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DNA and Genetics
The Basics
7th Science
Mr. Bombick
Major DNA Subunit
• Nucleotide contains a base, a
sugar, and a phosphate
• The four bases in DNA are
Adenine (A), Thymine (T),
Guanine (G), and Cytosine (C)
DNA Structure
• Each strand of the double stranded
DNA is comprised of nucleotides
connected by the phosphates
• The double strand of DNA is held
together by hydrogen bonding between
complementary bases
• A (adenine) is paired with T (thymine)
• G (guanine) is paired with C (cytosine)
DNA organization in the cell
• One double strand of DNA is
compacted within the nucleus by
coiling around specific proteins to
produce chromatin
• Chromatin is further compacted
into chromosomes
Information contained in DNA
• DNA contains many genes
• Genes are sections of DNA that
contain the code for proteins
• One gene is responsible for one
protein
• Proteins are responsible for the
different genetic traits in
individuals
DNA Mutations
• A mutation is a change in a
specific region of a gene
• The mutation results in an
alteration in the type of protein
(or whether a protein is made at
all) coded by the gene
• Mutations can be caused by a
variety of environmental or
physiological factors
DNA Replication
• DNA is replicated just prior to when a
non-sex cell undergoes cell division
• The double-stranded DNA is split down
the middle of the bases for a short
length of the DNA
• Nucleotides are inserted (with the
appropriate base pairing) into the
exposed single strand
• Completion of replication results in two
identical double-stranded DNA
molecules
Human Chromosome Organization
• Humans have 23 pairs of chromosomes (46
chromosomes)
• During non-germ cell division (mitosis) the
23 pairs of chromosomes are copied
resulting in each of the two new cells having
23 pairs of chromosomes
• During germ cell division (meiosis) each
germ (sperm or egg) ends up with only 23
chromosomes
• (use board to illustrate an example of
differences resulting from mitosis and
meiosis)
Importance of Meiosis in Genetics
• Meiosis results in each germ cell (egg
or sperm) having half of the
chromosomes of the normal cell (for
humans this would be 23 chromosomes)
• Fertilization (combination of egg and
sperm) will result in the normal number
of chromosomes in the cell (for humans
this would be 46 chromosomes)
• Therefore, half of the chromosomes
will come from the male and half of the
chromosomes will come from the
female
Basis of Genetics
• Probability
• Nature of the trait (gene or
allele)
• Behavior of chromosomes in
the process of meiosis
Movement of Chromosomes in Meiosis
• Meiosis is a process by which the
chromosome number is reduced by
one-half of the typical number of
chromosomes in a cell
• Therefore, each germ cell (egg or
sperm) contains one-half of the
number of chromosomes
• Fertilization will result in the
normal number of chromosomes
(for humans this equals 46)
Segregation of Alleles
• An allele is a form of a gene (a region of
the chromosome that “codes” for a
specific protein)
• In a simple situation, two types of
alleles might consist of dominant or
recessive alleles
• In meiosis the two alleles of the male or
female end up segregated so only one
allele is contained in the germ cell (egg
or sperm)
Types of Alleles
• Simplest case is where alleles are
dominant or recessive
• Alleles also may be co-dominant
where both alleles are expressed
in the offspring (blood type is a
typical example)
• Alleles also may exhibit incomplete
dominance (an example will follow
later in this presentation)
• Alleles may also be sex-linked
where the alleles are on the sex
chromosomes
Important Genetic Terminology
• Dominant Allele—results in the expression of
this trait in the offspring
• Recessive Allele—trait is only expressed in
offspring if there are two recessive alleles
• Heterozygous—presence of one dominant and
one recessive allele
• Homozygous—presence of two dominant or
two recessive alleles
• Phenotype—the resulting expression of the
two alleles in the offspring
• Genotype—the actual genetic composition of
the two alleles in the offspring
Punnett Square
• Graphical way of predicting possible
genetic outcomes (both genotypes and
phenotypes) in offspring from parents
• In a Punnett square the alleles from
each parent are shown on the top and
left side of the square
• The possible genotypes are then
calculated in the individual boxes in the
Punnett square
• Allows the calculation of ratios of
genotypes and phenotypes of the
offspring
Examples of Punnett Squares
(on the blackboard)
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Example
Example
Example
Example
Example
cross
of a single trait cross
of a cross with two traits
of a sex-linked cross
of co-dominant cross
of incomplete dominant
The End