1. Biology

2. DNA, Genes, & Chromosomes
Chapter 7
DNA, Genes, & Chromosomes
I. Molecule of Heredity
A. DNA or Protein
1. In 1952, Chase and Hershey experimented with bacteriophages to
determine which carried the genetic material.
a. Phages with radioactive protein did not cause bacteria to change.
b. Phages with radioactive DNA caused bacteria to become radioactive.

3. II. DNA Structure and Replication
A. DNA Model
1. James Watson and Francis Crick proposed the current double helix model
a. Double-helix – a spiral twisted ladder
b. The “sides” are composed of alternating sugar (deoxyribose) and
phosphate molecules.
1) Make up the “backbone” of DNA
c. The “rungs” are composed of nitrogen bases that are attached to
the sugar.
1) Purines Pyrimidines
Adenine Thymine
Guanine Cytosine

4. 2) Adenine always pairs with Thymine
Guanine always pairs with Cytosine
a) Held together by weak Hydrogen bonds
b) Nucleotide – the basic unit of DNA
1) Made up of a sugar, phosphate, and a nitrogen base.

5. B. Replication – process by which DNA copies itself.
1. An enzyme breaks the weak H-bonds between the base pairs and the DNA
double helix unwinds.
2. The two strands separate and “free nucleotides” pair up with the exposed
bases.
3. Bonds form between the sugar and phosphates of nucleotides and a new
strand is created.
4. Eventually two new DNA double helixes are formed.

7. III. Linked Genes – genes that are located on the same chromosome
A. Sex-linked Inheritance – determined by genes on the sex chromosomes
X - Female XX
Y - Male XY
- are not homologous
- do not contain matching genes
1. Autosomes – the 22 pairs of matching homologous chromosomes
(non-sex chromosomes)

8. 2. Sex-linked traits – a recessive trait that is normally carried on the
X chromosome.
a. Appear more often in males than females
b. Color-blindness – Normal vision is dominant over color-blind
XX - Normal XY - Normal
XcX - Carrier XcY - Colorblind
XcXc - Colorblind
B. Sex-limited traits – are only expressed in the presence of sex hormones
1. Normally only observed in one sex
a. Beard growth in humans
b. Colorful feathers or plumage in birds.

9. C. Sex-Influenced trait – are expressed in both sexes, but are expressed differently
1. Gene is found on the autosomes
2. Baldness is a dominant trait in the presence of male hormones, but is
recessive in the presence of female hormones.
a. A woman has to have two “bald genes” in order to be bald (BB)

10. IV. Human Genomes
A. Karyotype – photograph showing all of an organisms chromosomes.
1. Allows for the study of different shapes, sizes, and structure of
chromosomes.
B. Genome – the base sequence of all the DNA in an organism.
1. Human Genome Project – has sequenced all of the DNA in the
human body.
a. Hopes of identifying all disease causing genes, as well as genes
for other traits.

11. C. Nondisjunction –the failure of chromosomes to separate correctly during
cell division.
1. In meiosis, can produce abnormal sex cells
a. Monosomy – having only one copy of a chromosome.
b. Trisomy – having three copies of a chromosome.
1) Down’s Syndrome – Trisomy 21
c. In sex chromosomes
1) XO XXY XYY XXX
Xxy—Male with female traits, breast, little to no hair.
Xyy— boys, taller than normal, speech and learning issues.
Xxx girls, taller than normal, speech and learning issues
XO—only females(1:2000), lacking one x chromosome, 95% are infertile and do not go through puberty.

12. D. Polyploidy – nondisjunction occurs in all chromosome pairs.
1. Results in an organism with more than two sets of chromosomes.
a. Almost always results in death in animals.
b. Can produce more robust and hardier plants.
1) Wheat used to make bread