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Meiosis: Key Vocab
● You have many types of specialized cells in your body.
● They are classified in two ways:
○ Somatic Cells:
■ AKA: Body Cells
■ Found in body tissues and organs
■ DNA in body cells is not passed to offspring
■ ex). Nerve cells
○ Gametes:
■ AKA: Sex cells
■ Female: eggs
■ Male: Sperm
■ DNA in gametes can be passed to offspring
● Chromosome
○ A condensed DNA molecule with part (or all) of the genetic material of an
organism
○ Each species has a characteristic number of chromosomes per cell
■ Humans have 23 parts (46 total)
● Chromosome # for somatic cells and chromosomes # for gametes for the
following organisms:
○ Humans
■ Somatic cells: 46
■ Gametes: 23
○ Rat
■ Somatic cells: 42
■ Gametes: 21
○ Pea
■ Somatic cells: 14
■ Gametes: 7
○ Honeybee
■ Somatic cells: 32
■ Gametes: 16
○ Housefly
■ Somatic cells: 12
■ Gametes: 6
○ Cat
■ Somatic cells: 38
■ Gametes: 19
○ Fruit fly
■ Somatic cells: 8
■ Gametes: 4
● Chromosomes: two types
○ Autosomes
■ Chromosomes that carry traits (Characteristic)
● Chromosomes #1-22
○ Sex Chromosomes
■ Chromosomes with genes related to biological sex
● Chromosome #23- X and Y
○ XX - Female
○ XY - Male
○ Homologous chromosomes
■ Two chromosomes with the same genes, but may have different
versions of the genes (alleles)
● One is from the mother
● One is from the father
○ Diploid
■ Have two copies of each chromosome
● Somatic cells
● Homologous Chromosomes
○ Haploid (n)
■ Have only 1 copy of each chromosome
● Gametes
● Don’t have homologous chromosomes
○ Fertilization
■ Uniting of a sperm (n) and an egg (n) cell
● (n)+(n)=(2n)
■ Results in a diploid zygote=fertilizaed egg
● Mitosis vs Meiosis
○ Mitosis starts with one 2n cell and ends with two 2n cells
■ Somatic cells -> more somatic cells
○ Meiosis starts with one 2n cell and ends with four n cells
■ Somatic cells -> sex cells (gametes)
● Meiosis Result
○ Meiosis produces four sex cells that are genetically different from one
another and from the original cell
● Cells go through two rounds of division during meiosis
○ Produces genetically unique cells
○ Results in haploid cells
○ It takes place only at a certain time in an organism’s life cycle
○ Involved in sexual reproduction
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The process of Meiosis
● Meiosis is divided into two phases:
○ Meiosis I
■ Separates homologous chromosomes
● Mom's sets are separate from Dad’s sets
● Meiosis II
○ Separates sister chromatids
■ Mom sets get split up and Dad sets get split up
Meiosis I
Phase
Illustration
Description
Interphase
● G1 (Gap 1)
○ Cells grow and perform
many of their required
functions
● S (synthesis)
○ DNA is replicated
● G2 (Gap 2)
○ Makes Protein, grows
Prophase I
● Nuclear membrane breaks down
● Centrosomes move to opposite
sides of the cell
● Duplicated chromosomes
condense (interphase synthesis)
● Homologous Chromosomes pair
up
○ Crossing over happens
here
Metaphase I
● Homologous chromosome pairs
are randomly lined up in the
middle of the cell
● Chromosomes are lined up along
each side of the cell equator
Anaphase I
● Paired homologous
chromosomes separate from
each other and move toward
opposite sides of the cell
● Sister chromatids remain
together
Telophase I
● The nuclear membrane forms
again
● Spindle Fibers dissemble
● The cell undergoes cytokinesis
End Result: 2 cells that each have a unique combination of 23 duplicated
chromosomes coming from both parents.
● Genetic Diversity and Variation
○ Two mechanisms occur during meiosis I which introduce genetic variation
■ Crossing over
■ Independent assortment
● Crossing Over
○ When homologous pairs line up opposite of each other at the equator of
the cell, parts of the chromatids can become exchanged.
○ This results in a new combination of alleles
■ (Chromosomes transfer their genetic information and exchange it
with each other)
● Independent Assortment
○ Homologous pairs line up opposite each other at the equator of the cell
○ It’s random which side of the equator the paternal and maternal
chromosomes lie
○ These pairs are separated, so one of each homologous pair ends up with
in the daughter cell.
○ This creates many possible combinations of chromosomes in the daughter
cells produced.
Meiosis II
Phase
illustration
Description
Prophase II
● Nuclear membrane breaks
down
● Centrosomes move to
opposite sides of the cell
Metaphase II
● Spindle fibers align
chromosomes at the cell
equator
● Each chromosome still
has two sister chromatids
Anaphase II
● Sister chromatids are
pulled apart from each
other
● They move to opposite
sides of the cell
Telophase II
● Nuclear membranes form
around chromosomes
● Spindle fibers break apart
● Cell undergoes
cytokinesis
End Result: 4 haploid cells with a combination of chromosomes from both mother and
father
Mitosis vs Meiosis
Mitosis
Meiosis
Creates All the cells in your body
(except sex cells)
Sex cells only (egg/sperm)
Definition Process of cell division that
forms two new cells
(daughter cells) each with
the same number of
chromosomes
Process of the cell division
which four new cells are
created- each with half the
original number of
chromosomes
Round of cell division 1 cell division
2 cell division
Type of reproduction asexual
Sexual
end result Diploid cells (2n)
Genetic diversity? no
Purpose To repair damaged body
cells, growth
Haploid cells (n)
yes
Create sex cells that are
needed for reproduction
Variations
● Variation is the differences in the characteristics of individuals in a population
● Variation can be due to genetics, the environment, or a combination of both
● Different members of a population have different DNA, unless they are identical
twins. This means they have different versions of genes, called alleles, and these
alleles give rise to different characteristics
● If the species reproduce by sexual reproduction, the random mixing of alleles
from both parents results in extensive genetic variation within a population of a
species
● Some examples of variation in humans that are controlled only by genes include
eye color and blood group
● Some variation is due to the environmental or lifestyle of the organisms
● Many characteristics are due to a combination of both genetics and the
environment.
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Traits
● A trait is a specific characteristic of an organism
○ Can be determined by genes or the environment
○ Some traits are dominant over the environment
● Traits are observable characteristics passed down from parent to offspring
● An individual will have many traits they share in common with others
● 99.8% of your genetic information is the same as everyone else.
● It’s the 0.2% that makes you unique
● Traits are most common in the population NOT always dominant.
○ Ex). Polydactyly (extra finger/toes) - only 1 in 1000 people have this
condition
■ When polydactyly exhibits by itself, it is inherited as an autosomal
dominant
Heredity
● Genetics
○ Is a field of biology that studies how traits are passed from parents to their
offspring
○ The passing of traits from parents to offspring is known as heredity;
therefore, genetics is the study of heredity.
Reproduction
● Long before DNA had been discovered or the word ‘genetics’ had been invented,
people were studying the inheritance of traits from one generation to the next.
● Genetic inheritance of traits from one generation to the next
● Genetic inheritance occurs both in sexual reproduction and asexual reproduction
● Sexual Reproduction
○ Two organisms contribute DNA to produce a new organism
● Asexual Reproduction
○ One organism provides all the DNA and produces a clone of itself.
● In both, genetic material is passed from one generation to the next.
Gregor Mendel
● Gregor Mendel is known as the father of genetics
● He was born in 1822 in Austria
● He became a monk and worked in the monastery gardens
● He became fascinated with pea plants in the monastery gardens
● Mendel’s experiments with peas were able to show that genes are discrete units
that keep their separate inherits when passed from generation to generation.
● Mendel observed traits in his pea plants (7 to be exact)
● Traits are distinguishing characteristics that are inherited
○ Eye color
○ Leaf shape
○ Tail length
○ Widow’s peak
● Scientists knew that traits were inheritable (passed from one generation to the
next), but they didn’t know HOW.
● In genetics, we refer to the mating of two organisms as a cross
● Mendel noticed that when he crossed a purebred, white-flowered pea plant with a
purebred, purple-flowered pea plant, the resulting offspring looked like this:
●
●
●
●
● From these observations, Mendel drew three important conclusions which are
known as Mendel’s laws:
○ Law of segregation
○ Law of independent assortment
○ Law of dominance
Law of segregation
● Organisms inherit two copies of each gene, one from each parent.
● Genes are pieces of DNA that provide instructions to make a certain protein
● Genes determine your traits
● Features or characteristics that are passed on to you - or inherited - from your
parents
● Organisms donate only one copy of each gene in their gametes
Traits, Genes, alleles
● There can be different versions of genes called alleles
○ You receive one from each parent
○ Represented by letters
■ ex). Y for yellow color or y for green colors in peas
● The alleles that an organism receives from its parents can be the same
○ Homozygous
● A genotype refers to an organism’s combination of alleles
○ ex). BB, Bb, bb
● The physical characteristics, or traits, of an individual organism make up its
phenotype
○ ex). Blue eyes, smooth peas, tall plant, brown fur.
Dominant Alleles
● As Mendel learned, one allele might be dominant over another
● A dominant allele is the allele that is expressed when two different alleles or two
dominant alleles are present
Dominant/Recessive alleles
● An organism’s genotype might be:
○ Homozygous dominant (TT)
○ Heterozygous (Tt)
○ Homozygous recessive (tt)
Punnet Square
● A punnet square is showing possible offspring of a genetic cross
○ Monohybrid- crosses one trait (4 boxes)
○ Dihybrid - crosses two traits (16 boxes)
Resources
Somatic cells and Gametes:
Chromosomes:
Autosomes and Sex Chromosomes:
Homologous Chromosomes:
Diploid and Haploid:
Fertilization: