Transcript (Chapter 3):Reproduction and Chromosome Transmission

LECTURE 6
Reproduction and
Chromosome
Transmission
(Chapter 3)
Slides 1-3; 9-12; 25-44
On your own: Slides 4-8; 13-24
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INTRODUCTION
• In this chapter we will survey reproduction
at the cellular level
• We will examine chromosomes at the
microscopic level
– This examination provides us with insights to
help understand the inheritance patterns of
traits
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3.1 GENERAL FEATURES
OF CHROMOSOMES
• Chromosomes are structures within living
cells that contain the genetic material
– They contain the genes
• Biochemically, chromosomes are
composed of
– DNA, which is the genetic material
– Proteins, which provide an organized structure
– In eukaryotes the DNA-protein complex is
called chromatin
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3.1 GENERAL FEATURES
OF CHROMOSOMES
• First, let’s consider the distinctive cellular
differences between the two types of cells
– 1. Prokaryotes
• Bacteria and archaea
– 2. Eukaryotes
• Protists, fungi, plants and animals
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• Prokaryotes
– Do not contain a nucleus
– Usually contain a single type of circular
chromosome
• Found in the nucleoid
– Cytoplasm is enclosed by a plasma membrane
• Regulates nutrient uptake and waste excretion
– Outside the membrane is a rigid cell wall
• For protection from breakage
– May contain other structures
• Outer membrane
• Flagella
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Figure 3.1 (a) Bacterial cell
1 mm
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Ribosomes
in cytoplasm
Outer
Cell wall
membrane
Plasma
membrane
(also known
as inner
membrane)
Flagellum
Nucleoid
(where bacterial
chromosome is
found)
(a) Bacterial cell
This example is typical of bacteria such as Escherichia coli,
which has an outer membrane and flagella.
• Eukaryotes
– Have a nucleus
• Contains most of the genetic material in the form of linear
chromosomes
• Bounded by two membranes
– Have membrane-bounded organelles with specific
functions
• These include
• Mitochondria
– ATP synthesis
– Contain their own DNA
• Lysosomes
– Plays a role in degradation of macromolecules
• Golgi apparatus
– Plays a role in protein modification and trafficking
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Microfilament
Golgi
body
Nuclear
envelope
Nucleolus
Chromosomal
DNA
Nucleus
Polyribosomes
Ribosome
Rough endoplasmic
reticulum
Cytoplasm
Membrane protein
Plasma membrane
Smooth endoplasmic
reticulum
Lysosome
Figure 3.1 (b) Animal cell
Mitochondrial DNA
(b) Animal cell
Mitochondrion
Centriole
Microtubule
Cytogenetics
• The field of genetics that involves the
microscopic examination of chromosomes
• A cytogeneticist typically examines the
chromosomal composition of a particular
cell or organism
– This allows the detection of individuals with
abnormal chromosome number or structure
– This also provides a way to distinguish between
two closely-related species
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Eukaryotic Chromosomes Are
Inherited in Sets
• Most eukaryotic species are diploid
– Have two sets of chromosomes
• For example
– Humans
• 46 total chromosomes (23 per set)
– Dogs
• 78 total chromosomes (39 per set)
– Fruit fly
• 8 total chromosomes (4 per set)
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Eukaryotic Chromosomes Are
Inherited in Sets
• Members of a pair of chromosomes are called
homologs
– The two homologs form a homologous pair
• The two chromosomes in a homologous pair
– Are nearly identical in size
– Have the same banding pattern and centromere
location
– Have the same genes
• But not necessarily the same alleles
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Eukaryotic Chromosomes Are
Inherited in Sets
• The DNA sequences on homologous
chromosomes are also very similar
– There is usually less than 1% difference between
homologs
• Nevertheless, these slight differences in DNA
sequence provide the allelic differences in
genes
– Eye color gene
• Blue allele vs. brown allele
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• Mitosis was first observed microscopically in
the 1870s by the German biologist, Walter
Flemming
– He coined the term mitosis
• From the Greek mitos, meaning thread
• The process of mitosis is shown in Figure 3.8
• The original mother cell is diploid (2n)
– It contains a total of six chromosomes
– Three per set (n = 3)
• One set is shown in blue and the homologous set in red
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• Mitosis is subdivided into five phases
– Prophase
– Prometaphase
– Metaphase
– Anaphase
– Telophase
• Following are the stages of mitosis from
Figure 3.8
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• Chromosomes are
decondensed
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Two centrosomes,
each with centriole pairs
Nuclear
membrane
• By the end of
interphase, the
chromosomes have
already replicated
– But the six pairs of
sister chromatids are
not seen until prophase
• The centrosome, the
attachment point of
the mitotic spindle,
divides
Nucleolus
Chromosomes
INTERPHASE
• Nuclear envelope
dissociates into
small vesicles
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Microtubules
forming mitotic spindle
Sister chromatids
• Chromatids
condense into more
compact structures
• Centrosomes begin
to separate
• The mitotic spindle
apparatus is formed
– Composed of
mircotubules (MTs)
PROPHASE
• Microtubules are formed by rapid
polymerization of tubulin proteins
• There are three types of spindle
microtubules
– 1. Aster microtubules
• Important for positioning of the spindle apparatus
– 2. Polar microtubules
• Help to “push” the poles away from each other
– 3. Kinetochore microtubules
• Attach to the kinetochore , which is bound to the
centromere of each individual chromosome
– Refer to Figure 3.7
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Contacts the centromere
Contacts the other two
Inner
plate
Kinetochore
Centromeric
DNA
Middle
layer
Outer
plate
Kinetochore
microtubule
Contacts the kinetochore microtubule
Figure 3.7
Spindle pole:
a centrosome
with 2 centeriorles
Aster
microtubules
Kinetochore
Metaphase
plate
Sister
chromatids
Polar
microtubules
Kinetochore
microtubules
3-36
• Centrosomes move to
opposite ends of the cell,
forming the spindle poles
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Nuclear membrane
fragmenting into vesicles
• Spindle fibers interact with
the sister chromatids
• Kinetochore microtubules
grow from the two poles
Mitotic
spindle
– If they make contact with a
kinetochore, the sister
chromatid is “captured”
– If not, the microtubule
depolymerizes and retracts to
the centrosome
Spindle
• The two kinetochores on a
pair of sister chromatids are
attached to kinetochore
MTs on opposite poles
pole
PROMETAPHASE
• Pairs of sister
chromatids align
themselves along a
plane called the
metaphase plate
• Each pair of
chromatids is
attached to both
poles by kinetochore
microtubules
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Astral microtubule
Metaphase
plate
Polar
microtubule
Kinetochore
proteins attached
to centromere
METAPHASE
Kinetochore
microtubule
• The connection holding
the sister chromatids
together is broken
• Each chromatid, now an
individual chromosome,
is linked to only one pole
• As anaphase proceeds
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Chromosomes
– Kinetochore MTs shorten
• Chromosomes move to
opposite poles
– Polar MTs lengthen
• Poles themselves move
further away from each
other
ANAPHASE
• Chromosomes reach
their respective poles
and decondense
• Nuclear membrane
reforms to form two
separate nuclei
• In most cases, mitosis is
quickly followed by
cytokinesis
– In animals
• Formation of a cleavage
furrow
– In plants
• Formation of a cell plate
• Refer to Figure 3.9
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Nuclear
membrane
re-forming
Cleavage
furrow
Chromosomes
decondensing
TELOPHASE AND CYTOKINESIS
Figure 3.9
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Cleavage
furrow
S
G1
G2
150 um
© Dr. David M. Phillips/Visuals Unlimited
(a) Cleavage of an animal cell
Phragmoplast
Cell plate
10 um
© Ed Reschke
(b) Formation of a cell plate in a plant cell
• Mitosis and cytokinesis ultimately produce
two daughter cells having the same number
and complement of chromosomes as the
mother cell
• The two daughter cells are genetically
identical to each other
– Barring rare mutations
• Thus, mitosis ensures genetic consistency
from one cell to the next
• The development of multicellularity relies
on the repeated process of mitosis and
cytokinesis
3.3 SEXUAL REPRODUCTION
• Sexual reproduction is the most common
way for eukaryotic organisms to produce
offspring
– Parents make gametes with half the amount of
genetic material
• These gametes fuse with each other during
fertilization to begin the life of a new organism
• The process of forming gametes is called
gametogenesis
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• Some simple eukaryotic species are
isogamous
– They produce gametes that are morphologically
similar
• Example: Many species of fungi and algae
• Most eukaryotic species are heterogamous
– These produce gametes that are morphologically
different
• Sperm cells (male gametes)
– Relatively small and mobile
• Egg cell or ovum (female gametes)
– Usually large and nonmotile
– Stores a large amount of nutrients (animal species)
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• Gametes are typically haploid
– They contain a single set of chromosomes
• Gametes are 1n, while diploid cells are 2n
– A diploid human cell contains 46 chromosomes
– A human gamete contains only 23 chromosomes
• During meiosis, haploid cells are produced
from diploid cells
– Thus, the chromosomes must be correctly sorted
and distributed to reduce the chromosome
number to half its original value
• In humans, for example, a gamete must receive one
chromosome from each of the 23 pairs
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Tribble: B_D_S_XOXo
MENDEL'S LAWS
AND MEIOSIS
• Law of Segregation
– Refers to situations in which a single gene is being
followed through a cross
• Each diploid adult has two alleles for every gene but passes
only one allele to each of its haploid gametes
• AA makes gametes containing only A
• Aa makes gametes containing A or a (half of each)
• aa makes gametes containing only a
A
a
A
AA
Aa
a
Aa
aa
MENDEL'S LAWS
AND MEIOSIS
• Law of Independent Assortment
– Refers to situations in which more than one gene is being
followed through a cross
– Assumes that meiosis includes independent assortment
of homologues but NO CROSSING OVER
– Under these circumstances, the number of different
gametes produced depends only on the number of genes
being followed in the cross
# gametes = 2(# hybrid loci)
• For AaBb: # gametes = 22 = 4
AB
Ab
aB
Ab
• For AaBbCc: # gametes = 23 = 8
ABC
ABc
AbC
Abc
aBC
aBc
abC
abc
• For the following cross: AaBb x aabb
ab
AB
aB
Ab
ab
AaBb
aaBb
Aabb
aabb
MEIOSIS
• Like mitosis, meiosis begins after a cell has
progressed through interphase of the cell cycle
• Unlike mitosis, meiosis involves two successive
divisions
– These are termed Meiosis I and Meiosis II
– Each meiotic division is subdivided into
•
•
•
•
•
Prophase
Prometaphase
Metaphase
Anaphase
Telophase
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MEIOSIS
• Prophase I is further subdivided into five
stages known as
– Leptotene
– Zygotene
– Pachytene
– Diplotene
– Diakinesis
– Refer to Figure 3.10
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STAGES OF PROPHASE OF MEIOSIS I
LEPTOTENE
Nuclear
membrane
ZYGOTENE
PACHYTENE
DIPLOTENE
Bivalent
forming
Chiasma
DIAKINESIS
Nuclear membrane
fragmenting
tetrad
Synaptonemal
complex forming
Replicated chromosomes
condense.
Synapsis begins.
A bivalent has formed and
crossing over has occurred.
Synaptonemal complex
dissociates.
A physical exchange of
chromosome pieces
Figure 3.10
End of prophase I
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The Synaptonemal Complex
Synaptonemal complex
• Formed between
homologous chromosomes
• May not be required for pairing
• Precise role not clearly understood
Bound to
chromosomal
DNA of
homologous
chromatids
Figure 3.11
Lateral
element
Central
element
Chromatid
Transverse
filament
Provides link between
lateral elements
Figure 3.12
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Spindle apparatus complete
Chromatids attached via
kinetochore microtubules
MEIOSIS I
Centrosomes with centrioles
Mitotic spindle
Sister
chromatids
Bivalent
Synapsis of
homologous
chromatids and
crossing over
EARLY PROPHASE
LATE PROPHASE
PROMETAPHASE
Cleavage
furrow
Metaphase
plate
METAPHASE
Nuclear
membrane
fragmenting
ANAPHASE
TELOPHASE AND CYTOKINESIS
MEIOSIS II
Four haploid daughter cells
PROPHASE
PROMETAPHASE
METAPHASE
ANAPHASE
TELOPHASE AND CYTOKINESIS
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• Bivalents are organized
along the metaphase plate in
Meiosis I
Metaphase
plate
– Pairs of sister chromatids are
aligned in a double row, rather
than a single row (as in
mitosis)
Figure 3.12
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The arrangement is random
with regard to the (blue and
red) homologs
Kinetochore
Furthermore
 One pair of sister chromatids
is linked to one of the poles
 And the homologous pair is
Figure 3.13
linked to the opposite pole


The two pairs of sister chromatids
separate from each other.
However, the connection that
holds sister chromatids together
does not break.
Sister chromatids reach their
respective poles and decondense.
Nuclear envelope reforms to produce
two separate nuclei.
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Cleavage
furrow
Metaphase
plate
METAPHASE
Figure 3.12
ANAPHASE
TELOPHASE AND CYTOKINESIS
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• Meiosis I is followed by cytokinesis and then
meiosis II
• The sorting events that occur during meiosis
II are similar to those that occur during
mitosis
• However the starting point is different
– For a diploid organism with six chromosomes
• Mitosis begins with 12 chromatids joined as six pairs
of sister chromatids
• Meiosis II begins with 6 chromatids joined as three
pairs of sister chromatids
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Four haploid daughter cells
PROPHASE
PROMETAPHASE
METAPHASE
ANAPHASE
Figure 3.12
TELOPHASE AND CYTOKINESIS
• Mitosis vs Meiosis
– Mitosis produces two diploid daughter cells
– Meiosis produces four haploid daughter cells
– Mitosis produces daughter cells that are
genetically identical
– Meiosis produces daughter cells that are not
genetically identical
• The daughter cells contain only one homologous
chromosome from each pair
• The daughter cells contain many different
combinations of the single homologs
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Spermatogenesis
• The production of sperm
• In male animals, it occurs in the testes
• A diploid spermatogonial cell divides
mitotically to produce two cells
– One remains a spermatogonial cell
– The other becomes a primary spermatocyte
• The primary spermatocyte progresses
through meiosis I and II
– Refer to Figure 3.14a
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Meiois I yields two
haploid secondary
spermatocytes
Each spermatid
matures into a
haploid sperm cell
Meiois II yields four
haploid spermatids
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MEIOSIS I
MEIOSIS II
Primary
spermatocyte
(diploid)
Spermatids
(a) Spermatogenesis
Figure 3.14 (a)
Sperm cells
(haploid)
• The structure of a sperm includes
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– A long flagellum
– A head
• The head contains a haploid nucleus
– Capped by the acrosome
The acrosome contains
digestive enzymes
- Enable the sperm to
penetrate the protective layers
of the egg

In human males, spermatogenesis is
a continuous process

Sperm cells
(haploid)
A mature human male produces several
hundred million sperm per day
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Oogenesis
• The production of egg cells
• In female animals, it occurs in the ovaries
• Early in development, diploid oogonia
produce diploid primary oocytes
– In humans, for example, about 1 million primary
oocytes per ovary are produced before birth
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• The primary oocytes initiate meiosis I
• However, they enter into a dormant phase
– They are arrested in prophase I until the female
becomes sexually mature
• At puberty, primary oocytes are periodically
activated to progress through meiosis I
– In humans, one oocyte per month is activated
• The division in meiosis I is asymmetric
producing two haploid cells of unequal size
– A large secondary oocyte
– A small polar body
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• The secondary oocyte enters meiosis II, but is
arrested at metaphase II
• It is released into the oviduct
– An event called ovulation
• If the secondary oocyte is fertilized
– Meiosis II is completed
– A haploid egg and a second polar body are produced
• The haploid egg and sperm nuclei then fuse to
create the diploid nucleus of a new individual
• Note that only one of the cells produced in this
meiosis becomes an egg
• Refer to Figure 3.14b
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Unlike spermatogenesis,
the divisions in oogenesis
are asymmetric
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Secondary oocyte
Second polar body
Primary
oocyte
(diploid)
Egg cell
(haploid)
First polar body
(b) Oogenesis
Figure 3.14 (b)
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BIO 184, Exam 2
mean: 73%
30
25
20
15
10
5
0
A
B
C
D
F