Transcript Mitosis and Meiosis - Exploits Valley High

Unit 2 – Reproduction and Development
Cellular Reproduction
Human Karyotype
Key features of a chromosome:
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centromere (where spindle attaches)
telomeres (special structures at the ends)
arms (the bulk of the DNA).
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Chromatin: the long fibers that form
chromosomes and contain DNA, RNA and
various proteins. Found in the nucleus of cells.
Chromosome: condensed chromatin structure
formed when cells replicate (divide) (p.578)
Chromatid: one half of a chromosome. Two
sister chromatids are joined by a centromere to
form a chromosome
Chromatin and Chromosomes
Chromosomes come in 2 forms depending on the
stage of the cell cycle:
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The monad form consists of a single chromatid, a single
piece of DNA containing a centromere and telomeres at
the ends.
The dyad form consists of 2 identical chromatids (sister
chromatids) attached together at the centromere.
Chromosomes are in the dyad form before mitosis, and
in the monad form after mitosis.
The dyad form is the result of DNA replication: a single
piece of DNA (the monad chromosome) replicated to
form 2 identical DNA molecules (the 2 chromatids of the
dyad chromosome).
More Chromosomes
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Diploid organisms have 2 copies of each
chromosome, one from each parent. The
two members of a pair of chromosomes
are called homologues.
Each species has a characteristic number
of chromosomes, its haploid number n.
Humans have n=23, that is, we have 23
pairs of chromosomes. Drosophila have
n=4, 4 pairs of chromosomes.
Cell cycle
Cell cycle
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a continuous sequence of cell growth and division
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The cell cycle consists of two main stages
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1. Interphase – growth phase; includes G1, S phase,
and G2
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G1 (gap 1): cell carries out metabolic activities and
prepares for cell division
S phase: DNA is replicated
G2 (gap 2): centrioles replicate and cell prepares for
division
Cell cycle (continued)
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2. division stage – includes mitosis and
cytokinesis; shortest stage
Different cells have different timing for
their cells cycles; some take longer than
others to go thorough their cycle, and
they also spend different amounts of time
in each stage.
The 4 stages
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M = mitosis, where the cell divides into 2
daughter cells. The chromosomes go
from the dyad (2 chromatid) form to the
monad (1 chromatid) form.
S = DNA synthesis. Chromosomes go
from monad to dyad.
G1 = “gap”; where the cell spends most of
its time, performing its tasks as a cell.
Monad chromosomes
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G2 = Dyad chromosomes, cell getting
ready for mitosis.
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G1, S, and G2 are collectively called
“interphase”, the time between mitoses
Mitosis
Mitosis
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division of the cell’s nucleus where the daughter
cells receive the exact number of chromosomes
and genetic makeup as the parent cell
In order for an organism to grow, repair, and
maintain its function new cells are needed to
replace old ones.
Each cell that undergoes mitosis produces 2 new
cells. Mitosis allows the regeneration of
damaged tissue (like cuts) and to replace worn
out cells (like red blood cells)
Mitosis ensures that the same amount of genetic
information in each type of cell.
Mitosis
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Mitosis is division of Somatic cells (body cells);
not germ (sex) cell
Mitosis is ordinary cell division among the cells
of the body. During mitosis the chromosomes
are divided evenly, so that each of the two
daughter cells ends up with 1 copy of each
chromosome.
For humans: start with 46 dyad chromosomes in
1 cell, end with 46 monads in each of 2 cells.
Cytokinesis
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separation of the cytoplasm and the formation
of two new daughter cells; cytokinesis occurs
after telophase of mitosis
Parent cell – the original cell that divides during
mitosis to form two new daughter cells
Daughter cells – the cells produced during
mitosis of a parent cell
Stages of Mitosis
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Before mitosis begins, DNA is replicated
during interphase
Prophase:
--chromosomes condense
--nuclear envelope disappears
--centrioles move to opposite ends of
the cell
--spindle forms
Metaphase
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--chromosomes are lined up on cell
equator, attached to the spindle at the
centromeres
Anaphase
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--centromeres divide. Now chromosomes are
monads
--the monad chromosomes are pulled to
opposite poles by the spindle.
Telophase
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--cytokinesis: cytoplasm divided into 2
separate cells
--chromosomes de-condense
--nuclear envelope re-forms
--spindle vanishes
Meiosis
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Meiosis is the special cell division that converts
diploid body cells into the haploid gametes.
Only occurs in specialized cells.
Takes 2 cell divisions, M1 and M2, with no DNA
synthesis between.
In humans, start with 46 chromosomes (23
pairs) in dyad state. After M1, there are 2 cells
with 23 dyad chromosomes each. After M2
there are 4 cells with 23 monad chromosomes
each.
First Meiotic Division (M1)
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Prophase of M1 is very long, with a number of
sub-stages.
Main event in prophase of M1 is “crossing over”,
also called “recombination”.
In crossing over, homologous chromosomes pair
up, and exchange segments by breaking and
rejoining at identical locations.
Several crossovers per chromosome, with
random positions. This is the basis for linkage
mapping.
More M1
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Metaphase of meiosis 1 is very different from
metaphase in mitosis (or M2).
In metaphase of M1, pairs of homologous
chromosomes line up together. In mitosis and
M2, chromosomes line up as single individuals.
Anaphase of M1: the spindle pulls the two
homologues to opposite poles. However, the
centromeres don’t divide, and the chromosomes
remain dyads.
Telophase of M1: cytoplasm divided into 2 cells,
each of which has 1 haploid set of dyad
chromosomes
Second Meiotic Division (M2)
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Meiosis 2 is just like mitosis.
In prophase, the chromosomes condense and
the spindle forms.
Metaphase of M2: dyad chromosomes line up
singly on the cell equator.
Anaphase of M2: centromeres divide,
chromosomes are now monads which get pulled
to opposite poles.
Telophase: cytoplasm divided into 2 cells.
After M2: total of 4 cells from the original cell.
Each contains one haploid set of monad
chromosomes
Gametogenesis in Mammals
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Gametogenesis is the creation of the
sperm and egg cells from the products of
meiosis, through changes in the
cytoplasm.
in male mammals, sperm production is
continuous from puberty until death. All 4
meiotic products remodel their cytoplasm
and grow a long flagellum to become
spermatozoans, or sperm cells.
Gametogenesis in Female Mammals
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in female mammals, ovarian cells start meiosis 1 before birth, but
the process is arrested in prophase of M1.
Meiosis resumes after puberty, under hormonal control. A small
number of oocytes (cells undergoing meiosis) are shed from the
ovary during a human female’s menstrual cycle. Usually only 1
oocyte is shed in humans, but other mammals produce higher
numbers.
After ovulation, the oocyte finishes meiosis 1.
Meiosis 2 only occurs after fertilization.
During both meiotic divisions, the division of the cytoplasm is
asymmetric: one cell gets nearly all of the cytoplasm. This cell
becomes the egg. The other cell in both divisions is called a “polar
body”. One polar body is created in M1, and another in M2. In
some mammals, the first polar body divides so there are a total of 4
meiotic products, 1 egg plus 3 polar bodies. In humans, the first
polar body never undergoes M2, so the final meiotic products in
human females are a haploid egg, a haploid polar body, and a
diploid polar body.
Angiosperm Life Cycle
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Angiosperms are flowering plants.
All eukaryotes alternate between a diploid phase and a haploid
phase. In animals, the haploid phase is a single cell, the sperm or
the egg, and there is no haploid cell division.
In plants, there is a distinct haploid organism which has cell
divisions and a life of its own.
The plant diploid phase is called the sporophyte. In angiosperms
and most other land plants, the sporophyte is the large visible plant
body that we see.
The plant haploid phase is called the gametophyte. In lower plants,
such as club mosses, this phase is prominent. But in angiosperms,
the gametophyte stage is quite short and small.
Specifically, the male gametophyte, the pollen grain, consists of 3
haploid nuclei. These nuclei are derived from one haploid meiotic
product, by mitosis. Two of the nuclei are “sperm nuclei” and the
other controls the metabolism of the pollen grain.
The female gametophyte, the ovule, consists of 8 haploid nuclei.
These 8 nuclei are derived from one of the meiotic products.
Double Fertilization
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All angiosperms undergo “double fertilization”. It is a major defining
characteristic of angiosperms.
In double fertilization, 2 sperm (pollen) nuclei fertilize the ovule.
When the pollen grain lands on the stigma of a flower, it germinates a long
“pollen tube”, which grows down the style to the ovary, which contains the
ovules. The 2 sperm nuclei migrate down the pollen tube into the ovule.
Each ovule has a cell at one end that is pollinated by one of the sperm
nuclei. This fertilized cell is diploid, and it grows into the embryo and
ultimately into the sporophyte plant body.
The ovule also has 2 nuclei in the center, which join with the other sperm
nuclei to form a triploid tissue, the endosperm. The endosperm thus
contains 2 maternal haploid genomes plus one paternal haploid genome.
The endosperm develops into a nutritive tissue used by the germinating
seed.
After fertilization, both embryo and endosperm grow and develop into a
seed. After a while, development arrests and the seed dries out and forms
a hard coat. The seed contains a multicellular embryo and a multicellular
endosperm. The seed is a resting stage. When conditions are right, the
seed germinates: the embryo eats the endosperm until photosynthesis
begins.