Chapter 13
packet: DNA
and Protein
Synthesis Part II
DNA (Deoxyribonucleic
RNA
acid)
(Ribonucleic acid)
Sugar
Deoxyribose
Ribose
Bases
Strands
Adenine, thymine,
guanine, cytosine
Double-stranded
Adenine, uracil,
guanine, cytosine
Single-stranded
Helix
Yes
No
Location
Nucleus
Nucleus, cytoplasm
(outside nucleus)
Types
XXXXXXXXX
Messenger, transfer,
ribosomal
RNA
vs.
DNA
•Messenger RNA (mRNA) – carries
instructions for making a protein from the
nucleus to the ribosomes
•Ribosomes - a part of the cell where
proteins are made
•Ribosomal RNA (rRNA) - found in the
ribosomes
•Transfer RNA (tRNA) - transfers amino
acids to the ribosomes and “reads the mRNA
sequence, translating it into a sequence of
amino acids
Making Proteins
• DNA is found in the nucleus of a cell
• Proteins are made outside the nucleus at the
ribosomes.
Overview of gene expression
• Two processes are involved in the synthesis of
proteins in the cell:
• Transcription – making mRNA from the
information in DNA, which will take a copy of
the DNA code to the ribosome to direct the
making of protein; occurs in nucleus
• Translation – uses the information in mRNA to
make a specific protein, the sequence of bases
of mRNA is “translated” into a sequence of
amino acids; occurs in ribosome
• These processes are the same in all organisms
The Genetic Code
• DNA holds instructions to make a protein
• Instructions are copied into mRNA, which
will be used to make a protein
• Codon - each three-nucleotide sequence of
an mRNA molecule
• Each codon represents 1 amino acid
• There are 64 possible codons, and only 20
amino acids, so most amino acids have
more than one codon
Messenger RNA codons
•
•
•
•
•
Transcription
Purpose – Makes a copy of the DNA code
that can leave the nucleus and travel to the
ribosome to direct protein synthesis –
mRNA
Occurs in the nucleus
Occurs at only 1 gene at a time
Adenine in DNA pairs with uracil in RNA,
not thymine
Thymine in DNA pairs with adenine in
RNA
Steps:
1. RNA polymerase attaches to DNA at the start of
a gene
2. DNA unwinds and unzips
3. Complementary bases are added along DNA (½
of DNA will serve as a template)
• Once “stop” signal is reached, process ends,
DNA closes back up, and mRNA is released
Animation
Transcription and mRNA synthesis
Translation
• Protein constructed during this process
• Occurs at the ribosomes
• Key players in translation:
• mRNA (messenger RNA)
• Made during transcription, has codons
• Travels from nucleus to ribosome
• Contains copy of DNA code to make protein
• tRNA (transfer RNA)
• rRNA (ribosomal RNA)
Transfer RNA (tRNA)
• tRNA molecules bring amino acids to the
ribosomes
• Free-floating in the cytoplasm of the cell
•Anticodon – sequence
of 3 nucleotides on
tRNA (complementary
to mRNA)– it is this
sequence that
determines which amino
acid each tRNA has
•Complementary base pairing occurs between
anticodons of tRNA and codons of mRNA –
determines the sequence of amino acids to
construct the polypeptide.
•If mRNA codon is
AUG, tRNA anticodon
would be UAC
Ribosomal
RNA (rRNA)
• rRNA is made
in the
nucleolus (a
cell structure
found inside
the nucleus)
• Makes
ribosomes
(location in
cell where
proteins will
be made)
Steps:
1. Ribosome
attaches to start
codon on mRNA
2. tRNA brings amino acids to ribosome –
codon on mRNA pairs with anticodon on
tRNA
3. Amino acids are joined by peptide bonds
4. Stop codon is reached, mRNA is
released, and protein is released
Animation
Control of Gene Expression
• Cells differ in which genes are being expressed –
based on cell function – ex. nerve vs. muscle.
• Levels of control in
eukaryotes include:
• transcriptional
• posttranscriptional
• translational
• Posttranslational
• May rely on signals from
inside or outside of the cell
• Transcriptional control – involves rate at
which or even whether transcription
occurs at all; due to:
• The organization of the chromatin
(form that chromosomes take in nondividing cells)
• Regulator proteins called transcription
factors
• Signals from inside or outside the cell
•Posttranscriptional control – involves
processing of mature mRNA
•Translational control – involves life span
of mRNA and ability to bind to ribosomes
•Posttranslational control – involves
changes needed for polypeptide to become
functional
Gene Mutations
• Definition - a change in the sequence of
bases within a gene
• Causes –
• Mutations can be spontaneous or
caused by environmental influences
called mutagens.
• Mutagens include radiation (X-rays,
UV radiation), and organic chemicals
(in cigarette smoke and pesticides).
•Types –
•Frameshift mutations –
•one or more bases are inserted or
deleted from a sequence of DNA
•can result in nonfunctional proteins
•can result in no protein at all – stop
codon where there shouldn’t be one
•Point mutations –
•One base is substituted for another
•May result in change of amino acid
sequence
•May not affect protein at all
• Types of point mutations:
• Silent mutation - the change in the
codon results in the same amino acid
Ex: UAU  UAC both code for
tyrosine
• Nonsense mutation - a codon is
changed to a stop codon; resulting
protein may be too short to function
Ex: UAC  UAG (a stop codon)
• Missense mutation - involves the
substitution of a different amino acid, the
result may be a protein that cannot reach its
final shape
Ex: Hbs which causes sickle-cell disease
In general, mutations can have any of
the following effects:
•No change in proteins made or
appearance
•Wrong protein is made
•No protein in made
•New appearance may result