Chromosomes, Genes,
Alleles and Mutations
What is a chromosome?
What is a gene?
A section of DNA that codes for a polypeptide
Allele
A version of a gene.
Trait = tongue rolling
Allele= roller or non roller
genome
All your genes!
Humans have 23 pairs of homologous
chromosomes
Mutations
Mutations
• Changes to DNA are called mutations
•
•
•
•
change the DNA
changes the mRNA
may change protein
may change trait
DNA
TACGCACATTTACGTACG
mRNA
AUGCGUGUAAAUGCAUGC
protein
aa aa aa aa aa aa aa
trait
What Causes Mutations?
• Mistakes when DNA is copied
• Inherited mutations (only occur when the
mutation was in the gametes (sperm or egg)
• Environmental factors:
• Mutagens
• Chemical: Pesticides, chemicals in
cigarette smoke
• Physical: x-rays, uv rays from the sun
Are Mutations Helpful or Harmful?
• Mutations happen
regularly
• Almost all mutations are
neutral
• Many mutations are
repaired by enzymes
Are Mutations Helpful or Harmful?
• Some type of skin cancers and
leukemia result from somatic
mutations
• Some mutations may improve an
organism’s survival (beneficial)
Types of Mutations
Gene Mutation Animation
Gene Mutations
Change in the nucleotide
sequence of a gene
May only involve a single
nucleotide
May be due to copying
errors, chemicals, viruses,
etc.
Types of Gene Mutations
Point Mutations
Substitutions
Insertions
Cause a frame shift
Deletions
Point Mutation
Change of a single nucleotide
Includes the deletion, insertion, or
substitution of ONE nucleotide in a
gene
Point Mutation
Sickle Cell disease is
the result of one
nucleotide
substitution
Occurs in the
hemoglobin gene
Frameshift Mutation
Inserting or deleting one
or more nucleotides
Changes the “reading
frame” like changing a
sentence
Proteins built incorrectly
Frameshift Mutation
• Original:
• The fat cat ate the wee rat.
• Frame Shift (“a” added):
• The fat aca tat eth ewe era t.
Amino Acid Sequence Changed
Gene Mutation Animation
HHMI sickle cell video
Describe the relationship between the
following terms…
Gene, Allele, Chromosome, Genome and DNA
Explain how sickle cell anemia came about as
a result of a mutation and relate that to the
process of transcription and translation.
Explain why the sickle cell mutation is so common in populations of
African decent.
Meiosis Overview
Major Understandings
• One diploid nucleus divides by meiosis to produce four haploid nuclei.
• The halving of the chromosome number allows a sexual life cycle with
fusion of gametes.
• DNA is replicated before meiosis so that all chromosomes consist of
two sister chromatids.
• The early stages of meiosis involve pairing of homologous chromosomes
and crossing over followed by condensation.
• Orientation of pairs of homologous chromosomes prior to separation
is random.
• Separation of pairs of homologous chromosomes in the first division
of meiosis halves the chromosome number.
• Crossing over and random orientation promotes genetic variation.
• Fusion of gametes from different parents promotes genetic variation
Chromosome Mutations
• May Involve:
• Changing the
structure of a
chromosome
• The loss or gain
of part of a
chromosome
Chromosome Mutations
• Five types exist:
• Deletion
• Inversion
• Translocation
• Nondisjunction
• Duplication
Chromosome Mutation
Animation
Deletion
• Due to breakage
• A piece of a
chromosome is lost
Inversion
• Chromosome segment
breaks off
• Segment flips around
backwards
• Segment reattaches
Duplication
Occurs when a gene
sequence is repeated
Translocation
Involves two
chromosomes that aren’t
homologous
Part of one chromosome
is transferred to another
chromosomes
Translocation
Nondisjunction
Failure of chromosomes to separate
during meiosis
Causes gamete to have too many or too
few chromosomes
Disorders:
Down Syndrome – three 21st chromosomes
Turner Syndrome – single X chromosome
Klinefelter’s Syndrome – XXY chromosomes
Chromosome Mutation
Animation
KARYOTYPE
ANALYSIS
Chromosome Mutations
• Down Syndrome
• Chromosome 21 does not separate
correctly.
• They have 47 chromosomes in stead of
46.
• Children with Down Syndrome develop
slower, may have heart and stomach
illnesses and vary greatly in their degree
of inteligence.
Chromosome Mutations
• Cri-du-chat
• Deletion of material on 5th
chromosome
• Characterized by the cat-like cry
made by cri-du-chat babies
• Varied levels of metal handicaps
Sex Chromosome Abnormalities
• Klinefelter’s Syndrome
•
•
•
•
•
XXY, XXYY, XXXY
Male
Sterility
Small testicles
Breast enlargement
Sex Chromosome Abnormalities
• XYY Syndrome
• Normal male traits
• Often tall and thin
• Associated with antisocial and behavioral problems*
Sex Chromosome Mutations
• Turner’s Syndrome
• X
• Female
• sex organs don't mature at
adolescence
• sterility
• short stature
Sex Chromosome Mutations
• XXX
•
•
•
•
•
•
Trisomy X
Female
Little or no visible differences
tall stature
learning disabilities
limited fertility
Normal Male
2n = 46
45
Normal Female
2n = 46
46
Male, Trisomy 21 (Down’s)
2n = 47
47