Genetics: Cells, Chromosomes,
Cell Anatomy to Know:
1. Somatic Cell
2. Sex Cell
DNA Exists in Chromosomes
• Humans typically have 23 pairs of chromosomes for a total of 46
– 1 pair of sex chromosomes
– 22 pairs are autosomes The Chromosomes:
Each Cell Contains a Full Compliment of
The frog on the right was cloned from an
intestinal cell from the frog on the left
Mitosis: Somatic Cells Replicate to Grow or
Replace Old Cells
Meiosis: Sex Cells Replicate to Produce
Gametes (Egg and Sperm) for Reproduction
Meiosis Creates Variation: Independent Assortment
Meiosis Creates Variation: Crossing Over (a.k.a.
• DNA is exchanged
between the maternal
DNA Mutations: Another Source of Genetic
• A permanent structural alteration in DNA.
• In most cases, mutations either have no effect or cause
– Mutations with no effect occur because of the redundancy in
• Occasionally a mutation can improve an organism's
chance of surviving and reproducing. (Read: natural
• Mutations are essential for the survival of organisms.
Somatic and Germinal Mutations
• Somatic Mutations:
• Occur during mitosis
• Are not inherited
• Germinal mutations:
• Occur in sex cells
• Are inherited
• Occur during mitosis
• Are only transmitted to
the cells of the body that
descend from the mutated
• Have been linked to many
types of cancer
• On average, mutations
occur at any given locus
about once in every
million cell divisions Note the small island of albino
tissue in this man. This is the
result of a somatic mutation in
Down’s Syndrome: Trisomy 21
• Most individuals (95%) with trisomy 21
have 3 copies of chromosome 21
• Trisomy 21 (Down's Syndrome) occurs in
1/1,000 births for mothers below the age of
• increases to a frequency of 1/50 for mothers
over the age of 50.
Variation in sex chromosome number
• Y0= Lethal
• XXX= Super
• Other variations:
• Caused by the absence
of an X-
• Frequency: 1 in 2,500
• Causes short stature and
also associated with
• Males with an extra X
• There are also XXYY; XXXY;
XXXXY ; and XY/XXY variants.
• About 1 in 500 males are diagnosed
with this genetic anomaly.
• Males may have the inability to
produce sufficient amounts of
testosterone and experience infertility.
Breast development is sometimes
• Social problems, lower self esteem,
mood swings, and learning disabilites
are sometimes present
The XYY Syndrome: Born to kill?
• Men with a chromosome constitution 47,XYY.
• Approximately 1 of 1000 men
– Increased height (7cms above average)
– High activity levels and a tendency to a delayed
• Potentially dangerous? A 1961 study said so…
– Media hype on “super-males” with “criminality”
– Several court cases…XYY used as a defense
– Suggestions to screen babies so that adult anti-
social behavior could be anticipated.
• NOT TRUE!…many more studies showed NO
correlation between this syndrome and criminal
It was falsely rumored that
Richard Spect, the killer of 8
Chicago nursing students had
the XYY syndrome
• The genetic code—it
carries the instructions for
making living organisms.
• Double Helix Structure—
like an immensely long
ladder twisted into a
helix, or coil.
• The two halves of the
“ladder” are joined by
weak hydrogen bonds.
Watson and Crick
The Genetic Code
• A DNA nucleotide is made of a sugar, a
phosphate, and a BASE.
• The bases are the INFORMATION in the code.
• In base pairing, adenine always pairs with
thymine, and guanine always pairs with cytosine.
A = adenine
T = thymine
C = cytosine
G = guanine
What is a Gene?
• Definition: A sequence of
DNA that codes for a
• Human are estimated to have
• Language Analogy
– Letters = Bases
– Words = Codons
– Sentences = Gene
DNA Functions I: Replication
• The DNA unzips and
the two halves separate
• The empty sections of
each half are filled in by
• Each half is then
incorporated into a new
DNA Functions II: Protein Synthesis
• The DNA base sequence is read by
messenger RNA in the nucleus of
the cell (mRNA…uracil)
• mRNA leaves nucleus and transfers
base sequence information to
• In the ribosome, the base sequence
is read by transfer RNA (tRNA) and
the amino acids are placed in order.
• A chain of amino acids is a protein!
• The protein is then either secreted
from the cell or retained in the
cytoplasm where it become
Ribosomes producing proteins by reading mRNA
Genetic Code Redundancy
• Many mutations have no visible
effect because of the redundancy
in the genetic code.
– Different amino acids may serve the
same function (Lysine = Arginine).
– Different DNA may code for the
same amino acid (AGG and CGG
code for the amino acid Arginine).
Sequences of the same color code for
the same amino acid