Download Protein Synthesis and Mutation Notes

RNA
Protein Synthesis
Mutations
 RNA
differs from DNA in three ways:
• an RNA molecule consists of a single-strand of
nucleotides
• RNA has ribose (5-C sugar) rather than
deoxyribose
• RNA has the pyrimidine uracil instead of thymine
Function
mRNA
tRNA
rRNA
transmits
information from
DNA and serves as
a template for
protein synthesis
brings amino acids
to ribosomes for
protein synthesis
rRNA and proteins
make up ribosomes
Has 2 stages:
Transcription and Translation
 Transcription
is the first part of protein
synthesis.
* What does it mean to transcribe?
 Step
1: An enzyme called RNA polymerase
attaches to DNA and “unzips” it.
 Step 2: RNA polymerase then uses one
strand of DNA as a template from which
nucleotides are assembled into a strand of
mRNA.
Adenine (DNA and RNA)
Cystosine (DNA and RNA)
Guanine(DNA and RNA)
Thymine (DNA only)
Uracil (RNA only)
RNA
polymerase
DNA
RNA
 The
nucleotide sequence of DNA is
converted into an RNA nucleotide
sequence.
 Let’s say one strand of a DNA molecule
had this sequence
AGCCTACGTAAG
What would the mRNA sequence
produced be?
DNA:
AGCCTACGTAAG
mRNA: UCGGAUGCAUUC
 Proteins
are made by joining amino acids
together into a long chain. This chain is a
protein.
 The functions and properties of a protein
are determined by the order in which 20
different types of amino acids are joined.
 The “language” of mRNA is called the
genetic code.
 RNA
has 4 bases: adenine, uracil,
cytosine, and guanine.
 The “language” of RNA is written in only
4 letters – AUGC
 The code is read three letters at a time
 Each “word” is 3 letters long
 Each “word” of 3 nucleotides is called a
codon



The 4 different bases can
be arranged into 64
possible codons
(4X4X4=64), but make only
20 different amino acids.
This is because some
amino acids correspond to
several different codons.
Example:
Practice:
a) For what amino acid does
the codon CGG code?
b) For what amino acid does
the codon AUG code?
c) For what amino acid does
the codon UGA code?
Start Codon: AUG
Stop Codons:
UAA
UAG
UGA
 The
2nd stage of protein synthesis is
translation.
*What does it mean to translate?
 The sequence of nucleotides in mRNA
serves as instructions for the order in
which amino acids should be joined
together in a protein. Think of mRNA as
the “recipe” for a protein.
 Transcription
occurs in the nucleus.
 In translation, the mRNA travels out of the
nucleus, and moves through the
cytoplasm and attaches to a ribosome.
So, translation occurs at the ribosomes.
Steps of translation:
1. mRNA attaches to a ribosome
2. As each codon of the mRNA molecule moves
through the ribosome, the proper amino acid
is brought to the ribosome by tRNA
molecules.
3. Each tRNA molecule has 3 unpaired bases on
it. These bases are called anticodons.
4. The amino acids are strung together like a
chain (remember, a protein is a polymer) by
the ribosome, forming a long protein molecule.
5. The tRNA’s are released from the amino acids.
6. The protein chain grows longer and longer
until the ribosome reaches a stop codon on
the mRNA molecule. Then the protein is
released from the ribosome.
DNA:
TAC AAA TTT CAG AAC ACC TTC CAT ATT
mRNA:
AUG UUU AAA GUC UUG UGG AAG GUA UAA
tRNA:
UAC AAA UUU CAG AAC ACC UUC CAU AUU
Polypeptide:
met phe
lys
val
leu
try
lys
val
stop
•Gene
Mutation – produce
changes in a single gene
•Chromosomal Mutations –
produce changes in an entire
chromosome
A
gene mutation is a change in a DNA
sequence.
 Gene mutations that involve changes in
only one or just a few nucleotides are
called point mutations b/c they occur at a
single point in the DNA sequence.
 There are 3 types of point mutations:
substitutions, deletions, and insertions.
Substitution Mutations
 One base is changed to another
 Usually affect no more than a single
amino acid, but could still have an effect
on a protein.
Insertion Mutations
 One extra base is inserted into the DNA
sequence.
Deletion Mutations
 One base is left out of the DNA sequence.
 Since
the genetic code is read 3 letters at
a time, the message is shifted for every
codon that follows. Hence, insertions
result in frameshift mutations. They have
a dramatic effect on the amino acid
sequence, and thus, the protein.
* Question…during what event do these
mutations usually occur?
Deletion
Substitution
Insertion
Chromosomal mutations involve
changes in the number or structure of
chromosomes.
We discussed many of these last chapter.
Examples are Down syndrome, Turner
syndrome, Fragile-X syndrome, PraderWilli syndrome, and Klinefelter’s
syndrome.
Deletion
Duplication
Inversion
Translocation