Transcript RNA and Protein Synthesis

RNA and Protein
Synthesis
Chapter 12, section 3
The Structure of RNA
 “Central
Dogma” of Biology:
 DNA
is “read” by RNA which then
travels to the ribosomes to make
proteins.
 DNA
 RNA  Proteins
The Structure of RNA
RNA, like DNA, is made up of nucleotides
 However, there are 4 major differences
between DNA and RNA:

 RNA
has ribose instead of deoxyribose
 RNA has uracil instead of thymine
 RNA is single-stranded instead of doublestranded
 RNA is able to leave the nucleus.
3 Types of RNA



Messenger RNA (mRNA) – carries the message from
the DNA to the ribosomes
Ribosomal RNA (rRNA) – make up part of the
structure of a ribosome
Transfer RNA (tRNA) – transfers amino acids to the
ribosomes
Transcription
DNA  mRNA
 Making RNA from DNA (in the nucleus)
 RNA polymerase binds to DNA and
begins building a complementary strand of
mRNA

A
will now pair with U, T still pairs with A
 C and G still pair with each other

Why mRNA? mRNA can leave the
nucleus!
Transcription
The Genetic Code
DNA and RNA are “read” 3 bases at a time
 A codon is a 3 base sequence that codes
(“signals”) for a specific amino acid

 There
are 64 possible codons (3 letter combinations)
Proteins are made of amino acids
 There are 20 different amino acids

Practice Transcription…



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DNA – AGC TCC GAT GCA TAC TTG CCA
mRNA– UCG AGG CUA CGU AUG AAC GGU
DNA – GCC AGT GCT TAC GAA CTG AGT
mRNA– CGG UCA CGA AUG CUU GAC UCA
The Genetic Code

RNA sequence:
 UCGAGGCUA

Separate into codons:
 UCG-AGG-CUA

Identify the “signaled” amino acids:
 Serine-Arginine-Leucine
The Genetic Code
Translation



mRNA  Proteins
Making of proteins using the instructions from
the mRNA message (“translating the code”)
Occurs at the ribosomes
Translation
1.
2.
3.
Following transcription, mRNA leaves
the nucleus and enters the cytoplasm.
The mRNA attaches to the ribosome
The tRNA brings the proper amino acid
to the ribosome

Anticodon – sequence of bases on the
tRNA that pair with the mRNA
Translation
4.
5.
6.
The amino acids form a peptide bond to
hold them together. The next amino acid
is brought in and is attached
This continues until the ribosome reaches
a stop codon
The completed protein (polypeptide
chain) is then released
Translation
Mutations
Changes in the DNA sequence that affect
genetic information
 Gene mutations – result from changes in
a single gene. A gene carries the “recipe”
for a single protein.
 Chromosomal mutations – involve
changes in whole chromosomes

Mutations
Body-cell v. Sex-cell Mutation
 Somatic cell mutations are not passed on
to the next generation.
 Mutations that occur in sex cells are passed
on to the organism’s offspring and will be
present in every cell of the offspring.
Gene Mutations

Point mutations – a mutation that occurs
at a single point (only 1 nitrogen base)


Substitution – a single nucleotide is substituted for
another one (A instead of G)
Frameshift mutations– a mutation that
occurs when a nucleotide is inserted or
deleted
Insertion – a nucleotide is added
 Deletion – a nucleotide is removed

Gene Mutations

Insertions and deletions cause frameshift
mutations because they shift the “reading
frame” of the genetic message.
Gene Mutations