Title: 14.3 How Is the Information Content in DNA Transcribed to Produce RNA?
114.3 How Is the Information Content in DNA
Transcribed to Produce RNA?
- Transcription requires
- A DNA template for base pairings
- The four ribonucleoside triphosphates
(ATP,GTP,CTP,UTP) - An RNA polymerase
- Salts and pH buffer, if done in a test tube
214.3 How Is the Information Content in DNA
Transcribed to Produce RNA?
- Transcription produces mRNA, tRNA, and rRNA.
- These RNAs are encoded by specific genes.
- Eukaryotes also make several small RNAs,
including small nuclear RNA (snRNA), microRNA
(miRNA), and small interfering RNA (siRNA).
3Table 14.1
414.3 How Is the Information Content in DNA
Transcribed to Produce RNA?
- RNA polymerases catalyze synthesis of RNA
- Catalyze addition of nucleotides in a 5'-to-3'
direction - Processiveone enzyme-template binding results in
polymerization of hundreds of RNA bases - They do not need primers
5Figure 14.3 RNA Polymerase
614.3 How Is the Information Content in DNA
Transcribed to Produce RNA?
- Transcription occurs in three phases
- 1. Initiation
- RNA polymerase binds to a DNA sequence called a
promoter. - Promoters tell the enzyme where to start and
which strand of DNA to transcribe. - The promoter has an initiation site where
transcription begins.
7Figure 14.4 DNA Is Transcribed to Form RNA (A)
8Figure 14.4 DNA Is Transcribed to Form RNA (A)
914.3 How Is the Information Content in DNA
Transcribed to Produce RNA?
- Sigma factors and transcription factors are
proteins that bind to DNA sequences and to RNA
polymerase. - They help direct the polymerase onto the
promoter, and help determine which genes are
expressed at particular times.
1014.3 How Is the Information Content in DNA
Transcribed to Produce RNA?
- 2. Elongation
- RNA polymerase unwinds DNA about 10 base pairs at
a time reads template in 3' to 5' direction. - The transcript is antiparallel to the DNA
template strand. - RNA polymerases do not proofread and correct
mistakes.
11Figure 14.4 DNA Is Transcribed to Form RNA (B)
1214.3 How Is the Information Content in DNA
Transcribed to Produce RNA?
- RNA polymerase uses (ribo)nucleoside
triphosphates (NTPs) as substrates. - Two phosphate groups are removed from each
substrate molecule the energy released is used
to drive the polymerization reaction.
1314.3 How Is the Information Content in DNA
Transcribed to Produce RNA?
- 3. Termination
- Specified by a specific DNA sequence.
- Mechanism in eukaryotes is not well understood.
- In bacteria, the transcript forms a loop and
falls away from the DNA or a helper protein
binds to the transcript and causes it to detach
from the DNA.
14Figure 14.4 DNA Is Transcribed to Form RNA (C)
1514.3 How Is the Information Content in DNA
Transcribed to Produce RNA?
- The genetic code specifies which amino acids will
be used to build a protein. - Codon a sequence of three bases, something like
a three-letter word. - Each codon specifies a particular amino acid.
1614.3 How Is the Information Content in DNA
Transcribed to Produce RNA?
- How was the code deciphered?
- How could 20 code words (amino acids) be
written with only four letters (the four
bases)? - A triplet code seemed likely it could result in
4 4 4 64 codons.
1714.3 How Is the Information Content in DNA
Transcribed to Produce RNA?
- Nirenberg and Matthaei used simple artificial
mRNAs of known composition to identify the
polypeptide that resulted. - This led to the identification of the first three
codons.
18Figure 14.5 Deciphering the Genetic Code
1914.3 How Is the Information Content in DNA
Transcribed to Produce RNA?
- Later, scientists used artificial mRNAs only
three nucleotides long (one codon). - These would bind to a ribosome and a
corresponding tRNA carrying an amino acid. - Thus the codes for all the amino acids were
determined.
20Figure 14.6 The Genetic Code
2114.3 How Is the Information Content in DNA
Transcribed to Produce RNA?
- There are more codons than amino acids.
- AUG is the start codoninitiation signal for
translation. - Stop codonstermination signals, include UAA,
UAG, and UGA.
2214.3 How Is the Information Content in DNA
Transcribed to Produce RNA?
- For most amino acids, there is more than one
codon the genetic code is redundant. - The genetic code is not ambiguouseach codon
specifies only one amino acid.
2314.3 How Is the Information Content in DNA
Transcribed to Produce RNA?
- The genetic code is nearly universal
- The codons that specify amino acids are the same
in all organisms. - A few exceptions Within mitochondria and
chloroplasts, and in one group of protists, there
are codon differences.
2414.3 How Is the Information Content in DNA
Transcribed to Produce RNA?
- This common genetic code is also a common
language for evolution. - The code is ancient and has remained intact
throughout evolution. - The common code also allows genetic
engineeringhuman genes can be expressed in E.
coli, for example.
2514.3 How Is the Information Content in DNA
Transcribed to Produce RNA?
- The base sequence of the DNA strand that is
transcribed is complementary and antiparallel to
the mRNA codons. - The non-template DNA strand has the same sequence
as the mRNA and is called the coding strand. - By convention, DNA sequences are shown beginning
with the 5' end of the coding sequence.