1. STRUCTURE OF
MAMMALIAN CELL

2. SYNOPSIS ..
STRUCTURE OF MAMMALIAN CELL:
• THE CELL – IN BRIEF
• MEMBRANE STRUCTURE
• THE NUCLEUS
• THE NUCLEOLUS
• NUCLEAR ENVELOPE
• RIBOSOMES
• ENDOPLASMIC RETICULUM
• GOLGI APPARATUS

3. • LYSOSOMES
• PEROXISOMES
• MITOCHONDRIA
• CYTOSKELETON
▫ MICROFILAMENTS
▫ INTERMEDIATE FILAMENTS
▫ MICROTUBULES
• CENTROSOME

4. • The cell is the functional unit of all living
organism.
• Human cell consist of
Nucleus Cytoplasm
largest organelle contains a number of
organelles each with a
defined function

5. THE CELL – IN BRIEF
• All cells are bounded by an external lipid
membrane, called the plasma membrane or
plasmalemma PM, which serves as a dynamic
interface with the external environment.
• Functions :
▫ transfer of nutrients and metabolites,
▫ attachment of the cell to adjacent cells and
extracellular matrix, and
▫ communication with the external environment

6. • NUCLEUS - largest organelle and its
nucleoplasm is bounded by nuclear
membrane.
 contains the genetic material of the cell.
• CYTOPLASM - contains a variety of other
organelles, most of which are also bounded by
membranes.
• ENDOPLASMIC RETICULUM - extensive
system of flattened membrane-bound
tubules, saccules and flattened cisterns.
Widely distributed throughout the cytoplasm.

7. • GOLGI APPARATUS - discrete system of
membrane-bound saccules, typically located
close to the nucleus.
• MITOCHONDRIA - Scattered free in the
cytoplasm are a number of relatively
large, elongated organelles having smooth outer
membrane and a convoluted inner membrane
system.

8. • In addition to these major organelles, the cell
contains a variety of other membrane-bound
structures, including intracellular transport
vesicles and a lysosome .
• The cytoplasmic organelles are suspended in
cytosol.
• Within the cytosol, there is a network of minute
tubules and filaments, collectively known as the
cytoskeleton, which provides structural
support for the cell and its organelles, as well as
providing a mechanism for transfer of materials
within the cell and movement of the cell itself

10. MEMBRANE STRUCTURE
• Singer and Nicholson, in the early
1970s, proposed the fluid mosaic model of
membrane structure which is now generally
accepted.
• Cell membranes consist of a bilayer of
phospholipid molecules that are amphipathic

11.  Polar, hydrophilic (water-loving) head:
Derived from glycerol conjugated to a
nitrogenous compound (positive charge) via a
phosphate bridge (negative charge)
 Non-polar, hydrophobic (water-hating) tail:
Two long-chain fatty acids, each covalently linked
to the glycerol component of the polar head.
One of the fatty acids is a straight-chain saturated
fatty acid, while the other is an unsaturated fatty
acid which is 'kinked' at the position of the
unsaturated bond.

13. • The weak intermolecular forces that hold the
bilayer together allow individual phospholipid
molecules to move relatively freely within each
layer.
• The fluidity and flexibility of the membrane is
increased by the presence of unsaturated fatty
acids bridge.
• Cholesterol molecules are also present in the
bilayer in an almost 1:1 ratio with phospholipids

14. • Protein molecules make up almost half of the
total mass of the membrane.
intrinsic or integral extrinsic or peripheral
proteins proteins
transmembrane proteins
Span entire length/thickness of membrane to be
exposed to each surface.

15. Transmembrane proteins have a variety of
functions including
▫ cell-cell adhesion,
▫ cell matrix adhesion,
▫ communication and formation of pores or
channels for the transport of materials into and
out of the cell.

16. • On the external surface of the plasma membrane
polysaccharide layer termed glycocalyx,
involved in:
Cell recognition phenomena,
Formation of intercellular adhesions,
Adsorption of molecules to the cell surface
In some situations, provides mechanical and
chemical protection for the plasma membrane

17. The electron micrograph - high magnification view of the plasma membrane PM
of the minute surface projections (microvilli) MV of a lining cell from the
small intestine

19. NUCLEUS
• Largest organelle in the cell.
• Control centre of the cell, blueprint from which
all the other components of the cell are
constructed.
• This blueprint is stored in the form of
deoxyribonucleic acid (DNA) arranged in
the form of chromosomes

20. • The nucleus contains
a) DNA (<20% of its mass),
b) Protein called nucleoprotein and some
ribonucleic acid (RNA).
• Nucleoprotein is of two major types:
Histone proteins
▫ positively charged
▫ low molecular weight
▫ bind tightly to DNA
▫ control the coiling and expression of the genes
encoded by the DNA strand.
Non-histone proteins
▫ include enzymes for the synthesis of DNA and RNA
and regulatory proteins.

21. • All nucleoproteins are synthesised in the
cytoplasm and imported into the nucleus.
• Nuclear RNA includes newly synthesised
messenger, transfer and ribosomal RNA

22. • Nuclei are heterogeneous structures with
electron-dense electron-lucent areas
Heterochromatin Euchromatin
consist of tightly coiled part of the DNA that
inactive chromatin found is active in
in irregular clumps RNA synthesis
around the periphery of
nucleus.
In females, the inactivated
X chromosome Barr body,
at the edge of the nucleus

23. • Collectively, heterochromatin and euchromatin
are known as chromatin.
• Tend to clump in particular areas of the
nucleus, known as chromosome territories

24. ELECTRON MICROGRAPH - NUCLEUS

25. NUCLEOLUS
• Many nuclei, especially those of cells highly
active in protein synthesis, contain one or more
dense structures called nucleoli
sites of ribosomal RNA synthesis and ribosome
assembly

26. • Ultrastructurally, nucleoli are quite variable in
appearance.
• For eg, reticular nucleolonema with dense
filamentous components and paler
granular components.
▫ The filamentous components - sites of ribosomal
RNA synthesis, while
▫ Ribosome assembly takes place in the granular
components

27. ELECTRON MICROGRAPH - NUCLEOLUS

28. NUCLEAR ENVELOPE
• Encloses the nucleus, consists of two layers of
membrane with the intermembranous or
perinuclear space in between.
• The inner and outer nuclear membranes have
the typical phospholipid bilayer structure but
contain different integral proteins.

29. • The outer lipid bilayer is continuous with the
endoplasmic reticulum and has ribosomes on its
cytoplasmic face.
• On the inner aspect of the inner nuclear
membrane, there is an electron-dense layer of
intermediate filaments, the nuclear
lamina, consisting of polypeptides called lamins
that link inner membrane proteins and
heterochromatin.
• The nuclear envelope contains numerous nuclear
pores at the margins of which the inner and outer
membranes become continuous.

30. • Each pore contains a nuclear pore complex.
permit and regulate the exchange of
metabolites, macromolecules and ribosomal
subunits between nucleus and cytoplasm.
• Ions and small molecules diffuse freely through
the nuclear pore.
• Larger molecules, such as mRNA are moved
through the pore by an energy dependent
process.

31. ELECTRON MICROGRAPH SHOWING NUCLEAR PORES

32. RIBOSOMES
• Minute cytoplasmic organelles, each composed
of two subunits of unequal size.
• Each subunit consists of a strand of RNA
(ribosomal RNA) with associated ribosomal
proteins forming a globular structure.

33. • Ribosomes are often found attached to mRNA
molecules in small spiral-shaped aggregations
called polyribosomes or polysomes, formed
by a single strand of mRNA with ribosomes
attached along its length
• Each ribosome in a polyribosome is making a
separate molecule of the protein.
• Ribosomes and polyribosomes may also be
attached to the surface of endoplasmic
reticulum.

34. ENDOPLASMIC RETICULUM
• consists of an interconnecting network of
membranous tubules, vesicles and flattened sacs
(cisternae) which ramifies throughout the
cytoplasm.
• Much of its surface is studded with
ribosomes, giving a 'rough' appearance leading
to the name rough endoplasmic reticulum.

35. • Proteins destined for export, as well as lysosomal
proteins, are synthesised by ribosomes attached to
the surface of the rER and pass through the
membrane into its lumen.
• Integral membrane proteins are also synthesised on
rER and inserted into the membrane
• It is within the rER that proteins are folded to form
their tertiary structure, intrachain disulphide bonds
are formed and the first steps of glycosylation take
place.
• In contrast, proteins destined for the
cytoplasm, nucleus and mitochondria are
synthesised on free ribosomes.

36. ELECTRON MICROGRAPH – ROUGH ENDOPLASMIC RETICUCLUM

37. Smooth endoplasmic reticulum
• is continuous with and similar to rER except that it
lacks ribosomes.
• Functions - lipid biosynthesis (eg: cholesterol
and phospholipids)
- membrane synthesis and repair.
• In liver cells, smooth endoplasmic reticulum is rich
in cytochrome P450 and plays a major role in the
metabolism of glycogen and detoxification.
• In muscle cells, (sarcoplasmic reticulum) is
involved in the storage and release of calcium ions
that activate the contractile mechanism

38. ELECTRON MICROGRAPH SHOWING SMOOTH ENDOPLASMIC RETICULUM

39. GOLGI APPARATUS
• Consists of stacked, saucer-shaped membrane-
bound cisternae.
• The outermost cisternae take the form of a
network of tubules known as the cis and trans
Golgi networks.
• Proteins synthesised in the rough ER are
transported to the Golgi apparatus in coated
vesicles; the coat protein is known as coat
protein complex II (COP II).

40. a) On arrival at the convex forming face or cis
Golgi network of the Golgi apparatus, the
coat proteins disengage and the vesicles fuse
with the membrane of the forming face.
▫ In the Golgi apparatus the glycosylation of
proteins, begun in the rER, is completed by
sequential addition of sugar residues and the
proteins are packaged for transport to their final
destination

41. b) On arrival at the concave maturing face or
trans Golgi network, the proteins are
accurately sorted into secretory vesicles
destined for the extracellular space (e.g.
hormones, neurotransmitters, collagen) or the
plasma membrane (e.g. cell surface
receptors, adhesion molecules) or intracellular
organelles such as lysosomes.

42. • Secretory vesicles become increasingly
condensed as they migrate through the
cytoplasm to form mature secretory
granules, which are then liberated at the cell
surface by exocytosis.
• A group of membrane proteins called SNAREs
regulate docking and fusion of coated vesicles to
their target membrane

44. ELETRON MICROGRAPH SHOWING GOLGI APPARATUS

45. LYSOSOMES
• vary greatly in size and appearance
• recognised as membrane bound organelles
containing an amorphous granular material.
Phagolysosomes or secondary lysosomes:
• even more variable in appearance but are
recognisable by their diverse particulate
content, some of which is extremely electron-
dense.

46. • The lysosomal enzymes comprise more than 40
different degradative enzymes including
proteases, lipases and nucleases [acid
hydrolases] active at a pH of about 5.0.
protective mechanism for the cell; should
lysosomal enzymes escape into the cytosol where
they would be less active at the higher pH.

47. ELECTRON MICROGRAPH
OF LYSOSOME

48. PEROXISOMES (Syn: microbodies )
• small, spherical, membrane-bound organelles
• closely resemble lysosomes in size and
ultrastructure
• contain oxidases involved in certain catabolic
pathways (e.g. β oxidation of long-chain fatty
acids) which result in the formation of hydrogen
peroxide, a potentially cytotoxic by-product - kill
ingested microorganisms.

49. • also contain catalase, which regulates
hydrogen peroxide concentration, utilising it in
the oxidation of a variety of potentially toxic
substances including phenols and alcohol
• The peroxisomes of the liver and kidney are
particularly large and abundant - organs of lipid
metabolism and management of metabolic waste
products

50. Electron micrograph showing Smooth
and Rough Endoplasmic
Reticulum, Peroxisomes (P) and
Mitochondria(M)

51. CELLULAR PIGMENTS: LIPOFUSCIN AND
MELANIN:
• Lipofuscin - represents an insoluble
degradation product of organelle turnover.
• With increasing age, it accumulates as brown
granular material in the cytoplasm, particularly
of sympathetic ganglion cells.
• Melanin - responsible for skin colour.
Also present substantia nigra

52. N
LIPOFUSIN MELANIN

53. MITOCHONDRIA
• All cellular functions are dependent on a
continuous supply of energy, which is derived
from the sequential breakdown of organic
molecules during the process of cellular
respiration.
• The energy released during this process is
ultimately stored in the form of ATP molecules
which forms a pool of readily available energy
for all the metabolic functions of the cell.

54. • Mitochondria are the principal organelles
involved in cellular respiration, found in large
numbers in metabolically active cells (liver and
skeletal muscle)
• elongated, cigar-shaped organelles.
• very mobile, moving around the cell by means of
microtubules.
• localise at intracellular sites of maximum energy
requirement.
• The number of mitochondria in cells is highly
variable; liver cells contain as many as 2000
mitochondria

55. • Each mitochondrion consists of four
compartments:
The outer membrane
▫ relatively permeable, contains a pore-forming
protein, (porin), which allows free passage of
small molecules.
▫ contains enzymes that convert certain lipid
substrates into forms that can be metabolised
within the mitochondrion.
The inner membrane,
▫ thinner, is thrown into complex folds and tubules
(cristae) that project into the inner cavity.

56. The mitochondrial matrix.
▫ The matrix contains a number of dense matrix
granules, thought to be binding sites for
calcium, which is stored in mitochondria.
The intermembranous space
▫ contains a variety of enzymes

57. ELECTRON MICROGRAPH OF
MITOCHONDRIA

58. • Aerobic respiration
matrix on inner membrane
most of the enzymes The inner membrane contains
involved in oxidation molecules of the electron
of fatty acids and transport chain,and the
Kreb’s cycle enzymes involved in ATP
production

59. • Several unusual features
▫ Matrix contains one or more circular strands of
DNA resembling the chromosomes of bacteria.
▫ The matrix also contains ribosomes with a similar
structure to bacterial ribosomes.
▫ Mitochondria synthesise 37 of their own
constituent proteins.
▫ undergo self-replication

60. Energy Storage
• Energy storage GLYCOGEN ; LIPIDS
• Lipids are synthesised by all cells in order to
maintain the constant turnover of cell
membranes.
• as a means of storing excess energy as
cytoplasmic droplets, for lipid transport.

61. ELECTRON MICROGRAPH SHOWING GLYCOGEN GRANULES

62. LIPID DROPLETS SEEN AS UNSTAINED VACUOLE ELECTRON MICROGRAPH - LIPID

63. THE CYTOSKELETON AND CELL MOVEMENT
• Every cell has a supporting framework of minute
filaments and tubules, the cytoskeleton, which
maintains the shape and polarity of the cell.
• The cytoskeleton of each cell contains structural
elements of three main types,
▫ microfilaments,
▫ microtubules and
▫ intermediate filaments,
▫ as well as many accessory proteins responsible for
linking these structures to one another, to the plasma
membrane and to the membranes of intracellular
organelles.

64. I. Microfilaments.
▫ extremely fine strands (approximately 7 nm in
diameter) of the protein actin.
▫ Each actin filament consists of two strings of
bead-like subunits twisted together like a rope.
▫ The globular subunits are stabilised by calcium
ions and associated with ATP molecules to provide
energy for contraction.

65. ▫ Actin filaments are best demonstrated
histologically in skeletal muscle cells where they
form a stable arrangement of bundles with
another type of filamentous protein called
myosin.
▫ Cells not usually considered to be contractile also
contains the globular subunits of actin (G-actin)
which assemble readily into microfilaments (F-
actin) and then dissociate - providing structural
framework for cell.

66. • Membrane specialisations such as microvilli also
contain a skeleton of actin filaments.
• Actin, in association with various transmembrane
and linking proteins (predominantly filamin), forms
a robust supporting meshwork called the cell
cortex, which protects against deformation
• Actin role in :
a) cell movement,
b) pinocytosis
c) phagocytosis
d) bind to intrinsic plasma membrane proteins to
anchor them in position

67. CYTOSKELETON MICROFILAMENT

68. II. Intermediate filaments
(10-15 nm in diameter)
▫ purely structural function
▫ consist of filaments of protein that self-assemble
into larger filaments and bind intracellular
structures and to plasma membrane proteins.

69. ▫ In humans there are more than 50 different types of
intermediate filament, but these can be divided into
different classes.
Eg: cytokeratin intermediate filaments
(epithelial cells)
 vimentin is found in cells of mesodermal origin
 desmin in muscle cells
 neurofilament proteins in nerve cells
 glial fibrillary acidic protein in glial cells.
 Lamin intermediate filaments form a structural layer on
the inner side of the nuclear membrane

70. Micrograph shows an axon in transverse section wrapped in the
cytoplasm of a Schwann cell

71. III. Microtubules (24nm dia)
▫ made up of globular protein subunits which can
readily be assembled and disassembled to provide
for alterations in cell shape and position of
organelles.
▫ The microtubule subunits are of two types, alpha
and beta tubulin, which polymerise to form a
hollow tubule; when seen in cross-section, 13
tubulin molecules make up a circle.
▫ Microtubules originate from a specialised
microtubule organising centre, the centriole.

72. ▫ Microtubule-associated proteins (MAPs)
stabilise the tubular structure and include
capping proteins, which stabilise the growing
ends of the tubules.
▫ The motor proteins dynein and kinesin move
along the tubules towards and away from the cell
centre, respectively
▫ These motors attach to membranous organelles
and move them about within the cytoplasm
 Eg: The function of the spindle during cell division

73. CENTROSOME
• includes a pair of centrioles and the
centrosome matrix or pericentriolar
material.
• It is usually centrally located in the cell adjacent
to the nucleus and often surrounded by the
Golgi apparatus.
• The pair of centrioles are also known as a
diplosome.

74. • There are also fifty or more δ-tubulin ring
complexes, which form a nucleus for the
polymerisation of microtubules.
• Thus the centrioles act as a microtubule
organising centre.
• Microtubules radiate outwards from the
centrioles in a star-like arrangement, often
called an aster.
• Each centriole is cylindrical in form, consisting
of nine triplets of parallel microtubules.
• Structures apparently identical to centrioles
form the basal bodies of cilia and flagella

77. SYNOPSIS
CELL CYCLE AND CELL DIVISION:
• INTRODUCTION
• THE CELL CYCLE
a) MITOSIS
▫ PROPHASE
▫ METAPHASE
▫ ANAPHASE
▫ TELOPHASE
b) MEIOSIS
▫ 1ST MEIOTIC DIVISION
▫ 2nd MEIOTIC DIVISION
• COMPARISON OF MITOSIS AND MEIOSIS

78. INTRODUCTION
• The development of a single, fertilised egg cell to form a
complex, multicellular organism involves cellular
replication, growth and progressive specialisation
(differentiation) for a variety of functions.
• The fertilised egg (zygote) divides by a process known
as mitosis to produce two genetically identical daughter
cells, each of which divides to produce two more
daughter cells and so on
• The interval between mitotic divisions is known as the
cell cycle.

79. • All body cells divide by mitosis except for male
and female germ cells, which divide by meiosis
to produce gametes
• Facultative dividers - cells such as liver cells
that do not normally divide but retain the
capacity to undergo mitosis.

80. CELL CYCLE
• Historically, only two phases of the cell cycle
were recognised:
 a relatively short mitotic phase (M phase) and
a non-dividing phase (interphase).
• With the development of radioisotopes, it was
found that there is a discrete period during
interphase when nuclear DNA is replicated -
synthesis or S phase.

81. • Thus interphase may be divided into three
separate phases.
▫ First gap or G1 phase: cells differentiate and
perform their specialised functions.
▫ Second gap or G2 phase: cells prepare for mitotic
division.
▫ Synthesis or S phase
G0 phase - -terminally differentiated cells leave
the cell cycle after the M phase and enter a state of
continuous differentiated function.
Facultative dividers enter the G0 phase but retain
the capacity to re-enter the cell cycle

82. • In general, the S, G and M phases of the cell
cycle are relatively constant in duration, each
taking up to several hours to complete.
▫ whereas the G1 phase is highly variable, in some
cases lasting for several days or weeks.
• The G0 phase may last for the entire lifespan of
the organism.

84. MITOSIS
Division of somatic cells occurs in two phases.
• PHASE 1: chromosomes duplicated in S phase are
distributed equally between the two potential
daughter cells - mitosis.
mitosis is always equal and symmetrical
• PHASE 2: Dividing cell is cleaved into genetically
identical daughter cells by cytoplasmic division or
cytokinesis.
result in the formation of two daughter cells with grossly
unequal amounts of cytoplasm or cytoplasmic
organelles.

85. MITOSIS
• Continuous process; divided into four phases,
▫ Prophase,
▫ Metaphase,
▫ Anaphase and
▫ Telophase.
• Cell division requires the presence of mitotic
apparatus, (spindle of longitudinally arranged
microtubules extending between a pair of centrioles) at
each pole of the dividing cell.
• The mitotic apparatus is visible within the cytoplasm
only during the M phase of the cell cycle (disaggregates
shortly after completion of mitosis)

86. Prophase.
• Beginning defined as the moment when the
chromosomes (already duplicated during the
preceding S phase) first become visible within
the nucleus.
• The chromosomes become increasingly
condensed and shortened and the nucleoli
disappear.
• Dissolution of the nuclear envelope marks the
end of prophase.

87. • During prophase, the microfilaments and
microtubules of the cytoskeleton disaggregate
into their protein subunits
• In prophase the two pairs of centrioles migrate
towards opposite poles of the cell while
simultaneously a spindle of microtubules is
formed between them (interpolar
microtubules).

89. Metaphase.
• The nuclear envelope disintegrated
• The mitotic spindle moves into the nuclear area
and each duplicated chromosome becomes
attached, at a site called the kinetochore, to
another group of microtubules of the mitotic
spindle .
• Equatorial or metaphase plate -
chromosomes then become arranged in the
plane of the spindle equator.

90. • The kinetochore also controls entry of the cell
into anaphase so that the process of mitosis does
not progress until all chromatid pairs are aligned
at the cell equator.
• Metaphase checkpoint - prevents the
formation of daughter cells with unequal
numbers of chromosomes

92. Anaphase
• The splitting of the centromere marks this stage
of mitosis.
• The mitotic spindle becomes lengthened by
addition of tubulin subunits to its interpolar
microtubules
• The centrioles are thus pulled apart and the
chromatids of each duplicated chromosome are
drawn to opposite ends of the spindle.
• By the end of anaphase - two groups of identical
chromosomes are clustered at opposite poles of
the cell

94. Telophase.
• Chromosomes begin to uncoil and to regain their
interphase conformation.
• Nuclear envelope reassembles and nucleoli again
become apparent.
• Process of cytokinesis also takes place here.
• The plasma membrane around the spindle
equator becomes indented to form a
circumferential furrow around the
cell, (cleavage furrow)
▫ progressively constricts the cell until it is cleaved
into two daughter cells.

95. • A ring of microfilaments is present just beneath
the surface of the cleavage furrow and
cytokinesis occurs as a result of contraction of
this filamentous ring.
• In early G1 phase, the mitotic spindle
disaggregates and in many cell types the single
pair of centrioles begins to duplicate in
preparation for the next mitotic division.

97. MEIOSIS
• The process of sexual reproduction involves the
production by meiosis of specialised male and
female cells called gametes.
• Meiotic cell division is thus also called
gametogenesis.

98. • Each gamete contains the haploid number of
chromosomes ( i.e. one from each homologous pair.
• When the male and female gametes fuse at
fertilisation to form a zygote the diploid number of
chromosomes (46 in humans) is restored
• This mixing of chromosomes contributes to the
genetic diversity of the next generation.
• Crossing over - Further genetic diversity and
therefore evolutionary advantage

99. PROCESS OF MEIOSIS
a) First step, duplication of the chromosomes as for
mitosis.
b) This is immediately followed by crossing over of the
chromatids.
• Crossing over mixes up these paternally and maternally
derived alleles (alternative forms of the same gene) so
that the haploid gamete ends up with only one of each
chromosome pair but each individual chromosome
include alleles from each parent .
• The mechanism of crossing over - chiasma
formation..

100. c) 1st meiotic division –
involving separation of the pairs of chromatids still
joined together at the centromere.
Thus at the end of the first meiotic division, each
daughter cell contains a half complement of
duplicated chromosomes, one from each
homologous pair of chromosomes.
d) 2nd meiotic division -
involves splitting of the chromatids by pulling apart
the centromeres.
The chromatids then migrate to opposite poles of
the spindle.

102. • Thus, meiotic cell division of a single diploid
germ cell gives rise to four haploid gametes.
• In the male, each of the four gametes undergoes
morphological development into a mature
spermatozoon.

103. • In the female, unequal distribution of the
cytoplasm results in one gamete gaining almost
all the cytoplasm from the mother cell, while the
other three acquire almost none.
▫ the large gamete matures to form an ovum and
the other three, called polar bodies, degenerate

104. • During both the first and second meiotic
divisions, the cell passes through stages that
have many similar features to
prophase, metaphase, anaphase and telophase of
mitosis.
• Unlike mitosis, however, the process of meiotic
cell division can be suspended for a considerable
length of time.

105. PRIMITIVE GERM CELL
spermatogenia oogonia
After sexual maturity multiply by mitosis
spermatogonia multiply only by early fetal
continuously by mitosis development, thereby
to provide a supply of producing a fixed
cells which then complement of cells
undergoes meiosis to with the potential to
form male gamete undergo
gametogenesis

106. • The primitive germ cells of the male, the
spermatogonia, are present only in small
numbers in the male gonads before sexual
maturity.
• After this, spermatogonia multiply continuously
by mitosis to provide a supply of cells, which
then undergo meiosis to form male gametes.
• In contrast, the germ cells of the female, called
oogonia, multiply by mitosis only during early
fetal development, thereby producing a fixed
complement of cells with the potential to
undergo gametogenesis

107. • Chromosomes are not the only source of genes
in the germ cells.
• Mitochondria also contain DNA that codes for
some intrinsic mitochondrial proteins required
for energy production.
• Because the spermatozoa shed their
mitochondria at the time of fertilisation, only
maternal mitochondrial genes are passed on to
the offspring.
• A number of inherited diseases are known to be
transmitted through mitochondrial DNA

108. COMPARISON OF MITOSIS AND MEIOSIS
• Meiosis involves one reduplication of the
chromosomes followed by two sequential cell
divisions. Thus a diploid cell produces four
haploid germ cells (gametes).
• CROSSING OVER occurs only in meiosis, to
rearrange alleles such that every gamete is
genetically different. In contrast, the products of
mitosis are genetically identical

110. Thank you..