protein Synthesis
Information, Please
Section 12-3
• DNA contains the information that a cell needs to carry out
all of its functions. In a way, DNA is like the cell’s
encyclopedia. Suppose that you go to the library to do
research for a science project. You find the information in
an encyclopedia. You go to the desk to sign out the book,
but the librarian informs you that this book is for reference
only and may not be taken out.
1. Why do you think the library holds some books for reference only?
2. If you can’t borrow a book, how can you take home the information in it?
3. All of the parts of a cell are controlled by the information in DNA, yet
DNA does not leave the nucleus. How do you think the information in
DNA might get from the nucleus to the rest of the cell?
Go to
Section:
12-3 Vocabulary
• Messenger RNA- (mRNA)
– RNA molecule that carries
copies of instruction for the
assembly of amino acids into
proteins from DNA to the
rest of the cell.
• Ribosomal RNA- (rRNA)
– Type of RNA that makes up
the major part of the
ribosomes.
12-3 Vocabulary
• Transfer RNA (tRNA)
– Type of RNA molecule that
transfer amino acids to
ribosomes during protein
synthesis.
• Transcription– Process in which part of the
nuclotide sequence of DNA is
copied into complemetary
sequence in RNA.
Adenine (DNA and RNA)
Cystosine (DNA and RNA)
Guanine(DNA and RNA)
Thymine (DNA only)
Uracil (RNA only)
RNA
polymerase
DNA
RNA
12-3 Vocabulary
• RNA polymerase– Enzyme similar to DNA
polymerase that binds to DNA and
separates the DNA strands during
transcription.
• Promoter
– Region of DNA that indicates to
an enzyme where to bind to make
RNA.
12-3 Vocabulary
• Intron– Intervening sequence of DNA; does not code
for a protein.
• Exon– Expressed sequence of DNA; codes for a
protein.
1. The Structure of RNA
a. It consists of long chains of nucleotides, like
DNA
b. There are three main differences from DNA
RNA
DNA
1) Sugar is ribose
Sugar is deoxyribose
2) Single stranded
Double stranded
Contains uracil
Contains thymine
2.Types of RNA
a. Main function is protein synthesis
b. There are three main types of RNA
1)
Messenger RNA (mRNA): carry copies of
instructions for assembling amino acids
2)
Ribosomal RNA (rRNA): located on ribosomes,
where proteins are assembled
3)
Transfer RNA (tRNA): transfers each amino acid
to the ribosome as specified by mRNA
3. Transcription
a. Transcription: the DNA
nucleotide sequence is
copied into a complementary
RNA sequence
b. The DNA strands unzips, and
assembles nucleotides to
form an RNA strand from the
DNA template
c. mRNA is edited before
leaving the nucleus
d. Introns (intervening
sequences not needed in the
code) are cut out leaving
exons (expressed sequences
needed in the code)
e. mRNA is sent to the
ribosome
• Codon-
12-3 Vocabulary
– Three-nucleotide sequence on messenger RNA that
codes for a single amino acid.
• Translation– Decoding of mRNA message into a polypeptide chain.
• Anticodon
– Group of three bases on a tRNA molecule that are
complementary to an mRNA codon.
12-3 Part 2
1.Translation is the decoding of an mRNA
message into a polypeptide chain
(protein)
2.There are four main steps to translation
a. mRNA must be transcribed from DNA in the
nucleus and released into the cytoplasm
b. mRNA attaches to the ribosome at a start codon
1)
As each mRNA codon goes through the
ribosome, the proper amino acid is attached using the
genetic code
2)
The Genetic Code is a universal “language” for
reading mRNA
3)
tRNA molecules have amino acids attached to
one end and three unpaired bases at the other
4)
The three bases, called the anticodon, are
complementary of the mRNA codon
5)
The codons fit in place
Nucleus
Messenger RNA
Messenger RNA is transcribed in the nucleus.
Phenylalanine
tRNA
mRNA
Transfer RNA
Methionine
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
mRNA
Lysine
Start codon
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
Translation direction
The process continues until the ribosome reaches
one of the three stop codons. The result is a
growing polypeptide chain.
c. Polypeptide chain grows until ribosome
reaches a stop codon on the mRNA
1) It releases the mRNA, completing translation
2) This protein molecule can range from 50 to
3,000 amino acids
Concept Map
Section 12-3
RNA
can be
Messenger RNA
also called
which functions to
mRNA
Go to
Section:
Ribosomal RNA
Carry instructions
also called
which functions to
rRNA
Combine
with proteins
from
to
to make up
DNA
Ribosome
Ribosomes
Transfer RNA
also called
which functions to
tRNA
Bring
amino acids to
ribosome