Biology Ch. 11
DNA and Genes
11.1 DNA
 DNA controls the production of proteins

Living tissue is made up of protein, so DNA
determines an organism’s traits
 In 1952, experiments by Alfred Hershey
& Martha Chase demonstrated that DNA
is genetic material
 DNA stands for DeoxyriboNucleic Acid
11.1 The Makeup of DNA
 DNA is a polymer made up of
repeating nucleotides

Nucleotides have 3 parts
A
simple sugar

A
phosphate group

A
The sugar in DNA is deoxyribose
This is a phosphorus atom
surrounded by 4 oxygen atoms
nitrogenous base

There are 4 bases in DNA: adenine,
cytosine, guanine, and thymine
11.1 DNA chains
 In the DNA chain, nucleotides
bond between the phosphate
group of one nucleotide and the
deoxyribose sugar of the next
nucleotide


The phosphate groups and
deoxyribose sugars are called the
backbone
The nitrogenous bases stick out
from the backbone
11.1 Nitrogenous Bases
 The 4 nitrogenous bases in DNA are
adenine, guanine, cytosine and thymine
11.1 DNA structure
 In 1953, Watson & Crick proposed that
DNA was made of 2 strands of
nucleotides that were joined by the
nitrogenous bases

These bases paired in a certain way and
are held together by hydrogen bonds
 Adenine
pairs with thymine
 Guanine pairs with cytosine

These are called complementary base pairs
11.1 DNA 3-D structure

Watson &
Crick also
proposed
that the 2
strands were
joined, and
then twisted
to form a
shape called
a double
helix
11.1 Nucleotide Sequencing
 The order in which the nucleotides bond
determines the characteristics of that
organism

Evolutionary relationships, whether or not
2 people are related, and identity
confirmation of crime victims can all be
determined by analyzing this information
11.1 DNA Replication
 You have already learned that DNA must
replicate before cell division can occur

This process is called DNA replication
 An
enzyme separates the 2 strands of DNA
 Free nucleotides bond to the surface of each
separated DNA strand using the appropriate
base pairing (A-T & G-C)
 Another enzyme bonds these free
nucleotides together to make a new strand
 The result is 2 identical strands of DNA
11.2 RNA vs. DNA
 RNA is a nucleic acid similar to DNA, but
with a few minor differences:



RNA is a single strand instead of a double
strand like DNA
The sugar in RNA is ribose instead of the
deoxyribose in DNA
RNA has the nitrogenous base uracil,
instead of thymine
 Uracil
would
binds to adenine, just like thymine
11.2 Types of RNA
 There are 3 types of RNA and each type of
RNA has a different function in the cell:



Messenger RNA (mRNA)
 Brings instructions from the DNA in the nucleus to
a ribosome in the cytoplasm
Ribosomal RNA (rRNA)
 Binds to the mRNA and uses the instructions to
assemble the amino acids in the correct order
Transfer RNA (tRNA)
 Brings the amino acids to the ribosomes, where
they are assembled into a protein
11.2 Transcription
 In the nucleus, mRNA makes a copy of the DNA
in a process called transcription




First, the enzymes unzip the DNA strand
Then free RNA nucleotides bind to the
complementary nucleotide on one of the DNA
strands
Another enzyme binds the mRNA nucleotides
together
The mRNA strand then separates from the DNA
and the 2 DNA strands bind back together
11.2 Codons
 A codon is a series of 3 nitrogenous bases in mRNA that
code for a specific amino acid

There are 64 possible codons and 20 possible amino acids
 This means that each amino acid has more than one
possible 3 letter combination
 Table 11.1 on p. 292 shows all the possible codon
combinations and the amino acid associated with each codon
 There are also start and stop codons that signal the
beginning or end of a protein strand
11.2 Translation
 Translation is the process of converting
the information in a sequence of
nitrogenous bases in mRNA into a
sequence of amino acids in protein.


Translation occurs at the ribosomes in the
cytoplasm
In the cytoplasm, a ribosome attaches to
the strand of mRNA
11.2 transfer RNA
 The tRNA transfers the amino acids in
the cytoplasm to the ribosomes to be
made into proteins



There are 20 different amino acids
Each tRNA molecule attaches to only one
kind of amino acid
Each tRNA carries an anticodon that binds
to the codon in the mRNA
 The
anticodon is the complement to the
codon on the mRNA strand
11.2 The translation process
 a ribosome attaches to the starting end of the





mRNA strand
tRNA molecules, carrying specific amino acids
move towards the ribosome
The tRNA anticodons pair with the
complementary mRNA codon
The ribosome then slides down the mRNA
strand to the next codon and the process
repeats
The previous tRNA then separates, leaving the
amino acid and goes to get another amino acid
Eventually, a stop codon is reached, and the
amino acid chain is released from the ribosome
11.2 Proteins
 When amino acid chains are separated
from the ribosome, they twist and curl
into 3D shapes and become proteins

Each different protein strand forms the
same shape
 The process from DNA to mRNA to
proteins is called “the central dogma” of
biology

It is found in all organisms
11.3 Mutations
 Mutation- any change in the DNA
sequence

This can be caused by errors in replication,
transcription, cell division, or outside
factors
11.3 Cellular Mutations
 Mutations occur in 2 types of cells:


Reproductive cells- a change in the nucleotide
sequence in a sperm or egg cell
 These result in changes in the genes of that
individuals offspring
 It can harm the offspring, even causing the
embryo to die, or help the offspring
Body cells- changes in the DNA of a nonreproductive cell that are not passed on to offspring
 These can show no effect, damage individual
cells, or lead to the formation of cancer
11.3 Point Mutations
 A point mutation is a change in a single
base pair in DNA



It involves a switch in a single base pair
It results in the creation of a completely
different protein because one codon
changed, therefore one amino acid was
different
They are generally less harmful than
frameshift mutations
11.3 Frameshift Mutations
 A frameshift mutation is the addition or
deletion of a single base pair



This changes every codon after the
change, since the codons shift by one
base pair
This causes all of the amino acids following
the mutation to be different
They are generally more harmful than point
mutations
11.3 Chromosomal Mutations
 Chromosomal mutations are structural
changes in chromosomes


These mutations occur in all organisms,
but are most common in plants
These are often not passed on to offspring
because the embryo usually dies
 When
they do, they are unable to reproduce
11.3 Types of Chromosomal Mutations
 There are several types of chromosomal
mutations:
Nondisjunction- when chromosomes fail to
separate correctly during meiosis and results in
gametes with the incorrect number of
chromosomes
 Deletion- when part of a chromosome is left out
 Insertion-when part of a chromatid breaks off and
attaches to the sister chromatid
 Inversion- when part of a chromosome breaks off
and reattaches backwards
 Translocation- when part of a chromosome breaks
off and adds to another chromosome

11.3 Causes of Mutations
 Spontaneous mutations, such as errors in base
pairing, do not appear to have a specific cause
 Other mutations are caused by environmental
factors



Any agent that causes a change in DNA is called a
mutagen
Mutagens such as x-rays, UV light, and nuclear
radiation often causes deletions
Mutagens such as chemicals often cause
substitutions in the DNA chain