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Transcription
DNA g mRNA
Objectives
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Explain the purpose of transcription for a
cell
Tell how RNA differs from DNA
Name the three main types of RNA
Describe the process of transcription
Review

What was the purpose for DNA replication?
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What are the Nitrogen bases of DNA?
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A, G, C, T
What is the sugar for DNA?

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So cell division (mitosis & meiosis) can occur and
each new cell created has a full set of DNA
deoxyribose
What does the structure of DNA allow?

The strands are complementary of one another,
meaning one strand is a template for the other.
Central Dogma
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Soon after there discovery of
DNA structure, Watson and
Crick, stated that information
in molecular biology flows in
one direction. From DNA to
RNA to proteins.
The central dogma involves
three process:
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Replication copies DNA
Transcription converts a DNA
message into a temporary
‘disposable’ copy of DNA called
RNA
Translation interprets an RNA
strand into a string of Amino Transcription video
Acids which will turn into
specific proteins.

What is RNA?
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3 main differences
between DNA &
RNA?
RNA is like a “disposable copy” of
a segment of DNA.
RNA consists of long chains of
nucleotides (like DNA)
 Parts of a Nucleotide:
 5 carbon sugar
 Phosphate group
 Nitrogenous base
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RNA sugar- Ribose
RNA is single stranded
RNA contains Uracil (U) in
place of Thymine
Transcription makes 3 types of RNA
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Messenger RNA: mRNA
Ribosomal RNA: rRNA
Transfer RNA: tRNA
Function of mRNA
• Carries copies of gene
instructions for assembling amino
acids into proteins from nucleus
to ribosome (transcription)
Function of rRNA
 Forms part of ribosomes, a
cell’s protein factory
Large subunit
binds to tRNA
Binding Sites
Small subunit
binds to mRNA
Function of tRNA

Brings amino acids from the
cytoplasm to a ribosome to help
make the growing protein
Amino Acid
anticodon
Transcription
TRANSCRIPTION :The process of copying a
sequence of DNA to produce a complementary
(disposable) strand of RNA
•
•
•
•
During this process, a GENE – not an entire
chromosome – is transferred into an mRNA.
The cell needs this process because DNA has the
information/code needed to make proteins AND
DNA is too valuable to leave the nucleus, so it
makes a disposable copy!
THIS IS A MAJOR PART IN PROTEIN
SYNTHESIS!!!!!
Overview of Protein Synthesis:
• Ribosomes are made in nucleolus
•Ribosomes leave through the Nuclear pores on the nuclear membrane
•Some attach to ER, while others remain free floating
•Ribosomes begin attaching Amino Acids together to form proteins
•Proteins enter into RER to be modified and folded (shaped) into structures
•Vesicles form from RER and carry proteins to Golgi apparatus
•There proteins are
packaged, sorted and/ or
stored in Golgi
•Once the proteins are
ready to leave, vesicles
form to transport proteins
throughout the cell and to
other cells.

What are genes?

Genes are coded DNA
instructions that
control the production
of proteins within a
cell.
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The code is held with
the sequence of
nitrogen bases in DNA.
Like credit card
numbers, unique to
every person!
REMEMBER: that a
chromosome is made
up of multiple genes,
and transcription only
copies specific genes,
1 at a time.
Step 1:

RNA polymerase binds to promoters on
DNA strand and begins to unwind a
segment of DNA that corresponds to the
gene it is copying.
start site
transcription complex
nucleotides
Step 2:
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RNA polymerase, using only one strand of DNA as a
template, adds on free-floating RNA nucleotide
As RNA polymerase moves along the DNA, a
growing RNA strands hangs freely as it is
transcribed, and the DNA helix zips back together.
*A - U, C – G there are NO T’s
DNA
RNA polymerase
moves along the DNA
Step 3:
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Once the entire gene has been transcribed, the RNA
strand detaches completely from the DNA.
Exactly how RNA polymerase recognizes the end of
a gene is very complicated but we will discuss as it
reaching a Stop signal.
RNA polymerase
RNA polymerase is an enzyme that plays
multiple roles in transcription process
•
•
•
Unzips the DNA molecule where gene is found
Adds free floating nucleotides to make mRNA
strand
What it DOESN’T do is proof read it’s work
like DNA polymerase.
How does RNA
polymerase know
where to start and stop?
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Promoter sites- which
have specific nitrogen
base sequences (TAC)
Promoters are signals
in DNA to indicate
where an enzyme can
bind to make RNA
Stop signals/sites –
specific sequence of
nitrogen bases that
tell the RNA
polymerase to stop
and fall off.
one
gene
growing RNA strands
replication
transcription
DNA
translation