Download Science 9 Unit A 3.0

Section 3.0
DNA is the Inherited Material Responsible for
Variation
Goals for this Section:
1. Describe the relationship among
chromosomes, genes, and DNA
2. Explain how DNA stores genetic
information
3. Distinguish between cell division during
sexual and asexual cell reproduction
4. Investigate the transmission of
characteristics from parents to offspring
Goals – Continued…
5. Identify examples of different types
of inheritance
6. Identify examples of dominant and
recessive characteristics
3.1 – DNA – The Transmitter
of Genetic Code
• Variation within a species is
necessary for the survival of the
species
• The source of this variation is the
genetic material that is present in all
cells
DNA and Variation
• As you have seen, sexual
reproduction involves recombining
the genes of two parents in one
offspring
• However, variation also exists in
those species that reproduce
asexually
• This is because of slight changes, or
mutations in their DNA
What Produces Variation?
• Essentially, all variation results from slight
differences in the genetic code of each
individual
• This genetic code is contained in the
molecule known as DNA
• Canadian scientist Oswald Avery was one
of the first people to suggest that DNA
was a molecule that stored information
that coded for inherited characteristics
DNA
• DNA is an abbreviation for
deoxyribonucleic acid
• All living organisms carry DNA in
their cells
• In some cases, the DNA is found in
the nucleus, but it also may be found
elsewhere in the cell
• DNA was first identified in 1869, but
its role was not understood until
1944, when Avery confirmed its role
in passing on heritable traits
• However, at this time, the actual
structure of DNA was not understood
Watson & Crick
• In 1953, James
Watson and
Francis Crick
determined that
DNA has a
structure similar
to a spiral
staircase
http://www.chem.ucsb.edu
The Structure of DNA
• DNA consists of a
sugar-phosphate
backbone with
nitrogen base pairs
in-between
http://www.biologycorner.com
The Genetic Code
• The instructions for the
characteristics that an organism
displays are contained in the genetic
code
• The genetic code consists of the four
nitrogen base pairs
• The sequence of these nitrogen
bases makes up genes that code for
proteins made by the cell
Chromosomes
• A chromosome is a single strand of
DNA
• A single chromosome can hold
numerous genes
• Human cells contain 46
chromosomes, arranged into 23
pairs
• However, other species have
different numbers of chromosomes
Genes
• All genes come in pairs
• Each pair of genes carries DNA for
the same trait (for example, leg
length in a fly)
• These pairs of genes are always
found at the same position on a
chromosome
• However, the code for each gene in
the pair may be different
Alleles
• Offspring will inherit genes from both
of their parents
http://www.csulb.edu
Multiple Genes & Alleles
• Often a trait depends not only on a
single gene
• As well, there is often more than one
or two possible alleles for a single
trait
3.2 – Cell Division
• Cell division can be associated with
sexual or asexual reproduction
• The type of cellular division involved
will affect the number of
chromosomes present at the end of
the process of cellular division
Mitosis
• Mitosis is the
form of cellular
division used
during asexual
reproduction
http://www.bbc.co.uk
Meiosis
• Meiosis
produces
gametes
(sex cells)
http://www.bbc.co.uk
A Comparison of Mitosis
and Meiosis
Characteristic
Original number of
chromosomes per
cell
Final number of
chromosomes per
cell
Number of cell
divisions
Mitosis
Meiosis
3.3 – Patterns of
Inheritance
• For centuries humans have been
breeding animals to enhance
desired traits
• Over time, purebred strains of
various types of animals and crops
were developed
Genetics Terms
• Purebred
• Hybrid
More Genetics Terms
• Dominant Trait
• Recessive Trait
Example – Crossing
Purebred White & Black
Cats
• Assume the black coat colour is
dominant to white in cats
• What would be the result of breeding
a purebred white cat and a purebred
black cat?
Example – Breeding of
Hybrids
• Now assume that two of the
offspring from the previous example
are bred together
• What would be the results of this
breeding?
Other Patterns of
Inheritance
• Occasionally, some traits show
incomplete dominance, where a trait
is a mixture of the traits of the
parents
• For instance, if a white snapdragon
is crossed with a red snapdragon,
the offspring will be pink
• In some cases, because multiple
alleles may be present, an offspring
may not have traits similar to either
parent
• Many traits are in fact far too
complex to explain using simple
patterns of dominant and recessive
genes
Environmental Factors
• Although genes for a particular trait
may be present, it may not be
expressed because of environmental
factors
• To be expressed, genes must be
turned “on” or “off”, and this often
depends on factors in the
environment
Examples of Traits with
Environmental Factors