DNA contains genes that code for proteins. DNA is made up of nucleotides containing bases (A, T, G, C) bonded together. The double helix structure of DNA is maintained by hydrogen bonding between complementary bases (A-T and G-C). DNA is compacted and organized into chromosomes. Mutations in genes can alter protein production. DNA replicates before cell division to produce two identical DNA molecules. Meiosis reduces the chromosome number by half to produce gametes (eggs and sperm) for sexual reproduction and genetic variation in offspring.

1. DNA and Genetics
The Basics
7th Science
Mr. Bombick

2. 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)

3. 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)

4. 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

5. 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

6. 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

7. 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

8. 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)

9. 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

10. Basis of Genetics
• Probability
• Nature of the trait (gene or
allele)
• Behavior of chromosomes in
the process of meiosis

11. 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)

12. 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)

13. 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

14. 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

15. 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

16. Examples of Punnett Squares
(on the blackboard)
• Example of a single trait cross
• Example of a cross with two traits
• Example of a sex-linked cross
• Example of co-dominant cross
• Example of incomplete dominant
cross

17. The End