Download Meiosis - Building Directory

Meiosis
Genetics
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“Like begets like”
Heredity:
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The continuity of traits from one
generation to the next
This is why organisms resemble their
parents
Variation:
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Offspring exhibit individuality, different
somewhat from their parents and their
siblings
Genes
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Gene:
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Genome:
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A hereditary unit that codes for genetic information
A segment of DNA
The tens of thousands of genes we inherit from our
parents make up our genome
All of genes put together make up our GENOME
The DNA of a eukaryotic cell is subdivided into
chromosomes, located in the nucleus of every cell
A gene’s specific location along the length of a
chromosome is called the gene’s locus
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Like an address
Asexual Reproduction
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A single individual is
the sole parent and
passes copies of all its
genes on to its
offspring
Since the offspring
has the same genetic
info as its parent, it is
known as a clone
The only genetic
variation is due to
mutations
Sexual Reproduction
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Usually results in greater variation
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Two parents give rise to offspring that
have unique combinations of genes
inherited from both parents.
Offspring of sexual reproduction vary
genetically from their siblings and both
parents.
HOW??
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MEIOSIS!! 
The Human Life Cycle
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Gametes vs. Somatic Cells
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Gametes: Sperm or ova cells
Somatic Cells: Any other cell besides
sperm or ova (muscle, nerve, etc.)
Every human somatic cell contains
46 chromosomes
Human gametes contain 23
chromosomes
Chromosomes
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A chromosome is a long coiled-up strand of DNA
Humans have 23 pairs of homologous
chromosomes in each somatic cell
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Homologous Chromosomes:
 Chromosomes that have the same length,
centromere position, and staining pattern
 Carry genes controlling the same inherited
characteristics (ie. Eye color, attached ear lobes)
 One chromosome of each pair comes from each
parent
Only exception…
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Male sex chromosomes (XY) - nonhomologous
Female sex chromosomes (XX) -homologous
Haploid vs. Diploid Cells
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Haploid cells contain a single set of
chromosomes
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Abbreviated n
In humans, there are 23 chromosomes in a haploid cell
Gametes are the only haploid cells
Produced by meiosis
Diploid cells contain a double set of
chromosomes
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Abbreviated 2n
In humans, there are 46 chromosomes in a diploid cell
All cells other than gametes are diploid cells
Produced by mitosis
Haploid & Diploid Cells (Karyotype)
Meiosis
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Big Picture:
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Replication of chromosomes (DNA)
followed by two consecutive cell
divisions
Results in 4 different daughter cells,
each with only half as many
chromosomes as the parent (haploid)
Meiosis I:
Separating Homologous Chromosomes
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Interphase I:
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Chromosome
(DNA) replication
occurs
Meiosis I:
Separating Homologous Chromosomes
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Prophase I:
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Chromosomes begin to condense (you
can see them!)
Synapsis occurs:
 Homologous chromosomes come
together (4 sister chromatids total)
Chiasmata
 Chromatids cross over at various
places, known as chiasmata
 An exchange of genetic information
occurs
Nucleoli and nuclear envelope
disappear
Spindle microtubules begin to form
Meiosis I:
Separating Homologous Chromosomes
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Metaphase I:
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Chromosomes are
arranged on the
metaphase plate in
homologous pairs
Meiosis I:
Separating Homologous Chromosomes
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Anaphase I:
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The spindle fibers
move the
chromosomes
toward the poles
Each homologous
chromosome of a
tetrad goes to
opposite poles
Meiosis I:
Separating Homologous Chromosomes
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Telophase I:
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The chromosomes
reach the poles of
the cell
Cytokinesis occurs
 Animals:
cleavage furrow
 Plants: cell plate
Meiosis II:
Separating Sister Chromatids
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Prophase II:
 Spindle apparatus re-forms
(in some species)
 Chromosomes move towards
middle
Metaphase II:
 Chromosomes line up on the
metaphase plate
Meiosis II:
Separating Sister Chromatids
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Anaphase II:
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Centromeres of sister
chromatids separate
Sister chromatids move
toward opposite poles of
the cell
Telophase II/Cytokinesis:
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Nuclei form at opposite
poles of the cell
Cytokinesis occurs
4 haploid daughter cells
form
Mitosis vs. Meiosis
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Three key differences that occur during
meiosis:
1. Prophase I
 Crossing over of nonsister chromosomes
occurs, resulting in the exchange of genetic
information
2. Metaphase I
 Homologous pairs of chromosomes line up at
the metaphase plate, instead of individual
chromsomes
3. Anaphase I
 Sister chromatids of a chromosome remain
attached at the centromere and go to opposite
poles
Crossing Over Results in Genetic Variation
Random Fertilization
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Random nature of fertilization adds
to the genetic variation arising from
meiosis
A zygote is made up of an ovum
and a sperm cell
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8 million different possibilities for
sperm
8 million different possibilities for ovum
64 trillion different diploid possibilities