Download DNA - SchoolRack

Chapter 11
DNA
What is DNA?
• Living things need proteins to survive.
– most proteins are enzymes
• DNA provides the complete set of
instructions for making all the
proteins for an organism.
The structure of DNA
• DNA is very large - It is a
nucleic acid that is made up
of subunits called nucleotides.
• Nucleotides are made up of
three parts:
– A carbohydrate called
deoxyribose
– A phosphate group
– A nitrogen base
(4 possible types)
The structure of DNA
• There are 4 possible
types of nitrogen bases in
a nucleotide.
• The nitrogen bases are:
–
–
–
–
Adenine (A)
Guanine (G)
Cytosine (C)
Thymine (T)
• Nucleotides are named
from the nitrogen base
they have.
The structure of DNA
• DNA is composed of many
nucleotides joined together.
– the sequence (order) of the
nucleotides is the code for
making proteins
• The phosphate group of
one nucleotide bonds to the
deoxyribose sugar of an
adjacent nucleotide.
– This forms the skeleton of the
DNA molecule called the
sugar-phosphate backbone.
• The 4 nitrogen bases stick
out from the backbone like
teeth in a zipper.
The Double Helix
• Watson and Crick
discovered that DNA
was made up of two long
chains of twisted
nucleotides called the
double helix.
• Think of the double helix
like a twisted zipper.
• The bases will only bond to
its complementary base:
– Adenine bonds to Thymine
– Guanine bonds to Cytosine
– Because of the
complementary base pairing,
adenine and thymine always
exist in equal amounts; and
guanine and cytosine always
exist in equal amount.
– There are not always equal
amounts of A-T to G-C
however.
DNA Replication
•
Replication: The process by which a DNA
molecule makes an exact copy of itself.
•
Replication occurs during Interphase – S of
the cell cycle.
•
Replication begins by the DNA molecule
unzipping.
– This is done by an enzyme which cuts the
bonds between bases (A-T, G-C).
•
As the DNA unzips, another enzyme bonds
free-floating nucleotides to each of the
exposed nucleotides in the single strands.
•
Another enzyme bonds the sugar phosphate
backbone until each parent strand has a
new complementary daughter strand.
•
Because the new DNA molecules have one
old strand (parent), and one new strand
(daughter), we say that replication is a
semi-conservative process.
DNA is instructions for making proteins
• The sequence (or order) of nucleotides determines what type
of protein will be made.
• A gene is a segment of DNA that holds the instructions for
making a single protein.
• One chromosome can have thousands of genes in it.
Making proteins from genes
• DNA doesn’t make proteins It gets help from a molecule
called RNA.
• RNA differs from DNA in
three ways:
– It is usually single stranded
rather than double stranded.
– It has a ribose sugar rather
than a deoxyribose sugar.
– It contains the base uracil in
place of thymine.
Making proteins from genes
• There are 3 types of RNA that help to
build proteins.
– messenger RNA (mRNA) – is a copy of the
gene that is needed to make a protein.
– ribosomal RNA (rRNA or ribosome) –
reads the mRNA and assembles the protein.
– transfer RNA (tRNA) – brings the amino
acids to the ribosome for protein assembly.
Making proteins from genes
• A cell doesn’t always need every
protein to be made all the time
(that would be too much work).
• So the cell tells the DNA which
gene it needs copies of, in order
to make the protein that it needs
at a particular time.
• In other words, the gene is
“switched on” or activated
when the cell needs that protein.
Transcription
• Transcription – the
process in which
enzymes make an
mRNA copy of a
gene.
• The mRNA is then
moved into the
cytoplasm for
translation (the next
step).
Translation
• Translation – is the
process of reading the
mRNA and assembling
the protein.
• tRNA brings the amino
acids to the ribosome for
assembly into a protein.
• How does the tRNA know which
amino acid to bring to the ribosome?
• Each 3 bases in the mRNA (called a
codon), codes for a single amino acid.
• A tRNA molecule has three bases on it
that are complementary to the codon,
called an anticodon.
• Each tRNA carries only the amino acid
that it’s anticodon specifies.
• The process continues until a codon
that means “stop” is reached by the
ribosome. Then the ribosome releases
the amino acid chain.
• Once the amino acid chain is released,
it folds into a distinct shape and is a
protein.
Translation
Mutations
• Mutations are changes in a DNA
sequence. As a result, the protein that is
coded for is also changed.
• Sometimes DNA mutations are harmless
and have no effect on the cell. More often,
however, mutations can have negative and
even life-threatening results.
Mutations
• Mutations can occur in body cells and
gametes.
• If the mutation occurs in the gametes, the
mutation is passed on to the offspring.
• When a mutation occurs in a body cell, the
mutation is not inherited by the offspring,
but the mutation is passed to the daughter
cells when the mutated cell divides during
mitosis.
Cancer
• Some mutations
in body cells affect
the genes that
control mitosis.
When this
happens, the cell
divides
uncontrollably and
rapidly; this is
cancer.
• The following
pictures are of
breast cancer
and are graphic
in nature.
Types of Mutations
• A point mutation is a
change in a single base
pair (A-T, G-C).
• A point mutation
causes an incorrect
amino acid to be
inserted into the
growing amino acid
chain during
translation; this
results in a protein
that does not function
properly.
Types of Mutations
• A frameshift mutation
is the insertion or
deletion of a single base
(A, T, G, C).
• A frameshift mutation
causes each codon to
be out of place by one
letter; as a result every
amino acid from the
insertion / deletion is
incorrect.
Frameshift mutation
Chromosomal Mutations
• Much larger mutations can occur at the chromosome level.
• Entire pieces of chromosomes can be broken off and lost
during mitosis and meiosis. Often times, the broken pieces
then rejoin the chromosome incorrectly.
• Changes to the structure of the chromosome are called
chromosomal mutations.
Chromosomal Mutations
• Chromosomal mutations occur in all organisms,
but are most common in plants.
• Few chromosomal mutations are passed to
offspring, because the zygote with the
chromosomal mutations usually dies.
• In cases where the zygote lives, the mature
organism with a chromosomal mutation is often
sterile.
Causes of Mutations
• Some mutations are
spontaneous; they just
happen by chance during
DNA replication.
• However, many mutations
are caused by
environmental factors,
including:
• Radiation
(gamma rays, UV rays, Xrays)
• Chemicals
(formaldehyde, dioxins,
benzene, cyanide, asbestos)
• Any agent that can cause a
mutation is called a
mutagen.
Repairing DNA
• There are enzymes that proofread DNA and replace incorrect
nucleotide sequences with the correct sequences.
• The repair enzymes work well, but are not perfect. The greater
the damage, the harder it is to repair.
TEST – NEXT CLASS!
• Structure of DNA (double helix; nucleotide parts; 4
nitrogen bases and how they bond; Watson, Crick,
Wilkins, and Franklin; sugar-phosphate backbone)
• DNA replication (how and when it happens)
• Activation, Transcription, Translation (gene,
differences between RNA and DNA, mRNA, rRNA,
tRNA, codon, anticodon, amino acid, protein)
• Mutations (mutagen, point mutation, frameshift
mutation, chromosomal mutations, DNA repair)