Transcript Lesson Overview - SHS-Foundations-Biology-2014

Lesson Overview
12.1 Identifying the
Substance of Genes
Lesson Overview
Identifying the Substance of Genes
THINK ABOUT IT
How do genes work?
To answer that question, the first thing you need to know is what genes
are made of.
How would you go about figuring out what molecule or molecules go
into making a gene?
Lesson Overview
Identifying the Substance of Genes
The Role of DNA
What is the role of DNA in heredity?
Lesson Overview
Identifying the Substance of Genes
The Role of DNA
What is the role of DNA in heredity?
The DNA that makes up genes must be capable of storing, copying, and
transmitting the genetic information in a cell.
Lesson Overview
Identifying the Substance of Genes
The Role of DNA
The DNA that makes up genes must be capable of storing, copying, and
transmitting the genetic information in a cell.
These three functions are analogous to the way in which you might share a
treasured book, as pictured in the figure.
Lesson Overview
Identifying the Substance of Genes
Storing Information
The foremost job of DNA, as the molecule of heredity, is to store
information.
Genes control patterns of development, which means that the
instructions that cause a single cell to develop into an oak tree, a sea
urchin, or a dog must somehow be written into the DNA of each of these
organisms.
Lesson Overview
Identifying the Substance of Genes
Copying Information
Before a cell divides, it must make a complete copy of every one of its
genes, similar to the way that a book is copied.
Lesson Overview
Identifying the Substance of Genes
Copying Information
To many scientists, the most puzzling aspect of DNA was how it could
be copied.
Once the structure of the DNA molecule was discovered, a copying
mechanism for the genetic material was soon put forward.
Lesson Overview
Identifying the Substance of Genes
Transmitting Information
When a cell divides, each daughter cell must receive a complete copy of
the genetic information.
Careful sorting is especially important during the formation of
reproductive cells in meiosis.
The loss of any DNA during meiosis might mean a loss of valuable
genetic information from one generation to the next.
Lesson Overview
12.2 The Structure of DNA
Lesson Overview
Identifying the Substance of Genes
THINK ABOUT IT
The DNA molecule must somehow specify how to assemble proteins,
which are needed to regulate the various functions of each cell.
What kind of structure could serve this purpose without varying from cell
to cell?
Understanding the structure of DNA has been the key to understanding
how genes work.
Lesson Overview
Identifying the Substance of Genes
The Components of DNA
What are the chemical components of DNA?
Lesson Overview
Identifying the Substance of Genes
The Components of DNA
What are the chemical components of DNA?
DNA is a nucleic acid made up of nucleotides joined into long strands or
chains by covalent bonds.
Lesson Overview
Identifying the Substance of Genes
Nucleic Acids and Nucleotides
Nucleic acids are long, slightly acidic molecules originally identified in
cell nuclei.
Nucleic acids are made up of nucleotides, linked together to form long
chains.
The nucleotides that make up DNA are shown.
Lesson Overview
Identifying the Substance of Genes
Nucleic Acids and Nucleotides
DNA’s nucleotides are made up of three basic components: a 5-carbon
sugar called deoxyribose, a phosphate group, and a nitrogenous base.
Lesson Overview
Identifying the Substance of Genes
Nitrogenous Bases and Covalent Bonds
The nucleotides in a strand of DNA are joined by covalent bonds formed
between their sugar and phosphate groups.
Lesson Overview
Identifying the Substance of Genes
Nitrogenous Bases and Covalent Bonds
DNA has four kinds of nitrogenous bases: adenine (A), guanine (G),
cytosine (C), and thymine (T).
The nitrogenous bases stick out sideways from the nucleotide chain.
Lesson Overview
Identifying the Substance of Genes
Nitrogenous Bases and Covalent Bonds
The nucleotides can be joined together in any order, meaning that any
sequence of bases is possible.
Lesson Overview
Identifying the Substance of Genes
Solving the Structure of DNA
What clues helped scientists solve the structure of DNA?
Lesson Overview
Identifying the Substance of Genes
Solving the Structure of DNA
What clues helped scientists solve the structure of DNA?
The clues in Franklin’s X-ray pattern enabled Watson and Crick to build a
model that explained the specific structure and properties of DNA.
Lesson Overview
Identifying the Substance of Genes
Chargaff’s Rules
Erwin Chargaff discovered that the percentages of adenine [A] and
thymine [T] bases are almost equal in any sample of DNA.
The same thing is true for the other two nucleotides, guanine [G] and
cytosine [C].
The observation that [A] = [T] and [G] = [C] became known as one of
“Chargaff’s rules.”
Lesson Overview
Identifying the Substance of Genes
Franklin’s X-Rays
In the 1950s, British scientist Rosalind Franklin used a technique called
X-ray diffraction to get information about the structure of the DNA
molecule.
Lesson Overview
Identifying the Substance of Genes
Franklin’s X-Rays
X-ray diffraction revealed an X-shaped pattern showing that the strands
in DNA are twisted around each other like the coils of a spring.
The angle of the X-shaped pattern suggested that there are two strands
in the structure.
Other clues suggest that the nitrogenous bases are near the center of
the DNA molecule.
Lesson Overview
Identifying the Substance of Genes
The Work of Watson and Crick
At the same time, James Watson, an American biologist, and Francis
Crick, a British physicist, were also trying to understand the structure of
DNA.
They built three-dimensional models of the molecule.
Lesson Overview
Identifying the Substance of Genes
The Work of Watson and Crick
Early in 1953, Watson was shown a copy of Franklin’s X-ray pattern.
The clues in Franklin’s X-ray pattern enabled Watson and Crick to build
a model that explained the specific structure and properties of DNA.
Lesson Overview
Identifying the Substance of Genes
The Work of Watson and Crick
Watson and Crick’s breakthrough model of DNA was a double helix, in
which two strands were wound around each other.
Lesson Overview
Identifying the Substance of Genes
The Double-Helix Model
What does the double-helix model tell us about DNA?
Lesson Overview
Identifying the Substance of Genes
The Double-Helix Model
What does the double-helix model tell us about DNA?
The double-helix model explains Chargaff’s rule of base pairing and
how the two strands of DNA are held together.
Lesson Overview
Identifying the Substance of Genes
The Double-Helix Model
A double helix looks like a twisted ladder.
In the double-helix model of DNA, the two strands twist around each
other like spiral staircases.
The double helix accounted for Franklin’s X-ray pattern and explains
Chargaff’s rule of base pairing and how the two strands of DNA are
held together.
Lesson Overview
Identifying the Substance of Genes
Antiparallel Strands
In the double-helix model, the two strands of DNA are “antiparallel”—
they run in opposite directions.
This arrangement enables the nitrogenous bases on both strands to
come into contact at the center of the molecule.
It also allows each strand of the double helix to carry a sequence of
nucleotides, arranged almost like letters in a four-letter alphabet.
Lesson Overview
Identifying the Substance of Genes
Hydrogen Bonding
Watson and Crick discovered
that hydrogen bonds could
form between certain
nitrogenous bases, providing
just enough force to hold the
two DNA strands together.
Hydrogen bonds are relatively
weak chemical forces that
allow the two strands of the
helix to separate.
The ability of the two strands
to separate is critical to DNA’s
functions.
Lesson Overview
Identifying the Substance of Genes
Base Pairing
Watson and Crick’s model
showed that hydrogen bonds
could create a nearly perfect fit
between nitrogenous bases
along the center of the molecule.
These bonds would form only
between certain base pairs—
adenine with thymine, and
guanine with cytosine.
This nearly perfect fit between
A–T and G–C nucleotides is
known as base pairing, and is
illustrated in the figure.
Lesson Overview
Identifying the Substance of Genes
Base Pairing
Watson and Crick realized that
base pairing explained
Chargaff’s rule. It gave a reason
why [A] = [T] and [G] = [C].
For every adenine in a doublestranded DNA molecule, there
had to be exactly one thymine.
For each cytosine, there was one
guanine.
Lesson Overview
12.3 DNA Replication
Lesson Overview
Identifying the Substance of Genes
THINK ABOUT IT
Before a cell divides, its DNA must first be copied.
How might the double-helix structure of DNA make that possible?
Lesson Overview
Identifying the Substance of Genes
Copying the Code
What role does DNA polymerase play in copying DNA?
Lesson Overview
Identifying the Substance of Genes
Copying the Code
What role does DNA polymerase play in copying DNA?
DNA polymerase is an enzyme that joins individual nucleotides to produce
a new strand of DNA.
Lesson Overview
Identifying the Substance of Genes
Copying the Code
Base pairing in the double helix explained how DNA could be copied, or
replicated, because each base on one strand pairs with only one base on
the opposite strand.
Each strand of the double helix has all the information needed to
reconstruct the other half by the mechanism of base pairing.
Because each strand can be used to make the other strand, the strands
are said to be complementary.
Lesson Overview
Identifying the Substance of Genes
The Replication Process
Before a cell divides, it duplicates its DNA in a copying process called
replication.
This process ensures that each resulting cell has the same complete set of
DNA molecules.
Lesson Overview
Identifying the Substance of Genes
The Replication Process
During replication, the DNA molecule separates into two strands and then
produces two new complementary strands following the rules of base
pairing.
Each strand of the double helix of DNA serves as a template, or model, for
the new strand.
Lesson Overview
Identifying the Substance of Genes
The Replication Process
The two strands of the double helix separate, or “unzip,” allowing two
replication forks to form.
Lesson Overview
Identifying the Substance of Genes
The Replication Process
As each new strand forms, new bases are added following the rules of
base pairing.
If the base on the old strand is adenine, then thymine is added to the newly
forming strand.
Likewise, guanine is always paired to cytosine.
Lesson Overview
Identifying the Substance of Genes
The Replication Process
The result of replication is two DNA molecules identical to each other and
to the original molecule.
Each DNA molecule resulting from replication has one original strand and
one new strand.
Lesson Overview
Identifying the Substance of Genes
The Role of Enzymes
DNA replication is carried out by a series of enzymes. They first “unzip” a
molecule of DNA by breaking the hydrogen bonds between base pairs and
unwinding the two strands of the molecule.
Each strand then serves as a template for the attachment of
complementary bases.
Lesson Overview
Identifying the Substance of Genes
The Role of Enzymes
The principal enzyme involved in DNA replication is called DNA
polymerase.
DNA polymerase is an enzyme that joins individual nucleotides to produce
a new strand of DNA.
DNA polymerase also “proofreads” each new DNA strand, ensuring that
each molecule is a perfect copy of the original.
Lesson Overview
13.1 RNA
Lesson Overview
Identifying the Substance of Genes
THINK ABOUT IT
DNA is the genetic material of cells. The sequence of nucleotide bases
in the strands of DNA carries some sort of code. In order for that code to
work, the cell must be able to understand it.
What, exactly, do those bases code for? Where is the cell’s decoding
system?
Lesson Overview
Identifying the Substance of Genes
The Role of RNA
How does RNA differ from DNA?
Lesson Overview
Identifying the Substance of Genes
The Role of RNA
How does RNA differ from DNA?
There are three important differences between RNA and DNA: (1) the
sugar in RNA is ribose instead of deoxyribose, (2) RNA is generally singlestranded and not double-stranded, and (3) RNA contains uracil in place of
thymine.
Lesson Overview
Identifying the Substance of Genes
The Role of RNA
Genes contain coded DNA instructions that tell cells how to build proteins.
The first step in decoding these genetic instructions is to copy part of the
base sequence from DNA into RNA.
RNA, like DNA, is a nucleic acid that consists of a long chain of
nucleotides.
RNA then uses the base sequence copied from DNA to direct the
production of proteins.
Lesson Overview
Identifying the Substance of Genes
Comparing RNA and DNA
Each nucleotide in both DNA and RNA is made up of a 5-carbon sugar,
a phosphate group, and a nitrogenous base.
There are three important differences between RNA and DNA:
(1) The sugar in RNA is ribose instead of deoxyribose.
(2) RNA is generally single-stranded and not double-stranded.
(3) RNA contains uracil in place of thymine.
These chemical differences make it easy for the enzymes in the cell to
tell DNA and RNA apart.
Lesson Overview
Identifying the Substance of Genes
Comparing RNA and DNA
The roles played by DNA and RNA are similar to the master plans and
blueprints used by builders.
Lesson Overview
Identifying the Substance of Genes
Comparing RNA and DNA
A master plan has all the information needed to construct a building.
Builders never bring a valuable master plan to the building site, where it
might be damaged or lost. Instead, they prepare inexpensive,
disposable copies of the master plan called blueprints.
Lesson Overview
Identifying the Substance of Genes
Comparing RNA and DNA
Similarly, the cell uses DNA “master plan” to prepare RNA “blueprints.”
The DNA molecule stays safely in the cell’s nucleus, while RNA
molecules go to the protein-building sites in the cytoplasm—the
ribosomes.
Lesson Overview
Identifying the Substance of Genes
Functions of RNA
You can think of an RNA molecule, as a disposable copy of a segment
of DNA, a working copy of a single gene.
RNA has many functions, but most RNA molecules are involved in
protein synthesis only.
RNA controls the assembly of amino acids into proteins. Each type of
RNA molecule specializes in a different aspect of this job.
Lesson Overview
Identifying the Substance of Genes
Functions of RNA
The three main types of RNA are messenger RNA, ribosomal RNA, and
transfer RNA.
Lesson Overview
Identifying the Substance of Genes
Messenger RNA
Most genes contain instructions for
assembling amino acids into
proteins.
The RNA molecules that carry
copies of these instructions are
known as messenger RNA
(mRNA): They carry information
from DNA to other parts of the cell.
Lesson Overview
Identifying the Substance of Genes
Ribosomal RNA
Proteins are assembled on ribosomes,
small organelles composed of two
subunits.
These ribosome subunits are made up
of several ribosomal RNA (rRNA)
molecules and as many as 80 different
proteins.
Lesson Overview
Identifying the Substance of Genes
Transfer RNA
When a protein is built, a transfer
RNA (tRNA) molecule transfers each
amino acid to the ribosome as it is
specified by the coded messages in
mRNA.
Lesson Overview
Identifying the Substance of Genes
RNA Synthesis
How does the cell make RNA?
Lesson Overview
Identifying the Substance of Genes
RNA Synthesis
How does the cell make RNA?
In transcription, segments of DNA serve as templates to produce
complementary RNA molecules.
Lesson Overview
Identifying the Substance of Genes
Transcription
Most of the work of making RNA takes place during transcription.
During transcription, segments of DNA serve as templates to produce
complementary RNA molecules.
The base sequences of the transcribed RNA complement the base
sequences of the template DNA.
Lesson Overview
Identifying the Substance of Genes
Transcription
In prokaryotes, RNA synthesis and protein synthesis take place in the
cytoplasm.
In eukaryotes, RNA is produced in the cell’s nucleus and then moves to
the cytoplasm to play a role in the production of proteins. Our focus will
be on transcription in eukaryotic cells.
Lesson Overview
Identifying the Substance of Genes
Transcription
Transcription requires an enzyme, known as RNA polymerase, that is
similar to DNA polymerase.
Lesson Overview
Identifying the Substance of Genes
Transcription
RNA polymerase binds to DNA during transcription and separates the
DNA strands.
Lesson Overview
Identifying the Substance of Genes
Transcription
RNA polymerase then uses one strand of DNA as a template from
which to assemble nucleotides into a complementary strand of RNA.
Lesson Overview
13.2 Ribosomes and
Protein Synthesis
Lesson Overview
Identifying the Substance of Genes
THINK ABOUT IT
How would you build a system to read the messages that are coded in
genes and transcribed into RNA?
Would you read the bases one at a time, as if the code were a language
with just four words—one word per base?
Perhaps you would read them as individual letters that can be combined
to spell longer words.
Lesson Overview
Identifying the Substance of Genes
The Genetic Code
What is the genetic code, and how is it read?
Lesson Overview
Identifying the Substance of Genes
The Genetic Code
What is the genetic code, and how is it read?
The genetic code is read three “letters” at a time, so that each “word” is
three bases long and corresponds to a single amino acid.
Lesson Overview
Identifying the Substance of Genes
The Genetic Code
The first step in decoding genetic messages is to transcribe a nucleotide
base sequence from DNA to RNA.
This transcribed information contains a code for making proteins.
Lesson Overview
Identifying the Substance of Genes
The Genetic Code
Proteins are made by joining amino acids together into long chains, called
polypeptides.
As many as 20 different amino acids are commonly found in polypeptides.
Lesson Overview
Identifying the Substance of Genes
The Genetic Code
The specific amino acids in a polypeptide, and the order in which they are
joined, determine the properties of different proteins.
The sequence of amino acids influences the shape of the protein, which in
turn determines its function.
Lesson Overview
Identifying the Substance of Genes
The Genetic Code
RNA contains four different bases: adenine, cytosine, guanine, and uracil.
These bases form a “language,” or genetic code, with just four “letters”:
A, C, G, and U.
Lesson Overview
Identifying the Substance of Genes
The Genetic Code
Each three-letter “word” in mRNA is known as a codon.
A codon consists of three consecutive bases that specify a single amino
acid to be added to the polypeptide chain.
Lesson Overview
Identifying the Substance of Genes
How to Read Codons
Because there are four
different bases in RNA, there
are 64 possible three-base
codons (4 × 4 × 4 = 64) in
the genetic code.
This circular table shows the
amino acid to which each of
the 64 codons corresponds.
To read a codon, start at the
middle of the circle and move
outward.
Lesson Overview
Identifying the Substance of Genes
How to Read Codons
Most amino acids can be
specified by more than one
codon.
For example, six different
codons—UUA, UUG, CUU, CUC,
CUA, and CUG—specify leucine.
But only one codon—UGG—
specifies the amino acid
tryptophan.
Lesson Overview
Identifying the Substance of Genes
Start and Stop Codons
The genetic code has punctuation
marks.
The methionine codon AUG
serves as the initiation, or “start,”
codon for protein synthesis.
Following the start codon, mRNA
is read, three bases at a time, until
it reaches one of three different
“stop” codons, which end
translation.
Lesson Overview
Identifying the Substance of Genes
Translation
What role does the ribosome play in assembling proteins?
Lesson Overview
Identifying the Substance of Genes
Translation
What role does the ribosome play in assembling proteins?
Ribosomes use the sequence of codons in mRNA to assemble amino
acids into polypeptide chains.
Lesson Overview
Identifying the Substance of Genes
Translation
The sequence of nucleotide bases in an mRNA molecule is a set of
instructions that gives the order in which amino acids should be joined to
produce a polypeptide.
The forming of a protein requires the folding of one or more polypeptide
chains.
Ribosomes use the sequence of codons in mRNA to assemble amino
acids into polypeptide chains.
The decoding of an mRNA message into a protein is a process known as
translation.
Lesson Overview
Identifying the Substance of Genes
Steps in Translation
Messenger RNA is transcribed in the nucleus and then enters the
cytoplasm for translation.
Lesson Overview
Identifying the Substance of Genes
Steps in Translation
Translation begins when a
ribosome attaches to an
mRNA molecule in the
cytoplasm.
As the ribosome reads each
codon of mRNA, it directs
tRNA to bring the specified
amino acid into the ribosome.
One at a time, the ribosome
then attaches each amino acid
to the growing chain.
Lesson Overview
Identifying the Substance of Genes
Steps in Translation
Each tRNA molecule carries just
one kind of amino acid.
In addition, each tRNA molecule
has three unpaired bases,
collectively called the
anticodon—which is
complementary to one mRNA
codon.
The tRNA molecule for
methionine has the anticodon
UAC, which pairs with the
methionine codon, AUG.
Lesson Overview
Identifying the Substance of Genes
Steps in Translation
The ribosome has a second
binding site for a tRNA molecule
for the next codon.
If that next codon is UUC, a
tRNA molecule with an AAG
anticodon brings the amino acid
phenylalanine into the
ribosome.
Lesson Overview
Identifying the Substance of Genes
Steps in Translation
The ribosome helps form a
peptide bond between the first
and second amino acids—
methionine and phenylalanine.
At the same time, the bond
holding the first tRNA molecule
to its amino acid is broken.
Lesson Overview
Identifying the Substance of Genes
Steps in Translation
That tRNA then moves into a
third binding site, from which it
exits the ribosome.
The ribosome then moves to
the third codon, where tRNA
brings it the amino acid
specified by the third codon.
Lesson Overview
Identifying the Substance of Genes
Steps in Translation
The polypeptide chain
continues to grow until the
ribosome reaches a “stop”
codon on the mRNA
molecule.
When the ribosome reaches
a stop codon, it releases both
the newly formed polypeptide
and the mRNA molecule,
completing the process of
translation.
Lesson Overview
Identifying the Substance of Genes
The Roles of tRNA and rRNA in
Translation
Ribosomes are composed of roughly 80 proteins and three or four
different rRNA molecules.
These rRNA molecules help hold ribosomal proteins in place and help
locate the beginning of the mRNA message.
They may even carry out the chemical reaction that joins amino acids
together.
Lesson Overview
Identifying the Substance of Genes
The Molecular Basis of Heredity
What is the “central dogma” of molecular biology?
Lesson Overview
Identifying the Substance of Genes
The Molecular Basis of Heredity
What is the “central dogma” of molecular biology?
The central dogma of molecular biology is that information is transferred
from DNA to RNA to protein.
Lesson Overview
Identifying the Substance of Genes
The Molecular Basis of Heredity
Most genes contain instructions for assembling proteins.
Lesson Overview
Identifying the Substance of Genes
The Molecular Basis of Heredity
Many proteins are enzymes, which catalyze and regulate chemical
reactions.
A gene that codes for an enzyme to produce pigment can control the color
of a flower. Another gene produces proteins that regulate patterns of tissue
growth in a leaf. Yet another may trigger the female or male pattern of
development in an embryo.
Proteins are microscopic tools, each specifically designed to build or
operate a component of a living cell.
Lesson Overview
Identifying the Substance of Genes
The Molecular Basis of Heredity
Molecular biology seeks to explain living organisms by studying them at the
molecular level, using molecules like DNA and RNA.
The central dogma of molecular biology is that information is transferred
from DNA to RNA to protein.
There are many exceptions to this “dogma,” but it serves as a useful
generalization that helps explain how genes work.
Lesson Overview
Identifying the Substance of Genes
The Molecular Basis of Heredity
Gene expression is the way in which DNA, RNA, and proteins are
involved in putting genetic information into action in living cells.
DNA carries information for specifying the traits of an organism.
The cell uses the sequence of bases in DNA as a template for making
mRNA.
Lesson Overview
Identifying the Substance of Genes
The Molecular Basis of Heredity
The codons of mRNA specify the sequence of amino acids in a protein.
Proteins, in turn, play a key role in producing an organism’s traits.
Lesson Overview
Identifying the Substance of Genes
The Molecular Basis of Heredity
One of the most interesting discoveries of molecular biology is the nearuniversal nature of the genetic code.
Although some organisms show slight variations in the amino acids
assigned to particular codons, the code is always read three bases at a
time and in the same direction.
Despite their enormous diversity in form and function, living organisms
display remarkable unity at life’s most basic level, the molecular biology of
the gene.
Lesson Overview
13.3 Mutations
Lesson Overview
Identifying the Substance of Genes
THINK ABOUT IT
The sequence of bases in DNA are like the letters of a coded message.
What would happen if a few of those letters changed accidentally,
altering the message?
What effects would you predict such changes to have on genes and the
polypeptides for which they code?
Lesson Overview
Identifying the Substance of Genes
Types of Mutations
What are mutations?
Lesson Overview
Identifying the Substance of Genes
Types of Mutations
What are mutations?
Mutations are heritable changes in genetic information.
Lesson Overview
Identifying the Substance of Genes
Types of Mutations
Now and then cells make mistakes in copying their own DNA,
inserting the wrong base or even skipping a base as a strand is put
together.
These variations are called mutations, from the Latin word mutare,
meaning “to change.”
Mutations are heritable changes in genetic information.
Lesson Overview
Identifying the Substance of Genes
Types of Mutations
All mutations fall into two basic categories:
Those that produce changes in a single gene are known as gene
mutations.
Those that produce changes in whole chromosomes are known as
chromosomal mutations.
Lesson Overview
Identifying the Substance of Genes
Gene Mutations
Mutations that involve changes in one or a few nucleotides are known
as point mutations because they occur at a single point in the DNA
sequence. They generally occur during replication.
If a gene in one cell is altered, the alteration can be passed on to every
cell that develops from the original one.
Lesson Overview
Identifying the Substance of Genes
Gene Mutations
Point mutations include substitutions, insertions, and deletions.
Lesson Overview
Identifying the Substance of Genes
Substitutions
In a substitution, one base is changed to a different base.
Substitutions usually affect no more than a single amino acid, and
sometimes they have no effect at all.
Lesson Overview
Identifying the Substance of Genes
Substitutions
In this example, the base cytosine is replaced by the base thymine,
resulting in a change in the mRNA codon from CGU (arginine) to CAU
(histidine).
However, a change in the last base of the codon, from CGU to CGA for
example, would still specify the amino acid arginine.
Lesson Overview
Identifying the Substance of Genes
Insertions and Deletions
Insertions and deletions are point mutations in which one base is
inserted or removed from the DNA sequence.
If a nucleotide is added or deleted, the bases are still read in groups of
three, but now those groupings shift in every codon that follows the
mutation.
Lesson Overview
Identifying the Substance of Genes
Insertions and Deletions
Insertions and deletions are also called frameshift mutations because
they shift the “reading frame” of the genetic message.
Frameshift mutations can change every amino acid that follows the
point of the mutation and can alter a protein so much that it is unable to
perform its normal functions.
Lesson Overview
Identifying the Substance of Genes
Chromosomal Mutations
Chromosomal mutations involve changes in the number or structure of
chromosomes.
These mutations can change the location of genes on chromosomes
and can even change the number of copies of some genes.
There are four types of chromosomal mutations: deletion, duplication,
inversion, and translocation.
Lesson Overview
Identifying the Substance of Genes
Chromosomal Mutations
Deletion involves the loss of all or part of a chromosome.
Lesson Overview
Identifying the Substance of Genes
Chromosomal Mutations
Duplication produces an extra copy of all or part of a chromosome.
Lesson Overview
Identifying the Substance of Genes
Chromosomal Mutations
Inversion reverses the direction of parts of a chromosome.
Lesson Overview
Identifying the Substance of Genes
Chromosomal Mutations
Translocation occurs when part of one chromosome breaks off and
attaches to another.
Lesson Overview
Identifying the Substance of Genes
Effects of Mutations
How do mutations affect genes?
Lesson Overview
Identifying the Substance of Genes
Effects of Mutations
How do mutations affect genes?
The effects of mutations on genes vary widely. Some have little or no
effect; and some produce beneficial variations. Some negatively disrupt
gene function.
Mutations often produce proteins with new or altered functions that can be
useful to organisms in different or changing environments.
Lesson Overview
Identifying the Substance of Genes
Effects of Mutations
Genetic material can be altered by natural events or by artificial means.
The resulting mutations may or may not affect an organism.
Some mutations that affect individual organisms can also affect a species
or even an entire ecosystem.
Lesson Overview
Identifying the Substance of Genes
Effects of Mutations
Many mutations are produced by errors in genetic processes.
For example, some point mutations are caused by errors during DNA
replication.
The cellular machinery that replicates DNA inserts an incorrect base
roughly once in every 10 million bases.
Small changes in genes can gradually accumulate over time.
Lesson Overview
Identifying the Substance of Genes
Effects of Mutations
Stressful environmental conditions may cause some bacteria to increase
mutation rates.
This can actually be helpful to the organism, since mutations may
sometimes give such bacteria new traits, such as the ability to consume a
new food source or to resist a poison in the environment.
Lesson Overview
Identifying the Substance of Genes
Mutagens
Some mutations arise from mutagens, chemical or physical agents in
the environment.
Chemical mutagens include certain pesticides, a few natural plant
alkaloids, tobacco smoke, and environmental pollutants.
Physical mutagens include some forms of electromagnetic radiation,
such as X-rays and ultraviolet light.
Lesson Overview
Identifying the Substance of Genes
Mutagens
If these mutagens interact with DNA, they can produce mutations at
high rates.
Some compounds interfere with base-pairing, increasing the error rate
of DNA replication.
Others weaken the DNA strand, causing breaks and inversions that
produce chromosomal mutations.
Cells can sometimes repair the damage; but when they cannot, the
DNA base sequence changes permanently.
Lesson Overview
Identifying the Substance of Genes
Harmful and Helpful Mutations
The effects of mutations on genes vary widely. Some have little or no
effect; and some produce beneficial variations. Some negatively disrupt
gene function.
Whether a mutation is negative or beneficial depends on how its DNA
changes relative to the organism’s situation.
Mutations are often thought of as negative because they disrupt the
normal function of genes.
However, without mutations, organisms cannot evolve, because
mutations are the source of genetic variability in a species.
Lesson Overview
Identifying the Substance of Genes
Harmful Effects
Some of the most harmful mutations are those that dramatically change
protein structure or gene activity.
The defective proteins produced by these mutations can disrupt normal
biological activities, and result in genetic disorders.
Some cancers, for example, are the product of mutations that cause the
uncontrolled growth of cells.
Lesson Overview
Identifying the Substance of Genes
Harmful Effects
Sickle cell disease is a disorder associated with changes in the shape
of red blood cells. Normal red blood cells are round. Sickle cells
appear long and pointed.
Sickle cell disease is caused by a point mutation in one of the
polypeptides found in hemoglobin, the blood’s principal oxygencarrying protein.
Among the symptoms of the disease are anemia, severe pain,
frequent infections, and stunted growth.
Lesson Overview
Identifying the Substance of Genes
Beneficial Effects
Some of the variation produced by mutations can be highly
advantageous to an organism or species.
Mutations often produce proteins with new or altered functions that can
be useful to organisms in different or changing environments.
For example, mutations have helped many insects resist chemical
pesticides.
Some mutations have enabled microorganisms to adapt to new
chemicals in the environment.
Lesson Overview
Identifying the Substance of Genes
Beneficial Effects
Plant and animal breeders often make use of “good” mutations.
For example, when a complete set of chromosomes fails to separate
during meiosis, the gametes that result may produce triploid (3N) or
tetraploid (4N) organisms.
The condition in which an organism has extra sets of chromosomes is
called polyploidy.
Lesson Overview
Identifying the Substance of Genes
Beneficial Effects
Polyploid plants are often larger and stronger than diploid plants.
Important crop plants—including bananas and limes—have been
produced this way.
Polyploidy also occurs naturally in citrus plants, often through
spontaneous mutations.