RNA and Protein Synthesis
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The Function of DNA
• The DNA molecule
contains all of your
hereditary information in
the form of genes.
• Genes are portions of the
DNA molecule that code
for the production of
specific types of proteins.
• However, DNA is confined
to the nucleus, while
proteins are made by
ribosomes in the
cytoplasm.
• Thus, a messenger
molecule is needed.
http://www.accessexcellence.org/RC/VL/GG/images/genes.gif
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Comparing DNA and RNA
• RNA is the nucleic acid that
acts as a messenger
between DNA and the
ribosomes.
• The RNA produced during
transcription is structurally
different from DNA in 3
basic ways:
– 1. The sugar in RNA is ribose
whereas the sugar in DNA is
deoxyribose.
– 2. RNA is single stranded
while DNA is double stranded.
– 3. RNA contains a base called
uracil instead of thymine.
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http://www.dkimages.com/discover/previews/769/85011519.JPG
The Role of RNA
• RNA molecules have many functions, but
in the majority of cells, most RNA
molecules are involved in protein
synthesis.
• Protein synthesis is the assembly of
amino acids into proteins.
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Types of RNA
• The RNA produced during transcription is modified into 3 basic
types:
– 1. messenger RNA (mRNA) is responsible for copying one strand of DNA
in the nucleus and carrying that information to the ribosomes in the
cytoplasm.
– 2. ribosomal RNA (rRNA) makes up a large part of the ribosome and is
responsible for reading and decoding mRNA.
– 3. transfer RNA (tRNA) carries amino acids to the ribosome where they
are joined to form proteins.
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http://images.encarta.msn.com/xrefmedia/zencmed/targets/illus/ilt/T068340A.gif
Protein Synthesis
• During protein synthesis, genes copied
onto RNA are expressed by the production
of specific types of proteins.
• Protein synthesis involves two processes:
– 1. Transcription – the process where a portion
of the DNA sequence is copied into a
complementary RNA sequence.
– 2. Translation – the decoding of an mRNA
message into a polypeptide chain (protein).
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Transcription and Translation
• Transcription occurs
on the DNA in the
nucleus.
• Transcription Demo
• Translation is
completed by the
ribosomes located in
the cell’s cytoplasm.
• All three types of RNA
work together during
translation to produce
polypeptides
(proteins).
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Decoding mRNA
• The sequence of bases in an mRNA molecule
serves as instructions for the order in which amino
acids are joined to produce a polypeptide.
• Ribosomes decode these instructions by using
codons, sets of 3 bases that each code for 1 amino
acid.
• Each codon is matched to an anticodon, or
coplementary sequence on the tRNA to determine
the order of the amino acids.
• Translation Demo
http://www.gwu.edu/~darwin/BiSc150/One/codon.gif
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Using a Codon Chart
http://www.safarikscience.org/biologyhome/7_dna/codon_qu
estion.png
• A codon chart can be used to determine
the sequence of the amino acids in the
polypeptide.
• The mRNA bases or CODONS are used
to find the amino acid.
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Decoding Practice
• For the following examples, give the appropriate
mRNA sequence and amino acid sequence.
(Remember: U replaces T in mRNA.)
Example 1:
DNA:
TAC GCA TGG AAT
mRNA:
AUG CGU ACC UUA
Amino Acids:
Met
Arg
Thr
Leu
Example 2:
DNA:
CGT GGA GAT ATT
mRNA:
GCA CCU CUA UAA
tRNA:
CGU GGA GAU AUU
Amino Acids:
Ala
Pro
Leu
stop
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What is a gene mutation?
• Sometimes during replication, an error is
made that causes subsequent changes in
the mRNA and proteins that are made
using that DNA.
• These errors or changes are called
mutations.
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Types of Gene Mutations
• Point mutation:
– changes in only 1 or a few nucleotides of
DNA.
– Substitutions, in which one base is changes
to another, are a type of point mutation.
• Frameshift mutation:
– When one base is deleted (deletion) or
added (insertion) into a DNA sequence.
– This causes the entire sequence of codons to
shift over by one base.
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What are Chromosomal Mutations?
• Mutations involving a long segment of
DNA.
– These mutations can involve deletions,
insertions, or inversions of sections of DNA.
– In some cases, deleted sections may attach to
other chromosomes, changing both the
chromosomes that lost the DNA and the one
that gained it, also called a chromosomal
rearrangement.
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What is the significance of
mutations?
• Mutations in DNA cause changes in the
sequence of amino acids which ultimately
creates changes in proteins and their
function.
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