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DNA
Chapter 8
• Every cell in your body has
its own DNA that is identical
to every other cell in your
body
• Your DNA is different from
everyone else’s DNA, unless
you have an identical twin
• DNA is found inside the
nucleus of every cell that has
a nucleus
• If a cell does not have a
nucleus, DNA is found
floating around in the
cytoplasm
• What we have been calling
genes are really DNA
• Chromosomes are made up
of DNA
• DNA is short for
deoxyribonucleic acid
• DNA is a molecule that
makes up genes and
determines the traits of all
living things
• ALL living things contain
DNA
• DNA is made up of units called
nucleotides
• A nucleotide consists of a
nitrogenous base, a phosphate
group and a 5 carbon sugar
called deoxyribose (hence the
name deoxyribonucleic acid)
• DNA looks very much like a
twisted ladder
• A ladder has two sides and
many rungs
• The sides of DNA are made up
of two different chemicals – a
sugar (deoxyribose) and an acid
(phosphate group)
• The sugar and acid alternate
on each side of the DNA
• The rungs of the DNA ladder
are made up of chemicals
called nitrogen bases
• There are four different
nitrogen bases in DNA
• The 4 bases are adenine,
thymine, guanine & cytosine
• Adenine only fits with Thymine
• Cytosine only fits with Guanine
• There are only two bases on
each DNA rung
• Adenine (A) & Guanine (G) are
Purines
• Cytosine (C) & Thymine (T) are
Pyrimidines
• A Purine is always paired with a
Pyrimidine
• Purines are bigger than
Pyrimidines
• Between each base pair are
hydrogen bonds which hold
the two bases together
• The ladder of DNA is
twisted into a spiral, or helix
• DNA is called a double helix
• It is called a double helix
because it is made up of two
strands that are twisted
• Scientists did not know much
about DNA until the 1900’s
• Many different experiments
provided info about DNA (8.1,
pg 226-228)
• The order of nitrogen
bases in DNA is the code
for all your traits
• If the order is changed in
any way, there will be a
different trait
• This works much the same
way as the Alphabet
• The word pen means one
thing, but if we change the E
to an I, it means something
totally different
• If we change the P to an H, it
means something else
• However, if we change the E
in pen to a D, the word
would make no sense at all
• It is the same with DNA –
sometimes changes make the
DNA say something else, &
sometimes it makes no sense
• When DNA is changed to make
no sense at all, it could mean
death or a severe illness
• The difference is there are only
4 letters in the DNA alphabet –
but the words are much longer
Question 1
•If one strand of DNA is:
AT G G CAT G C
What is the other strand
going to look like?
Question 2
•If one strand is:
G G C TAGAGAT
What is the other
strand?
DNA & Chromosomes
• Prokaryotic cells lack a nucleus,
so the DNA is located in the
cytoplasm
• Prokaryotes have one circular
“chromosome” that contains
most or all of the cells DNA
• Eukaryotic cells have at least
1000 times as much DNA as
prokaryotic cells
• Eukaryotic DNA is not free in
the cytoplasm, but is in the
nucleus
• The number of chromosomes
varies in eukaryotes
Size & Length of DNA
• DNA molecules are VERY long
• E. coli DNA is 4,639,221 base
pairs long = approx. 1.6mm
• An E. coli cell is only 1.6mm!
• The DNA must fit in a space one
one-thousandth of its length!
Chromosome Structure
• The DNA must be folded
numerous times to fit into the
cell’s nucleus
• A human cell contains almost
1000 times as many base pairs
as E. coli – human DNA must be
folded even more!
• Eukaryotic chromosomes
contain protein & DNA (form a
substance called chromatin)
• The DNA is tightly coiled
around proteins called histones
• The DNA & histones form a
nucleosome
• Nucleosomes coil together to
form a thick fiber and
supercoils
• The supercoils form a
chromosome
• We saw chromosomes in
Chapter 5 (figure 5.5)
DNA Replication
• DNA must be replicated before
mitosis (and meiosis)
• How does that occur?
• Each strand of DNA has the info
to make the other strand (B/C
of base pairing)
• During DNA replication, the
DNA molecule separates into
two strands, then produces two
new complimentary strands
following the rules of base
pairing
• In eukaryotes, DNA replication
occurs in numerous places on
the DNA
• The place where the DNA is
opening and replication is
occurring is called the
replication fork
• An important enzyme that helps
in DNA replication is called
DNA Polymerase
• There are other enzymes that
“unzip” the DNA and then zip
the new strands back up again
RNA & Protein
Synthesis
Chapter 8.4 & 8.5
How does DNA code for traits?
• DNA must remain in the
nucleus – it is too fragile, & too
important to be in the
cytoplasm
• DNA codes for proteins, but
proteins are made in the
cytoplasm
• How can this be?
• DNA has a helper molecule
to leave the nucleus and go
into the cytoplasm to make
proteins
• This molecule is ribonucleic
acid – or RNA
RNA Structure
• RNA is similar to DNA in
that it is made of nucleotides
• There is no Thymine in RNA
– instead there is a base
called Uracil (U), which is
similar to thymine
RNA Structure
• RNA is single stranded, not
double stranded (not a
double helix)
• The sugar in RNA is ribose,
not deoxyribose
RNA Structure
• You can think of RNA as a
disposable copy of DNA or a
working copy of a single gene
• One gene on DNA can make
hundreds or thousands of RNA
copies of that gene, which can in
turn make that many proteins
Types of RNA
• There are 3 types of RNA, each
with a specific structure and
function
• Messenger RNA (mRNA) is the
copy of the DNA
• Ribosomal RNA (rRNA) makes
up the ribosomes
Types of RNA
•Transfer RNA (tRNA) is
used in the making of
proteins and brings the
amino acids to the
ribosome
Making Proteins
• There are two main stages of
making proteins – Transcription
& Translation
• Transcription is going from
DNA to RNA
• Translation is going from RNA
to protein
Transcription
• Transcription occurs in the
nucleus
• RNA polymerase is an enzyme
that is needed, similar to DNA
polymerase
• Only one strand of DNA is used
to make RNA
Transcription
• Transcription is similar to DNA
replication, but the result is
single stranded with Uracils in
the place of Thymines
• The resulting RNA will be
complimentary to the DNA
strand used
Transcription
• RNA polymerase knows where
genes start because of pieces of
DNA called promoters
• Promoters have specific base
sequences – they act as signals
to the RNA polymerase
Transcription
• Similar pieces of DNA act as
signals for the RNA polymerase
to stop transcription
• The RNA made from
transcription needs to be edited
before use
Transcription
• Large pieces of RNA will be cut
out, called introns
• The remaining pieces of RNA
are called exons and are spliced
back together with enzymes
The Genetic Code
• Proteins are made by joining
amino acids together
• How can an mRNA molecule
make a protein if RNA is
made of nucleotides?
The Genetic Code
• The genetic code has only four
letters (A,U,G,C)
• But, each word in the code is
three letters long
• Each three letter word makes a
specific amino acid
The Genetic Code
• The 3-letter words are called
codons
• For instance, the codon AUG
always codes for the amino acid
methionine
The Genetic Code
• The mRNA sequence
AUGGCGUGU can be broken
up into its codons
• AUG – GCG – UGU
• The amino acid sequence is:
–methionine, alanine, cysteine
The Genetic Code
• Figure 8.13 shows the genetic
code (page 244)
• Here are some other Genetic
Code charts
• (You will have this on a test)
Translation
•Translation occurs
outside of the
nucleus, in the
cytoplasm of the cell,
on the Ribosome
Translation
• Translation requires the 3
types of RNA
• mRNA = messenger RNA
• tRNA = transfer RNA
• rRNA = ribosomal RNA
Translation
• Attached to each tRNA is an
Amino Acid
• Amino acids make up the polypeptides
• There is also an anti-codon on
the tRNA
Translation
• The anti-codon on the tRNA
matches up with the codon on
the mRNA
Translation – Step by Step
• First, the mRNA must be
transcribed from the DNA,
leave the nucleus & go to the
cytoplasm
• Then, the mRNA attaches to a
ribosome in the cytoplasm
Translation – Step by Step
• The mRNA moves through the
ribosome in one direction & for
each codon, the proper amino acid
is attached to the polypeptide
chain via the tRNA
• The anti-codon on the tRNA
matches up with the codon on the
mRNA
Translation – Step by Step
• For example – the start codon
on the mRNA is A U G
• The anti-codon is U A C
• Then, a second tRNA with the
next amino acid attaches to a
different site on the ribosome
Translation – Step by Step
• The ribosome forms a peptide
bond between the 1st and 2nd
amino acid
st
• The bond between the 1 tRNA
and its amino acid is broken
• The ribosome then moves to the
rd
3 codon
Translation – Step by Step
• The polypeptide continues to
grow in this fashion until the
ribosome reaches a stop codon
(UAA, UAG or UGA)
• The mRNA molecule is then
released & the polypeptide is
released into the cytoplasm
Genes and Proteins
• The polypeptide made in
translation will be modified by
the golgi or the rough ER and
made into a fully functional
protein
• Proteins control almost
everything that living cells do!
Genes and Proteins
• Why is it important that the
mRNA is read in one direction
only?
• Decode the following DNA
strand from left to right
GACAAGTCCACAATC
Genes and Proteins
• The mRNA strand is:
CUGUUCAGGUGUUAG
• The amino acid sequence is
leucine-phenylalaninearginine-cysteine-stop
Genes and Proteins
• Now, read the mRNA strand
right to left
GAUUGUGGACUUGUC
• The amino acid sequence is
aspartic acid-cysteineglycine-leucine-valine
Genes and Proteins
• The resulting proteins are
completely different and will do
different things – maybe even
not function at all!
• Next, we’ll talk about mutations
and how they affect proteins
and traits
Mutations
• Every now and then cells make
a mistake in copying DNA or
making RNA
• These mistakes are called
mutations
• Mutations come in all “shapes
& sizes”
Mutations
• Gene mutations result from
changes in a single gene
• Chromosomal mutations
involve changes in a whole
chromosome
Mutations
• Most gene mutations involve
just one nucleotide, but some
involve many
• Mutations that affect only one
nucleotide are called point
mutations
Mutations
• When one nucleotide is switched
out for another, it will generally
change only one amino acid in
the protein
• If one nucleotide is inserted or
deleted, more severe changes
occur
Mutations
• Mutations that add or delete a
nucleotide are called frame-shift
mutations
• They affect every amino acid
after the mutation – very bad
• HOW???
Example - substitution
• DNA: TCT ACA ACC ACG
• RNA: ??
• Amino Acid: ??
• New DNA: TCTTCAACCACG
• NEW RNA: ??
• New Amino Acids: ??
Example - substitution
• DNA: TCT ACA ACC ACG
• RNA: AGA UGU UGG UGC
• Amino Acid: arginine – cysteine
– tryptophan – cysteine
NEW
• DNA: TCT TCA ACC ACG
• RNA: AGA AGU UGG UGC
• Amino Acids: arginine – serine
– tryptophan – cysteine
Example - Deletion
• DNA: TTCGTCATGCACATC
• RNA = ??
• Amino Acids = ??
• New DNA: TTCGTCTGCACATC
• New RNA = ??
• New Amino Acids = ??
Example - Deletion
• DNA: TTCGTCATGCACATC
• RNA: AAG CAG UAC GUG
UAG
• Amino Acids: lysine-glutaminetyrosine-valine-stop
NEW
• DNA: TTCGTCTGCACATC
• RNA: AAG CAG ACG UGU
AG
• Amino Acids: lysine-glutaminethreonine-cysteine-…
Chromosomal Mutations
• There are different types of
chromosomal mutations
• A deletion is when part of the
chromosome is deleted
• A duplication is when a part pf
the chromosome is duplicated
Chromosomal Mutations
• An inversion is when parts of a
chromosome are switched
around
• A translocation is when pieces
of one chromosome end up on a
different chromosome