Transcript 2.7 DNA Transcription_translation

Essential Idea

Genetic information in DNA can be
accurately copied and can be translated to
make the proteins needed by the cell.
Understandings






Transcription is the synthesis of mRNA copied from the
DNA base sequences by RNA polymerase.
Translation is the synthesis of polypeptides on
ribosomes.
The amino acid sequence of polypeptides is determined
by mRNA according to the genetic code.
Codons of three bases on mRNA correspond to one
amino acid in a polypeptide.
Translation depends on complementary base pairing
between codons on mRNA and anticodons on tRNA.
Skills and Applications



Skill: Use a table of the genetic code to deduce which
codon(s) corresponds to which amino acid.
Skill: Use a table of mRNA codons and their
corresponding amino acids to deduce the sequence of
amino acids coded by a short mRNA strand of known
base sequence.
Skill: Deducing the DNA base sequence for the mRNA
strand.
DNA, RNA, Protein Synthesis &
GENES
RNA and Protein Synthesis
Copyright Pearson Prentice Hall
IB Assessment Statement
Compare the structure of RNA and DNA.
Copyright Pearson Prentice Hall
DNA, RNA, PROTEIN SYNTHESIS & GENES
Genes are coded DNA instructions that control the
production of proteins.
Genetic messages can be decoded by copying part
of the nucleotide sequence from DNA into RNA.
RNA contains coded information for making
proteins.
The Structure of RNA
The Structure of RNA
RNA consists of a long chain of nucleotides.
Each nucleotide is made up of a 5-carbon sugar,
a phosphate group, and a nitrogenous base.
The Structure of RNA
There are three main differences between RNA and
DNA:
1.
The sugar in RNA is ribose instead of
deoxyribose.
2.
RNA is generally single-stranded.
3.
RNA contains uracil in place of thymine.
Copyright Pearson Prentice Hall
DNA vs. RNA
Copyright Pearson Prentice Hall
Types of RNA
Types of RNA
There are three main types of RNA:
1.
2.
3.
messenger RNA
ribosomal RNA
transfer RNA
Types of RNA
Messenger RNA (mRNA) carries copies of
instructions for assembling amino acids into
proteins.
Types of RNA
Ribosome
Ribosomal RNA
Ribosomes are made up of proteins and ribosomal
RNA (rRNA).
Types of RNA
Amino acid
Transfer RNA
During protein construction, transfer RNA (tRNA)
transfers each amino acid to the ribosome.
3.5.2 Outline DNA transcription in terms of the
formation of an RNA strand complementary to the
DNA strand by RNA polymerase.
Copyright Pearson Prentice Hall
Transcription
Transcription
RNA molecules are produced by copying part of a
nucleotide sequence of DNA into a complementary
sequence in RNA. This process is called
transcription.
Transcription requires the enzyme RNA
polymerase.
Transcription
During transcription, RNA polymerase
binds to DNA and separates the DNA
strands.
RNA polymerase then uses one strand of
DNA as a template from which nucleotides
are assembled into a strand of RNA.
Practice Base Pairing of DNA to mRNA
DNA Template
Strand
Adenine
Thymine
Guanine
Cytosine
A
A
G
T
C
A
T
G
C
Complementary
RNA Strand
Uracil U
Adenine
Cytosine
Guanine
U
U
C
A
G
Transcription Summary
• During Transcription, only one strand of DNA double helix serves as a
template for the synthesis of mRNA.
• The strand of DNA that is the template for mRNA is called the antisense.
• The strand of DNA that is NOT a template for mRNA is called the sense
strand.
• RNA polymerase binds only to regions of DNA known as promoters.
• Promoters are signals in DNA that indicate to the enzyme where to bind
to make RNA.
This model illustrate the process of transcription that takes place in the
nucleus. The DNA base sequence of the gene is copied into messenger
RNA (mRNA)
1. The DNA helix is opened at the position of the
gene.
2. The helix is unwound by RNA polymerase
3. RNA nucleotides are found in the nucleus space.
4. One of the DNA chains act as a template for mRNA
Copyright Pearson Prentice Hall
This model illustrate the process of transcription that takes place in the
nucleus. The DNA base sequence of the gene is copied into messenger
RNA (mRNA)
5. Free nucleotides base pair with DNA nucleotides
6. The phosphodiester bonds on the mRNA chain are
formed by RNA polymerase
7. mRNA is a single polynucleotide chain but the base
thymine is replaced by Uracil.
Copyright Pearson Prentice Hall
This model illustrate the process of transcription that takes place in the
nucleus. The DNA base sequence of the gene is copied into messenger
RNA (mRNA)
8 After the mRNA is complete the molecule detach's
from the DNA and leaves the nucleus for the
cytoplasm ribosomes.
9 The DNA helix reforms.
Copyright Pearson Prentice Hall
Transcription
RNA
RNA polymerase
DNA
IB Assessment Statement
Describe the genetic code in terms of codons
composed of triplets of bases.
Copyright Pearson Prentice Hall
The Genetic Code
The Genetic Code
The genetic code is the “language” of mRNA
instructions.
The code is written using four “letters” (the
bases: A, U, C, and G).
The Genetic Code
A codon consists of three consecutive nucleotides
on mRNA that specify a particular amino acid.
The Genetic Code
Each codon specifies a particular amino acid
that is to be placed on the polypeptide chain.
Some amino acids can be specified by more
than one codon.
The Genetic Code
The Genetic Code
There is one codon AUG that can either specify
the amino acid methionine or serve as a “start”
codon for protein synthesis.
There are three “stop” codons that do not code for
any amino acid. These “stop” codons signify the
end of a polypeptide.
Practicing Translation from mRNA to Amino Acid
mRNA
Amino Acid
AUG
Methionine
CCC
Proline
AGG
Arginine
GGA
Glycine
GCA
Alanine
AGC
Serine
AGU
Serine
More on the genetic code:
The genetic code:
• A polynucleotide is a sequence of bases
• Bases are either A T G or C
• There are 4 bases which operate in sets of 3 (a
triplet).= 43possible triplets of DNA =64 triplets
• There are 20 common amino acids
• Therefore 64 triplets are mapped to 20 amino acids
• However there is a 'punctuation' triplets.
• Therefore the mapping of the code is 64: 21
Copyright Pearson Prentice Hall
• The genetic code is first transcribed
into mRNA
• The mRNA codons can be mapped to
a specific amino acid. The mapping is
64 triplets: 64 codons: 21
• DNA is a degenerate code since there
are more than one triplet or codon that
maps to an amino acid or punctuation.
• mRNA codon AUG codes for
Methionine and is a START signal for
translation.
• mRNA codon UAA, UAG, UGA are all
stop codons punctuating the code.
• GGU, GGC, GGA and GGG all code
for amino acid glycine.
Copyright Pearson Prentice Hall
IB Assessment Statement:
Explain the process of translation, leading to
polypeptide formation.
Copyright Pearson Prentice Hall
Translation
Translation is the
decoding of an mRNA
message into a
polypeptide chain
(protein).
Translation takes place
on ribosomes.
During translation, the
cell uses information
from messenger RNA to
produce proteins.
Translation
Translation
Messenger RNA is
transcribed in the nucleus,
and then enters the cytoplasm
where it attaches to a
ribosome.
Nucleus
mRNA
Translation
Translation begins when an mRNA molecule
attaches to a ribosome.
As each codon of the mRNA molecule moves through
the ribosome, the proper amino acid is brought into
the ribosome by tRNA.
In the ribosome, the amino acid is transferred to the
growing polypeptide chain from the cytoplasm.
Translation
Each tRNA molecule carries only
one kind of amino acid.
In addition to an amino acid, each
tRNA molecule has three unpaired
bases.
These bases, called the anticodon,
are complementary to one mRNA
codon.
The Structure and Function of
Transfer RNA
A tRNA molecule consists of a single RNA strand
that is only about 80 nucleotides long CA
C
Flattened into one plane to reveal its base pairing, a
tRNA molecule looks like a cloverleaf
3
Amino acid
attachment site
5
Hydrogen
bonds
Anticodon
Two-dimensional structure
Amino acid
attachment site
5
3
Hydrogen
bonds
3
Anticodon
Three-dimensional structure
5
Anticodon
Symbol used in this book
Because of hydrogen
bonds, tRNA actually
twists and folds into a
three-dimensional
molecule
tRNA is roughly L-shaped
Translation
The ribosome binds new tRNA molecules and amino
acids as it moves along the mRNA.
Phenylalanine
Methionine
Ribosome
mRNA
Start codon
tRNA
Lysine
Ribosomes
Ribosomes facilitate specific coupling of tRNA
anticodons with mRNA codons in protein synthesis
The two ribosomal subunits (large and small) are
made of proteins and ribosomal RNA (rRNA)
Lysine
tRNA
5’
3’
Translation direction
mRNA
Ribosome
Protein Synthesis (building a polypeptide):
•
Ribosomes are the site of polypeptides
synthesis.
• This involves linking amino acids together
through condensation (dehydration) reactions.
Lysine
tRNA
5’
3’
Translation direction
mRNA
Ribosome
Elongation of the Polypeptide Chain
At the ribosome, amino acids are added one by
one to the preceding amino acid
A peptide bond is formed between the amino
acids.
Translation
•The process continues until the ribosome
reaches a stop codon.
Polypeptide
Ribosome
tRNA
mRNA
Copyright Pearson Prentice Hall
Copyright Pearson Prentice Hall
Copyright Pearson Prentice Hall
Copyright Pearson Prentice Hall
Genes and Proteins
Codon
The sequence of
bases in DNA is
used as a
template for
mRNA.
The codons of
mRNA specify the
sequence of
amino acids in a
protein.
Codon Codon
Single strand of DNA
Codon Codon Codon
mRNA
Alanine Arginine Leucine
Amino acids within
a polypeptide
http://www.wiley.com/legacy/college/boyer/
0470003790/animations/translation/transl
ation.htm
https://www.youtube.com/watch?v=itsb2Sq
R-R0
Copyright Pearson Prentice Hall
IB Assessment Statement
Discuss the relationship between one gene and one
polypeptide. Originally, it was assumed that one
gene would invariably code for one polypeptide,
but many exceptions have been discovered.
Copyright Pearson Prentice Hall
Genes and Proteins
Genes and Proteins
Genes contain instructions for assembling
proteins.
Many proteins are enzymes, which catalyze and
regulate chemical reactions.
Proteins are each specifically designed to build or
operate a component of a living cell.
Copyright Pearson Prentice Hall
One Gene One Polypeptide:
Theory: One gene is transcribed and translated to produce
one polypeptide.
Some proteins are composed of a number of polypeptides
and in this theory each polypeptide has its own gene.
e.g. haemoglobin is composed of 4 polypeptides (2 of each
type) and there is a gene for each type of polypeptide.
This theory, like so many in biology has exceptions. e.g.
1) Some genes code for types of RNA which do not produce
polypeptides.
2) Some genes control the expression of other genes.
Copyright Pearson Prentice Hall
Animation on one gene one polypeptide:
http://www.youtube.com/watch?v=Thj6jq7
mYkE
Copyright Pearson Prentice Hall
Copyright Pearson Prentice Hall
Copyright Pearson Prentice Hall
Copyright Pearson Prentice Hall
Copyright Pearson Prentice Hall
Copyright Pearson Prentice Hall
Copyright Pearson Prentice Hall
END OF SECTION