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.
Cell Cycle
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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.
G1 = “gap”; where the cell spends most of
its time, performing its tasks as a cell.
Monad chromosomes
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S = DNA synthesis. Chromosomes go
from monad to dyad.
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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 mitosis
Mitosis
Parent cell and daughter cells
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 is in each type of cell.
Mitosis
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Mitosis is division of Somatic cells (body cells);
not germ (sex) cell
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Mitosis is ordinary cell division among the cells
of the body.
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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.
Stages:
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Prophase
Metaphase
Anaphase
Telophase
Mitosis
All Phases Of Mitosis
Interphase
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The first part of the interphase
stage is called gap 1 (G1); cells
carry out metabolic activities to
prepare for cell division.
S phase; DNA gets replicated.
gap 2 (G2); cells prepare to
undergo division.
The division stage includes 2
processes: mitosis – the division
of the nucleus, and cytokinesis –
the division of the cytoplasm.
These two processes are the
shortest events in the cell cycle.
Prophase
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During prophase, the chromatin
coils and forms thick condensed
chromosomes.
Each chromosome is made up of
two sister chromatids (two DNA
molecules that are identical to
each other). These chromatids
are held together by a
centromere.
Prophase
--chromosomes condense
--nuclear envelope disappears
--centrioles move to opposite ends of
the cell
--spindle forms
Metaphase
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During metaphase, the spindle fibers
attach to the centromere of the two
replicated chromosomes.
The chromatids are then guided by
the spindle fibers to the middle of
the cell – the cells equator.
A spindle fiber from one pole is then
attached to one chromatid and
another from the opposite pole is
attached to the other chromatids at
the centromere.
Metaphase
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chromosomes are lined up on cell equator,
attached to the spindle at the centromeres
Anaphase
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During anaphase, the
centromere splits apart and
the chromatids are pulled to
the opposite poles of the cell
by the spindle fibers.
The pull is caused by the
shortening of the
microtubules that make up
the spindle fibers.
Anaphase
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Centromeres divide (chromosomes are monads)
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The monad chromosomes are pulled to opposite
poles by the spindle.
Telophase
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Telophase begins when the
chromatids have all reached the
opposite poles within the cell.
At this point each of the chromatids
is a “single” – a non-replicated
chromosome.
The chromosomes begin to unwind;
the spindle fibers break down and
disappear because they are no
longer needed at this stage.
The nucleus reappears and a
nuclear membrane forms around
each new set of chromosomes.
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 a special type of cell division that
can occur only in reproductive organs.
Meiosis produces reproductive cells called
gametes (either eggs or sperm).
These gametes are haploid cells, which
means they contain only one copy of each
type of chromosome that the diploid parent
cell contains
* Haploid – only one
* Diploid – contains two
Meiosis
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Takes 2 cell divisions, Meiosis 1 and
Meiosis 2, 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.
Meiosis
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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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
Prophase I
During prophase I, each pair
of chromosomes that carry
the same gene (homologous
chromosomes) become
aligned. These homologous
pairs are called tetrads.
During the pairing process,
crossing over of
chromosomes can occur,
where non-sister chromatids
exchange segments of
chromosomes.
Metaphase I
Metaphase I follows prophase I.
This is when the spindle fiber
attaches to the centromere of each
chromosome. A spindle fiber from
one pole attaches to one pair of
sister chromatids and a spindle
fiber from the opposite pole
attaches to the other pair.The
spindle fibers pull each tetrad to
the equator, but they do not line
up like in mitosis.
Anaphase I
During anaphase I, the
homologous chromosomes
separate and move to the
opposite poles of the cell. They
are then pulled apart by the
shortening of the spindle fibers.
However, here they do not split
as they did in mitosis and so
the sister chromatids stay
together. This means that only
one chromosome from each
pair will move to each pole of
the cell.
Telophase I
When telophase I occurs, cell
division goes directly to meiosis
II. If telophase does not occur,
the homologous chromosomes
begin to uncoil and then the
spindle fibers disappear. The
cytoplasm gets divided, and
nuclear membrane forms around
each group and two cells are
formed.
* Telophase I doesn’t occur in all cells
Meiosis II
The phases of meiosis II are the same as
mitosis. The two cells from telophase I go
through prophase II , Metaphase II, anaphase II
and telophase II. Each of these cells beginning
meiosis II is haploid but they do consist of
replicated chromosomes. At the end of meiosis
II, the daughter cells are still haploid but each
cell contains single, un-replicated chromosomes.
The daughter cells will develop into gametes in
animals and either gametes of spores in plants.
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
Identify the Stage
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 stopped 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 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.
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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.
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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.
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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.