Chapter 4
Sec 1 – What does
DNA Look Like?
DNA stands for…
Deoxyribonucleic
acid
What does DNA do?
DNA stores and passes on genetic information
from one generation to the next
1950s
Rosalind Franklin and Maurice Wilkins took x-ray
photographs of DNA
1953
James Watson and Francis Crick determine that
DNA is a double helix
So saying that DNA is a double
helix is the same as saying it’s like a
twisted ladder.
The Sides of the Ladder are made
up of millions of alternating sugars
and phosphates.
• The sugar is named
deoxyribose.
• The two sides of the
ladder are held
together by rungs
attached to the sugars.
Rungs of ladder –
each rung contains 2 bases
•Bases contain nitrogen so we call
them nitrogenous bases
• The 2 bases of each rung are
connected by a hydrogen bond
•There are 4 bases to choose from:
Adenine
Thymine
Guanine
Cytosine
Erwin Chargraff discovered
that the amount of adenine
always equals the amount of
thymine, AND the amount of
guanine always equals the
amount of cytosine
He concluded that a rung
can either have A and T OR
it can have C and G but no
other combinations are
possible.
Complementary Base Pairing
Adenine always pairs with thymine.
Guanine always pairs with cytosine.
Thymine always pairs with adenine.
Cytosine always pairs with guanine.
So, if you know ONE side of the DNA,
you should be able to figure out the
other!
DNA – Master Molecule (15:20)
What is a nucleotide?
A nucleotide is a small piece of DNA
which contains:
•1 base
•1 sugar
•1 phosphate
Lots of nucleotides link up together
to make a big DNA molecule.
DNA Replication
• Process in which DNA makes an exact copy of
itself
Interphase before Mitosis
• Occurs during __________________________
DNA Replication is semi-conservative.
Steps of DNA
Replication
1. When it is time to
replicate, the DNA unzips
(at the hydrogen bonds)
2. New complementary
nucleotides move in to
match BOTH halves of
the DNA and form
hydrogen bonds with the
old nucleotides.
3. Product: 2 identical DNA
molecules!
Important:
The 2 new DNA’s are
identical to each other
AND identical to the
original DNA because
the bases are
complementary.
Identical
base sequences
DNA replication animation
Chapter 4
Sec How DNA
Works
Protein Synthesis
DNA carries a code for
each type of protein that
your cells need.
DNA is like the blueprints for a house.
How genes work
Genes carry the instructions for making proteins.
Whenever a cell needs a particular protein, that gene
is triggered to make a protein.
There are 2 steps to making a protein:
1.
Transcription – the gene for the protein that
you want is copied in the nucleus so that the
original DNA never has to leave the nucleus.
The copy goes to the ribosome.
2.
Translation – the copy of the gene is used by
the ribosome to make a protein
Transcription is like taking the
blueprints and copying a single section
about one specific part of the house.
The copy of the gene leaves the
nucleus and goes to the ribosome.
Translation
The second part of protein synthesis
in which the amino acids that the
mRNA codes for are linked together
in the ribosome.
Translation is like taking the single section of
blueprints that you copied and using it to build one
portion of the house.
Translation:
The ribosome
reads the code
on the copy of
the gene and
assembles all of
the amino acids
that the code
calls for.
The amino acids link up together and
make a protein.
Codon = 3 letters on mRNA
Anticodon = 3 complementary
letters on tRNA (transfer RNA)
The anticodon on tRNA
matches up with the
codon on mRNA and
brings with it the amino
acid that the codon
codes for!
Peptide Bond = bond that connects
2 amino acids
• As the tRNA’s bring amino acids to the
ribosome, the amino acids connect to each
other with peptide bonds.
• Many amino acids connected together makes
a protein.
This is a molecule of messenger
RNA.
It was made in the nucleus by
transcription from a DNA molecule.
codon
A U G G G C U U AAA G C A G U G C A C G U U
mRNA molecule
A ribosome on the rough
endoplasmic reticulum attaches to
the mRNA molecule.
ribosome
A U G G G C U U AAA G C A G U G C A C G U U
Amino acid
tRNA molecule
A transfer RNA molecule arrives.
It brings an amino acid to the first three bases
(codon) on the mRNA.
anticodon
The three unpaired bases (anticodon) on the
tRNA link up with the codon.
UAC
A U G G G C U U AAA G C A G U G C A C G U U
Another tRNA molecule comes into place,
bringing a second amino acid.
Its anticodon links up with the second codon on
the mRNA.
UAC
A U G G G C U U AAA G C A G U G C A C G U U
Peptide bond
A peptide bond forms between the two amino
acids.
A U G G G C U U AAA G C A G U G C A C G U U
The first tRNA molecule releases its amino acid and
moves off into the cytoplasm.
A U G G G C U U AAA G C A G U G C A C G U U
The ribosome moves along the mRNA to the next
codon.
A U G G G C U U AAA G C A G U G C A C G U U
Another tRNA molecule brings the
next amino acid into place.
A U G G G C U U AAA G C A G U G C A C G U U
A peptide bond joins the second and third
amino acids to form a polypeptide chain.
A U G G G C U U AAA G C A G U G C A C G U U
The process continues.
The polypeptide chain gets longer.
This continues until a termination (stop)
codon is reached.
The polypeptide is then complete.
A U G G G C U U AAA G C A G U G C A C G U U