8.4 Transcription
L 8.4 : Transcription
Objectives:
1- Describe the relationship between
RNA and DNA
2- Identify the three kinds of RNA and
their functions
3- Compare transcription to replication
8.4 Transcription
Warm up!!
1- Summarize the process of replication by
using a cycle diagram.
2- DNA replication has a proofreading
function. What does it mean?
 After the discovery of
DNA structure.
Francis Crick
DEFINED the
central dogma.
 The central dogma
states that
information flows in
one direction from
DNA to RNA to
proteins.
Information flows in
one direction:
1. Replication copies
DNA
2. Transcription
converts DNA into
RNA
3. Translation
interprets RNA
message into string of
amino acids or
proteins
Central
Dogma
•
The central dogma includes three processes.
– Replication
– Transcription
replication
– Translation
transcription
• RNA acts as an
intermediate link
between DNA in the
nucleus and proteins in
the cytoplasm.
translation
What is a gene?
A section of DNA that contains instructions on
how to make a specific protein.
Each gene has a locus (location) on DNA
Alleles are different forms of a gene
DNA located in the nucleus only
What is transcription?
Process of copying a
sequence of DNA (gene) to
produce strand of RNA
RNA is a copy of a gene,
not the entire strand of
DNA.
Strand of RNA is
complementary to DNA
strand
Small segment can exit
nucleus into cytoplasm
• Like DNA, RNA is a chain of nucleotides.
• RNA differs from DNA in three major ways.
– RNA has a ribose sugar.
– RNA has uracil instead of thymine.
– RNA is a single-stranded structure.
Transcription makes three types of RNA.
Transcription copies DNA to make a
strand of RNA.
• Transcription is catalyzed by RNA polymerase.
– RNA polymerase and other proteins form a
transcription complex.
1- The transcription complex recognizes the
start of a gene and unwinds a segment of it.
start site
transcription complex
nucleotides
2- Nucleotides pair with one strand of the DNA.
RNA polymerase bonds the nucleotides together.
The DNA helix winds again as the gene is transcribed.
DNA
RNA polymerase
moves along the DNA
3- The RNA strand detaches from the DNA once the
gene is transcribed.
RNA
4. Complete mRNA strand breaks off
and moves out of nucleus.
• Transcription makes three types of RNA.
– Messenger RNA (mRNA) carries the message that
will be translated to form a protein.
– Ribosomal RNA (rRNA) forms part of ribosomes
where proteins are made.
– Transfer RNA (tRNA) brings amino acids from the
cytoplasm to a ribosome.
The transcription process is similar to
replication.
Transcription and replication both
involve complex enzymes and
complementary base pairing.
The two processes have different
end results.
 Replication copies
all the DNA;
transcription copies
a gene.
 Replication makes
one copy;
transcription can
make many copies.
one
gene
growing RNA strands
DNA
Practice transcribing……
Don’t forget to swap U for T
DNA: A C G T T A C A G
RNA: U G C A A U G U C
Define Replication:
Create the complementary strand for the DNA
sequence:
(original) A G C C T C A G T T C A G
(new)
Define Transcription:
Translate the complementary strand from above into
mRNA:
(new)
(mRNA)