Download Topic 3.5 Transcription (9-13)

TOPIC 3.5: TRANSCRIPTION AND
TRANSLATION (PROTEIN
SYNTHESIS)
ASSESSMENT STATEMENTS
3.5.1 Compare the structure of RNA and DNA
 3.5.2 Outline DNA Transcription in terms of the
formation of an RNA strand complementary to
the DNA strand by RNA polymerase
 3.5.3 Describe the genetic code in terms of codons
composed of triplets of bases
 3.5.4 Explain the process of translation, leading
to polypeptide formation
 3.5.5 Discuss the relationship between one gene
and one polypeptide

PROTEIN SYNTHESIS INTRODUCTION

The control that DNA has over a cell is by a
process called Protein Synthesis

DNA controls the proteins produced in a cell


Some of these proteins are Enzymes
The Production or Lack of Production of a particular
enzyme can have a dramatic effect on the overall
biochemistry of the cell

DNA indirectly controls the biochemistry of carbohydrates,
lipids, and nucleic acids by the production of enzymes
PROTEIN SYNTHESIS INTRODUCTION

Protein synthesis has two major sets of reactions
Transcription (DNA to RNA)
 Translation (RNA to Polypeptides (protein))


Both Processes require RNA
COMPARING DNA AND RNA
DNA
RNA
Contains a 5 carbon sugar
Contains a 5 carbon sugar
5 carbon sugar is deoxyribose
5 carbon sugar is ribose
Each nucleotide has one of four
nitrogenous bases
Each nucleotide has one of four
nitrogenous bases
The nitrogenous bases are
cytosine, guanine, adenine, and
thymine
The nitrogenous bases are
cytosine, guanine, adenine, and
uracil
Double-stranded molecule
Single-stranded molecule
TRANSCRIPTION PRODUCES RNA MOLECULES

Genes-are the sections of DNA that codes for
polypeptides


DNA is found inside nucleus but proteins are
synthesized outside the nucleus in the cytoplasm


Each gene is a specific sequence of nitrogenous bases
found in a DNA molecule
Messenger RNA (mRNA) is an intermediary
molecules which caries the message of DNA to the
cytoplasm where he enzymes, ribosomes, and amino
acids are found
In the nucleoplasm contains free RNA
nucleotides along with free DNA nucleotides
TRANSCRIPTION PRODUCES RNA MOLECULES

Transcription Process:

Begins when an are of DNA
of one gene becomes
unzipped (similar to the
unzipping of DNA
replication)
Only one strand of the two
DNA strands will be used as a
template to create the mRNA
molecule.
 RNA polymerase—enzyme
used as the catalyst for this
process
 RNA polymerase moves along
the strand of DNA acting as
the template, nucleotides float
into place by complementary
base pairing

TRANSCRIPTION PRODUCES RNA MOLECULES

The complementary base pairs are the same as in
double-stranded DNA, which an exception that
adenine on DNA is now paired with uracil on the
newly forming mRNA.
TRANSCRIPTION PRODUCES RNA MOLECULES

Facts about Transcription:
Only one of the two strands of DNA is ‘copied’, the
other strand is not used
 mRNA is always single-strand and shorter than the
DNA that it is copied form as it is a complementary
copy of only one gene
 The presence of thymine in a molecule identifies it as
DNA
 The presence of uracil in a molecule identifies it as
RNA

THE GENETIC CODE IS WRITTEN IN TRIPLETS

The mRNA molecule produced by transcription
represents a complementary copy of one gene of
DNA
This sequence of nucleotides making up the length of
the mRNA is typically enough information to make
one polypeptide
 The message written into the mRNA molecule is the
message that determines the order of the amino acids

THE GENETIC CODE IS WRITTEN IN TRIPLETS

The Genetic Code is written in a language of
three bases
Three bases is enough to code for 1 of the 20 amino
acids
 Triplet-any set of three bases that determine the
identity of one amino acids
 When a triplet is found on the mRNA molecule, it is
called a codon or codon triplet

TRANSLATION RESULTS IN THE PRODUCTION
OF A POLYPEPTIDE

There are three different types of RNA molecules.

They are all single stranded and each is transcribed
from a gene of DNA
mRNA: each mRNA is a complementary copy of
DNA gene and is enough genetic information to
code for a single polypeptide
 rRNA: ribosomal RNA, each ribosome is compsed
of rRNA and ribosomal protein
 tRNA: transfer RNA, each type of tRNA transfers
1 of the 20 amino acids to the ribosome for
polypeptide formation

TYPICAL TRNA

The three bases in the
middle loop are called the
anticodon bases and they
determine which of the 20
amino acids is attached to
the tRNA
TRANSLATION RESULTS IN THE PRODUCTION
OF A POLYPEPTIDE

Once an mRNA molecule has been transcribed,
the mRNA detaches from the single-strand DNA
template and floats free in the nucleoplasm.

At some point, the mRNA will float through one of
the many holes in the nuclear membrane (nuclear
pores) and will then be in the cytoplasm
TRANSLATION PROCESS
The mRNA will locate a ribosome and align with
it so that the first two codon triplets are within
the boundaries of the ribosome
 The next step is the introduction of the tRNA.
The tRNA must be complementary to the first
codon triplet of the mRNA molecule

Than, the first amino acid is brought into the
translation process
 While the first tRNA ‘sits’ in the ribosome holding
the first amino acid

The second tRNA floats in and brings a second (specific)
amino acid
 The second tRNA matches its three anticodon bases with
the second codon triplet of the mRNA

TRANSLATION PROCESS

Two specific amino
acids are now being
held side by side

An enzyme now
catalyses a
condensation reaction
between the two amino
acids and the resulting
covalent bond between
them is called a
peptide bond
TRANSLATION PROCESS

The next step in the
translation process
involves the
breaking of the bond
between the first
tRNA molecule and
that amino acid that
it transferred in

The first tRNA floats
away into the
cytoplasm and
invariably reloads
with another amino
acid of the same type
TRANSLATION PROCESS

The ribosome that has
only one tRNA in it now
moves one codon triplet
down the mRNA
molecule
This creates room for a
third tRNA to float in
bringing with it a third
specific amino acid
 This process continues
until the ribosome gets to
the last codon triplet—the
final codon triplet will not
code for an amino acid,
but signals a stop codon

TRANSLATION PROCESS

The entire
polypeptide breaks
away from the final
tRNA molecule, and
becomes a free
floating polypeptide
in the cytoplasm of
the cell
THE ONE GENE/ONE POLYPEPTIDE
HYPOTHESIS
Beadle and Tatum set out to provide
experimental proof of the connection between
genes and enzymes.
 They hypothesized that if there really was a oneto-one relationship between genes and specific
enzymes, it should be possible to create genetic
mutants that are unable to carry out specific
enzymatic reactions.

THE ONE GENE/ONE POLYPEPTIDE
HYPOTHESIS

In the last few years, researchers have discovered
that at least some genes are not quite that
straight forward.

For example, one gene may lead to a single mRNA
molecule, but the mRNA molecule may be modified in
many different ways

Each modification may result in the production of a
different polypeptide during the translation portion of
protein synthesis