Meiosis and Genetics
Organization of genetic material
Prokaryotes: DNA is circular,
not associated with proteins
Eukaryotes: DNA is linear,
associated with proteins
Somatic Cell Cycles
• Somatic (body) cells
have a distinct cell
cycle with four phases:
–
–
–
–
G1: growth
S : DNA is replicated
G2: growth
M phase: cells divide
by mitosis
Chromosome Structure
Has two
chromatids
(replicated
chromosomes)
Chromatids held
together by a
region of DNA called
a centromere
Stages of Mitosis
Most eukaryotic cells
are diploid:
--two sets of
chromosomes, one from
each parent
--chromosomes from
parents are similar, but
not identical:
homologous
chromosomes
Karyotype
--Humans have 46
chromosomes, or 23
pairs of chromosomes
Mitosis summary
• Daughter cells have the same number
of chromosomes as parental cells
• Daughter cells have the same DNA
content as parental cells
• Daughter cells have identical DNA
structure as the parental cells
• Mitosis starts with diploid cells and
produces diploid cells
Meiosis
Occurs only in organisms that use sexual reproduction
Specialized cell division that only occurs in germ cells.
The product cells of meiosis are the gametes (egg,
sperm).
Overview of Meiosis
• Germ cells duplicate
their DNA ONCE-prior to meiosis
• Two stages of
meiosis, each with
cell division
• Germ cells are
diploid; gametes are
haploid
From GlaxoSmithKline:http://genetics.gsk.com/graphics/meiosis-big.gif
Prophase I
• During meiosis I (prophase I), there is exchange
of genetic material between chromosomes:
genetic recombination.
• Genetic recombination may allow for a
competitive advantage by rearranging genetic
material from generation to generation.
Meiosis I
Key points about Meiosis I
• Germ cells start out diploid
• Germ cells duplicate their DNA
• Homologous chromosomes exchange genetic
material during prophase I
• Meiosis I ends with the separation of the
homologs and the physical division of the cells
• Products of meiosis I are not diploid because
they do not have homologous chromosome
pairs
• Products of meiosis I are not haploid (yet)
Key points about Meiosis II
• No DNA duplication prior to meiosis II
• Separation of the attached chromatids
(replicated chromosomes)
• Four products (gametes) are genetically
NOT identical to each other!
• Four products (gametes) are haploid—no
homologous chromosome pairs
Mitosis/Meiosis
Meiosis to Genetics....
• Meiosis produces gametes with a
haploid chromosome number.
• During fertilization, these gametes unite
to form a diploid zygote, which then
develops by successive cell divisions
into an organism.
• Thus, organisms inherit two sets of
genetic information: one from each
gamete (parent).
Chromosomal Diseases
• Gametes have abnormal
chromosome numbers and
mutations
• Offspring inherit extra
chromosome or are missing a
chromosome
• Caused by problems with
meiosis
Nondisjunction of chromosomes during meiosis
Based on: Brooks/Cole – Thomson Learning
Nondisjunction and Trisomy 21
Some Characteristics of Trisomy 21
(Down Syndrome)
Genetics:
Introduction
Each organism displays
certain traits which are
inherited from previous
generations.
The monk Gregor Mendel,
through his studies of pea
plants, discovered a
mechanism for the
inheritance of specific traits.
Genes and
Alleles
• Gene: unit of information
about a specific trait,
passed from parent to
offspring
• Allele: all of the different
forms of the gene, in
diploid organisms, each
gene has at least two
alleles.
Alleles Continued
• Allele combinations
– homozygous: when both alleles are identical
– heterozygous: when each allele is different
• Types of alleles
– Dominant alleles: capital letter (D)
– Recessive alleles: lowercase letter (d)
When paired, the dominant allele will mask the
effect of the recessive allele
Examples of Inheritance
Dominant and Recessive Traits
Brown eyes are dominant
Tongue rolling ability is
recessive
Mendel's Law of Segregation
For a given trait (gene), the pair of alleles in each parent separate
such that the offspring only inherits one allele.
Separation of alleles occurs during the meiotic divisions that
produce the gametes.