Lesson 3 – Gene
Expression
Part 1 - Transcription
Warm-UP

Take turns going around your table saying the name of an enzyme or protein
involved in DNA Replication, and have someone else say what it does.
Learning Goals

Determine the gene to be the molecular unit of heredity.

Identify the structure of a gene as a stretch of DNA

Determine the first stage of expression as the transcription of DNA to mRNA.
Epidermolysis Bullosa



Butterfly Children
Genes code for
polypeptides. Mutations
in genes can cause
changes in polypeptide
sequence.
https://www.youtube.co
m/watch?v=Opot60OfjQQ
Jonathan Pitre has a genetic disorder
known as Epidermolysis Bullosa, that
can be traced back to a gene coding
for a protein that helps holds skin
cells together.
Gene Expression

Gene expression refers to
the transfer of genetic
information from DNA to
protein (where it is
expressed).

To be expressed, DNA is
transcribed to RNA and
then translated to protein.
This theory is called the
central dogma of
molecular biology.
The “Central Dogma of molecular
biology” is a phrase coined by Francis
Crick, in a sort of tongue-in-cheek
manner. It is not seen as strictly true
anymore.
A Two Step Process

In transcription: A
DNA strand serves
as a template for
the synthesis of an
mRNA strand.

In translation: An
mRNA strand
serves as a
template for the
synthesis of a
protein
Transcription of a DNA template produces an
RNA molecule that is a copy of the genetic
information. The nucleotide sequence of this
RNA molecule is then translated using the
genetic code so that the protein coded for by
the gene is produced.
The Structure of a Gene

Not every part of a gene is
transcribed to RNA, and not all that is
transcribed remains in the mature
mRNA strand.

Read the analogy for the structure of
a gene.
RNA Refresher

Transcription involves the
production of an RNA
molecule from a DNA
template. RNA is a singlestranded polymer of
nucleotides. Recall that it
contains the bases Adenine
(A), Uracil (U), Guanine
(G), and Cytosine (C).
RNA has a different
base (Uracil rather
than Thymine) and a
different sugar
(Ribose rather than
Deoxyribose). It is
more stable as a
single stranded
molecule.
Type of RNA

RNA serves more functions in a cell than DNA does, and
there are many types based on slight changes in structure
and sequence.
Template vs. Coding Strand

Remember that DNA is double
stranded, and only one strand of
DNA will be transcribed. This
strand is called the template
strand (as it serves as the
template for the RNA molecule).
The strand that is not transcribed
is called the coding strand (as it
shares the same code as the RNA
molecule).
In this example, the top DNA strand is th
coding strand and the bottom DNA stra
is the template strand. Note how the
coding strand shares the same code as
mRNA other than the thymines being
replaced by uracils.
Initiation of Transcription

RNA Polymerase is an
enzyme that catalyzes
the synthesis of RNA.
Transcription begins
when RNA polymerase
recognizes a
promoter region on
the DNA.

Once the RNA
polymerase complex
is bound to the DNA,
it unwinds and opens
a section of the
double helix.
The Promoter is just a recognition site that
allows Polymerase to bind and open the
double helix. It does not get transcribed into
mRNA.
Elongation

RNA Polymerase pairs
complementary NTPs to the
template strand, catalyzing
the creation of an RNA
strand.

RNA is more stable as a single
strand, and so it falls off,
and the DNA reanneals
behind it.
RNA polymerase can only build the new
RNA strand in a 5’ to 3’ direction (the same
way DNA polymerase III builds a DNA
strand).
Termination of Transcription

Specific nucleotide sequences
in the DNA template strand
serve as a termination signal
to transcription. When the
RNA Polymerase reaches the
signal, it detaches from the
DNA strand.

The new mRNA strand is
released from the
transcription assembly, and
the DNA double helix reforms
and remains unchanged
Review of Transcription

Chromosomes contain
thousands of genes, and they
can be transcribed repeatedly
and simultaneously.
mRNA in Eukaryotes

In Eukaryotes, the mRNA needs to move
from the nucleus to the
cytoplasm, but must first
undergo modification and
processing so it is recognized as
being useful outside the
nucleus.

Addition of 5’ Cap

Addition of 3’ Poly-A tail

Splicing out of intron sequences
5’ Cap

A special modified
guanosine nucleotide is
added to the 5’ end of
the RNA transcript so it
can be recognized by
ribosomes in the
cytoplasm.
Poly-A Tail

Hundreds of adenine
nucleotides are added on
the 3’ end of the mRNA
molecule. These help with
export from the nucleus, and
keep the mRNA from being
degraded in the cytoplasm.

Over time, the tail gets
shorter, eventually causing
the mRNA to be broken down
in the cytoplasm.
Intron Removal

Introns (Intervening
sequences) are removed
from the mRNA strand, and
the exons are joined
together. This is called
splicing, and is regulated
by proteins.

Exons can be joined
together in different ways,
meaning one gene can
create different mRNA
molecules, and thus
different polypeptides.
https://www.youtube.com/watch?v=itsb2SqR-R0
Review
1.
List three differences between DNA and RNA.
2.
How is RNA involved in protein synthesis?
3.
List and describe the three stages of transcription.
4.
Predict where mRNA is transported in the cell after
transcription is finished.
5.
How do cells synthesize multiple strands of mRNA from
one gene simultaneously? Why is this advantageous?
6.
Would errors in transcription be more or less damaging
than errors in DNA replication? Explain your answer.