Transcript Cells, Mitosis and Meiosis

Genetics--Nuclear Division &
Inheritance
Mitosis & Meiosis
Introduction
• What is Genetics?
• What level of organization is being
examined?
– molecular, cellular, species, or population
• What is the purpose of this lab?
Somatic versus Reproductive Cells
• Somatic cells:
– all except the reproductive
cells
– 2 copies of each
chromosome
– diploid
– example: in humans, the
diploid number is 46
• Reproductive or sex
cells:
– haploid
– 1 copy of each
chromosome
Nuclear Division
• Mitosis
–
–
–
–
prior to mitosis chromosomes are duplicated
nuclear division in somatic cells
genetic info from “parent” to daughters
cancer: uncontrolled cell division
Nuclear Division
• Meiosis
– prior to meiosis: chromosomes are duplicated
– nuclear division in the sex cells
– genetic diversity
Cytokinesis
•
•
•
•
division of the cytoplasm
occurs at the end of mitosis
occurs at the end of meiosis I
occurs at the end of meiosis II
Interphase
Mitosis
Interphase
• no nuclear or cell division
• DNA replication, chromosome duplication,
and cell growth
• “chromosomes,” not visible inside nucleus
• but can see nucleolus inside nucleus
Mitosis
• Four phases of mitosis:
– prophase
– metaphase
– anaphase
– telophase
• End result of mitosis: 2 daughter cells
Prophase
• Duplicated chromosomes condense and
become visible as sister chromatids joined
at the centromere
• nuclear membrane disintegrates; neither the
nucleus nor the nucleolus is distinct
• spindle apparatus of the cell is formed
Prophase
• Chromatin condenses
into discrete
chromosomes
Early prophase
– two identical “sister
chromatids”
– joined at the
centromere
Prophase
Metaphase
• Chromosomes are aligned along the cell’s
equator at their centromeres
• spindle fibers are visible
Metaphase
Chromosomes convene on the metaphase plate, an imaginary
“equator” located between the two poles of the spindle.
Centromeres are aligned, and sister chromatids of each
chromosome straddle the metaphase plate.
Metaphase plate
Spindle fibers
Anaphase
• Migration of the chromosomes:
– centromeres split
– move along the spindle fibers towards opposite
poles
– pulling the sister chromatid
• Each chromatid -- a chromosome
Anaphase
The sister chromatids separate from each other and move
along the spindle fibers towards opposite poles. Each
chromatid is now considered a chromosome.
Spindle poles
Early anaphase
Anaphase
Anaphase
By the end of anaphase, each pole has an equivalent and
complete set of chromosomes.
Late anaphase
Telophase
• Chromosomes are aggregated at the poles
and begin to thin out and extend in length
• new nuclear membrane forms; nucleolus
and nucleus begin to reappear
• spindle disintegrates
• cytoplasm divides (cytokinesis occurs);
daughter cells begin to form
Telophase
• The “daughter” nuclei
begin to form at the
two poles
Telophase
• Nuclear membranes
begin to form
• chromosomes lose
their distinct
appearance.
Cytokinesis begins,
separating the cytoplasm into
2 “daughter cells”.
Late telophase
Telophase
• formation of 2
daughter cells
• Each daughter cell has
same genetic material
as parent
Daughter Cells
Meiosis: Reproductive or Sex Cells
• Gametes
– female gametes: “eggs” or “ova”
(ovum, singular)
– male gametes: “sperm”
– HAPLOID, have 1 copy of each
chromosome (i.e., 1/2 as many
chromosomes as somatic cells)
– example, in humans, the haploid
number is 23.
Meiosis in the Lily Anther
• Lily anther:
– male reproductive
organ in flowering
plants.
– meiosis takes place
– result: haploid pollen
grains
Meiosis in the Lily Anther
• A very thin cross-section of a lily anther
• stain it
• examine with a compound light microscope:
Meiosis in the Lily Anther
Each anther contains 4 pollen sacs where meiosis takes place.
Meiosis in the Lily Anther
• Study the various phases
of meiosis in prepared
slides of the lily anther.
• look at the cells inside the
pollen sacs
• locate the different phases.
Stages of Meiosis
Meiosis consists of meiosis I and meiosis II and both have 4
stages leading up to the actual cell division.
Meiosis I:
Meiosis II:
Prophase I
Prophase II
Metaphase I
Metaphase II
Anaphase I
Anaphase II
Telophase I
Telophase II
Meiosis: Prophase I
Chromatin condenses into discrete chromosomes.
Chromosome is actually composed of 2 sister chromatids.
Homologous
chromosomes come
together as pairs.
Meiosis: Metaphase I
Metaphase plate
• Chromosomes are
arranged on the
metaphase plate, still
in homologous pairs.
• Spindle fibers from
each pole attach to one
chromosome of each
pair of the homologue.
Spindle fibers
Meiosis: Anaphase I
• each homologous pair of
chromosomes separate
and move toward
opposite poles.
• each chromosome still
consists of 2 sister
chromatids joined
together at the
centromere.
Meiosis: Telophase I
• Chromosomes
have moved to
opposite poles.
• Cytokinesis
separates the
cytoplasm into 2
daughter cells.
• Each cell is now
haploid.
Haploid
Haploid
Meiosis: Prophase II
In prophase II, the chromatin again condenses so the
chromosomes have a distinct shape.
Meiosis: Metaphase II
In metaphase II, chromosomes align on the metaphase plate.
Meiosis: Anaphase II
• sister chromatids
separate, and move
towards opposite
poles
• sister chromatids are
now individual
chromosomes.
Meiosis: Telophase II
• nuclei begin to form
at opposite poles, and
cytokinesis occurs
• 4 daughter cells, each
with the haploid
number of
chromosomes.
Haploid
Haploid
Haploid
Haploid
Meiosis I: Review
Prophase I
Anaphase I
Metaphase I
Telophase I
Review: Meiosis II
Prophase II
Metaphase II
Anaphase II
Telophase II
Chromosomes and Genes
• one long strand of
DNA
• associated proteins
– “chromatin”
• thousands of genes
• gene
– discrete unit of
heredity
• locus
– physical location of a
gene
Homologous Chromosomes,
Genes, and Alleles
• Chromosome pair:
– “homologous
chromosomes”
• pair has genes at the
same loci
– “alleles”
– may be the same or
different
Human Genetics
• Genotype
– heterozygous
– homozygous
• Phenotype
• Observe traits and identify which form you
possess
• Make a table
Heredity and Genetic Disorders
• Video, “Healthy,
Heredity, and Genetic
Disorders”
Extra 21st chromosome
Mechanics of Mitosis
• Answer questions
– use instructional materials available
• video: Mechanics of Mitosis
• posters
• Photo Atlas of Biology
Mitosis in the Onion Root Tip
• Today’s lab exercises
– examine the tip of an
onion root
– find and identify the 4
different phases of
mitosis.
• Root tips are useful
material because the
cells are frequently
dividing as the root
grows.
Growth and Mitosis in Onion
Root Tips
A) Examine onion (Allium) root tip under low
power objective (10x)
– answer questions
B) Examine same slide under high power
objective (40x)
– follow instructions
Meiosis: The basis of Genetic
Inheritance and Sexual
Reproduction
• Mechanics of Meiosis
– Answer questions
• use instructional materials available
– video: Mechanics of Meiosis
– posters
– Photo Atlas of Biology
Meiosis and Gamete Formation
in Lily Anthers
• Observe first and second meiotic division in
demonstration slides
Note:
•DO NOT move the slides out of position
•Use only FINE focus