division of genetic material mitosis and meiosis - …classpages.warnerpacific.edu/bdupriest/bio...
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DIVISION OF
GENETIC MATERIAL
Mitosis and Meiosis
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Lecture Overview
Orientation to cells
What is the genetic material?
Chromosome structure
Orientation to the life cycle of a cell
Cell cycle
How do cells reproduce?
Mitosis and regulation of cell cycle
Meiosis and gamete formation
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CELLS
Fig. 2-1
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CHROMOSOMES
A chromosome…
…is a molecule of DNA
…can exist as chromatin (diffuse) or as condensed
chromosomes
…is a unit of inheritance
Exist in homologous pairs in diploid organisms
Diploid: 2 complete sets of chromosomes (2n)
Homologs: Pairs of like chromosomes
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Exceptions to the Rules
Not all organisms (or cells) are diploid
Haploidy
Polyploidy
Alternation of generations
Not all chromosome pairs are homologous
Sex-determining chromosomes (XX vs. XY)
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Chromosome Comparisons Between Species
Common Name Scientific Name Diploid Number
Fruit fly Drosophila melanogaster 8
Garden pea Pisum sativum 14
Cat Felis catus 38
Human Homo sapiens 46
Yeast Saccharomyces cerevisiae 32
Chicken Gallus gallus 78
Geometrid moth Nemoria sp. 224
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Basic Chromosome Structure
Centromere
Constricted region of DNA
Point of adherence of sister chromatids
Formation point of kinetochore
Attachment point for spindle fibers
Locus (Loci)
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Basic Chromosome Structure
Classified by location of centromere
Fig. 2-3
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Basic Chromosome Structure
Classified by location of centromere
Fig. 2-3
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Karyotype (Karyogram)
Fig. 2-4
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Clicker question: What is the
functional significance of the
centromere?
• A) It’s where transcription begins
• B) It’s where spindle fibers attach during mitosis
• C) It’s where chromosomes begin condensing
• D) It’s where chromosomes attach to the
plasma membrane
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CELL CYCLE
Fig. 2-5
Interphase
Mitosis
Karyokinesis
Cytokinesis
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Mitosis
Result of mitosis:
Production of 2 genetically identical daughter cells
Reasons for mitosis:
Growth
Tissue repair
Replacement of old cells
Asexual reproduction of protists, fungi, plants,
some animals
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Mitosis
• Parent cells must provide their daughter cells with:
– Hereditary instructions encoded in DNA
– Enough metabolic machinery to start up their own operations.
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Mitosis
• http://www.johnkyrk.com/mitosis.html
• Cell division occurs by precise steps which
distribute one set of chromosomes to each
of two daughter cells
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Clicker question: At which
phase of the cell cycle are
chromosomes NOT paired as
sister chromatids? • A) G1
• B) G2
• C) Prophase of mitosis
• D) Metaphase of mitosis
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Clicker question: At which
phase of the cell cycle are sister
chromatids formed?
• A) G1
• B) S
• C) Prophase of mitosis
• D) Anaphase of mitosis
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Stages of Mitosis
Prophase
Prometaphase
Metaphase
Anaphase
Telophase
Fig. 2-7
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Prophase
Centriole migration (in animals)
Spindle fiber formation
Nuclear envelope breakdown
Chromatin condenses to chromosomes
Fig. 2-7
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Interphase Prophase
Fig. 2-7
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Prometaphase
Attachment of spindle fibers
to kinetochores; one fiber
to each sister chromatid
Chromosome migration
toward equatorial plane
Fig. 2-7
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Figure 2-8
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Prometaphase
Fig. 2-7
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Metaphase
Chromosomes are
aligned at equatorial
plane (a.k.a. the
metaphase plate)
Fig. 2-7
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Metaphase
Fig. 2-7
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Anaphase
Disjunction: Sister chromatids separate at centromere and are pulled to opposite ends of the cell
After separation, sister chromatids are now considered separate chromosomes
Fig. 2-7
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Anaphase
Fig. 2-7
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Telophase
“Reverse prophase”
Two new nuclei form
Cytokinesis follows or is simultaneous
Animals: Furrowing
Plants: Cell plate formation
Fig. 2-7
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Telophase (two different preparations)
Fig. 2-7
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Clicker question: In which phase
of the cell cycle do
chromosomes become visible?
• A) G1
• B) S
• C) Prophase of mitosis
• D) Anaphase of mitosis
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Clicker question: In which phase
of mitosis do sister chromatids
separate from each other?
• A) G1
• B) S
• C) Prophase of mitosis
• D) Anaphase of mitosis
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Clicker question: In which phase
of mitosis chromosomes begin
to move toward the equatorial
plate? • A) Prophase
• B) Prometaphase
• C) Metaphase
• D) Anaphase
• E) Telophase
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Clicker question: What process
occurs either simultaneously
with or following telophase?
• A) DNA synthesis
• B) Breakdown of nuclear envelope
• C) Karyokinesis
• D) Cytokinesis
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Cell Cycle Regulation
Why is the regulation of cell replication so
vitally important?
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Cell Cycle Regulation
Length of the cell cycle
Fig. 2-6
Fig. 2-5
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Cell Cycle Regulation: 3 Checkpoints
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Cyclin-cdk complexes
Cyclin-dependent kinases
Encoded by “cell division cycle” (cdc) genes
Cyclins
Proteins that bind and activate cdk proteins, which
then activate other target proteins
Made and degraded in cyclic fashion
Cyclin-cdk complexes help cells transition through
checkpoints
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Mitosis Review
http://www.sumanasinc.com/webcontent/animations/content/mitosis.html
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Clicker question: What is the
outcome of mitosis?
• A) Two daughter cells genetically identical to
original (parent) cell
• B) One daughter cell genetically identical to
parent cell
• C) Four daughter cells genetically similar to –
but not identical to – the parent cell
• D) Infinite numbers of daughter cells all
genetically identical to parent cell
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Clicker question: What prevents
mitosis from occurring in
defective cells?
• A) cdk-cyclins
• B) S phase
• C) spindle fibers
• D) checkpoints
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Clicker question: What can
happen when a cell is arrested
at a checkpoint?
• A) the cell dies
• B) the cell fixes the problem
• C) the cell starts the cell cycle over
• D) A and B only
• E) A, B and C
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Meiosis
Produces genetically unique haploid (n) gametes
Reduces the genetic material by half
Two divisional phases
Allows genetic recombination
Gametes combine for sexual reproduction
Increases genetic diversity within a population
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Meiosis: Summary of Key Events
Meiosis I
Synapsis: pairing of homologous
chromosomes
Crossing over: exchange of chromosomal
material between homologs
Reductional division: number of
chromosomes is cut in half
2n n
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Meiosis
Meiosis II
Equational division
Number of chromosomes remains the same
n n
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Clicker question: What is the
outcome of meiosis?
• A) Two daughter cells genetically identical to
original (parent) cell
• B) One daughter cell genetically identical to
parent cell
• C) Four daughter cells containing half the
genetic material of the parent cell
• D) Infinite numbers of daughter cells all
containing half the genetic material of the
parent cell
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Stages of Meiosis
Prophase I
Prometaphase I
Metaphase I
Anaphase I
Telophase I / Prophase II
Prometaphase II
Metaphase II
Anaphase II
Telophase II Fig. 2-7
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Stages of Prophase I
1: Leptonema (leptotene stage)
Chromatin begins to condense into chromomeres
Fig. 2-9
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Stages of Prophase I
2: Zygonema (zygotene stage)
Homologs pair roughly
Form bivalents (# = n)
Fig. 2-9
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Stages of Prophase I
3: Pachynema (pachytene stage)
Formation of synaptonemal
complex
Synapsis
Chromatids become apparent
Tetrads
Crossing over between non-sister
chromatids
Fig. 2-9
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Stages of Prophase I
4: Diplonema (diplotene stage)
Sister chromatids begin to separate from each other
Formation of chiasmata
Attached points reveal where crossover occurred
Fig. 2-9
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Stages of Prophase I
5: Diakenesis
Nuclear envelope breakdown
Spindle fibers attach to centromeres
Maximal chromosome contraction
Chiasmata move to ends of
chromosomes (terminalization)
Fig. 2-9
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Metaphase I
Homologs aligned at
metaphase plate
Homologs held together
by chiasmata between
non-sister chromatids
Fig. 2-10
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Anaphase I
Segregation (disjunction)
of tetrads
Dyads are formed
Point of reductional
division (2n n)
Fig. 2-10
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Telophase I
Daughter cells are n
Not all species undergo
this stage – some skip
from Anaphase I to
Prophase II
Fig. 2-10
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Prophase II Metaphase II
Akin to mitosis
All chromosomes align at
metaphase plate
What’s the difference
between metaphase and
metaphase II?
Fig. 2-10
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Anaphase II
Dyads segregated
Monads = individual
chromosomes
Equational division
(n n)
Fig. 2-10
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Telophase II & Cytokinesis
Gamete formation (n)
Compared to parent cell,
1/2 the number of
chromosomes but still 1 of
each type
Contain combinations of
maternal & paternal DNA
How many potential
gametes from one parent
cell? Are they all
genetically distinct, or are
any the same as another?
Fig. 2-10
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Clicker question: During
crossing over, genetic material
is exchanged between…
• A) sister chromatids
• B) non-sister chromatids of non-homologous
chromosomes
• C) non-sister chromatids of homologous
chromosomes
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Clicker question: During
metaphase I, what line up next
to each other at the metaphase
plate? • A) sister chromatids
• B) homologous pairs of chromosomes
• C) non-homologous pairs of chromosomes
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Clicker question: During
anaphase II, what is it that
separate from each other?
• A) sister chromatids
• B) homologous chromosomes (with sister
chromatids attached)
• C) non-homologous chromosomes (with sister
chromatids attached)
• D) homologous chromosomes (w/ only one
copy of the chromosome
• E) non-homologous chromosomes (w/ only one
copy of the chromosome)
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Mitosis / Meiosis Comparison
Fig. 2-11
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Mitosis / Meiosis Comparison
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GAMETE FORMATION
Spermatogenesis
Oogenesis
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Spermatogenesis
Occurs in testes
Continuous process
beginning at puberty,
continuing throughout life
Fig. 2-11
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Oogenesis
Occurs in ovaries
Not continuous or
equivalent in humans
Primary oocyte formation:
Meiosis initiated as embryo
Arrested in prophase I
Resumes at sexual maturity
Fig. 2-11
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Occurs in ovaries
Not continuous or
equivalent in humans
Primary oocyte
Secondary oocyte formation:
Meiosis arrested at metaphase
II until fertilization
Upon fertilization, meiosis
resumes to completion, then
egg and sperm nuclei combine
Oogenesis
Fig. 2-11
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Occurs in ovaries
Not continuous or
equivalent in humans
Primary oocyte
Secondary oocyte
First & second polar bodies
Equal division of genetic
material but not cytoplasm
Not used for reproduction
Oogenesis
Fig. 2-11
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Clicker question: What is the
genetic content of secondary
spermatocytes?
• A) diploid
• B) haploid
• C) triploid
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Clicker question: What is the
genetic content of a primary
oocyte?
• A) diploid
• B) haploid
• C) triploid
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Meiotic Division & Genetic Variation
Potential combination of chromosomes in a
gamete = 2n
n = haploid number of chromosomes
If n = 2 then 4 possibilities
If n = 23 then 8.4x106 possible combinations
Combine both parental gametes = 7x1013 possibilities
Does not include variation due to crossing over!!!