Lecture 4
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Interphase
G1 checkpoint
Cell cycle main checkpoint.
If DNA is damaged, apoptosis
will occur. Otherwise, the cell
is committed to divide when
growth signals are present
and nutrients are available.
S
(growth and DNA
replication)
G
1
G1
(growth)
Insert figure 9.1 here
G
M
0
G2
(growth and final
preparations for
division)
G
2
G2 checkpoint
Mitosis checkpoint.
Mitosis will occur
if DNA has
replicated properly.
.
Apoptosis will
occur if the DNA is
damaged and
cannot be repaired.
M
MCAT Prep Exam
Reproduction
M checkpoint
Spindle assembly
checkpoint. Mitosis
will not continue if
chromosomes are
not properly aligned.
© SPL/Photo Researchers, Inc.;
PowerPoint® Lecture Slides are prepared by Dr. Isaac Barjis, Biology Instructor
1
The Cell Cycle

An orderly set of stages from the first
division to the time the daughter cells divide

Just prior to next division:


The cell grows larger

The number of organelles doubles

The DNA is replicated
The two major stages of the cell cycle:

Interphase

Mitosis
2
The Cell Cycle
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Interphase
G1 checkpoint
Cell cycle main checkpoint.
If DNA is damaged, apoptosis
will occur. Otherwise, the cell
is committed to divide when
growth signals are present
and nutrients are available.
S
(growth and DNA
replication)
G1
G0
G1
(growth)
M
G2
(growth and final
preparations for
G2
division)
G2 checkpoint
Mitosis checkpoint.
Mitosis will occur
if DNA has
replicated properly.
Apoptosis will
occur if the DNA is
damaged and
cannot be repaired.
M
M checkpoint
Spindle assembly
checkpoint. Mitosis
will not continue if
chromosomes are
not properly aligned.
3
Interphase




Most of the cell cycle is spent in
interphase (as much as 90% of their time)
Cell performs its usual functions
Time spent in interphase varies by cell
type
Nerve and muscle cells do not complete
the cell cycle (remain in the G0 stage)
4
Interphase

Interphase consists of: G1, S and G2 phases

G1 Phase:





Recovery from previous division
Cell doubles its organelles
Cell grows in size
Accumulates raw materials for DNA synthesis (DNA
replication)
Passage into S phase is governed by restriction points or
check points
5
Interphase

Interphase consists of: G1, S and G2 phases

S Phase:





DNA replication
Proteins associated with DNA are synthesized
Chromosomes enter with 1 chromatid each
Chromosomes leave with 2 identical chromatids each (called
sister chromatids)
G2 Phase:


Between DNA replication and onset of mitosis
Cell synthesizes proteins necessary for division
6
Mitotic (M) Stage

Includes:


Mitosis (karyokinesis)

Nuclear division

Daughter chromosomes distributed to two daughter
nuclei
Cytokinesis

Cytoplasm division

Results in two genetically identical daughter cells
7
Cell Cycle Control

Cell cycle controlled by internal and external
signals
 A signal is a molecule that either stimulates or
inhibits a metabolic event.

External signals


Growth factors
Internal signals




Family of proteins called cyclins
Increase and decrease as cell cycle continues
Without them cycle stops at G1, M or G2 (checkpoints)
Allows time for any damage to be repaired
8
Chromosome Number

The diploid (2n) number includes two sets
of chromosomes of each type

Humans have 23 different types of
chromosomes

Each type is represented twice in each body cell
(Diploid)



One set of 23 from individual’s father (paternal)
Other set of 23 from individual’s mother (maternal)
Only sperm and eggs have one of each type
(haploid)
9
Chromosome Structure

At end of S phase:


Each chromosome internally duplicated
Consists of two identical DNA chains



Sister chromatids (two strands of genetically identical
chromosomes)
Attached together at a single point (called centromere)
During mitosis:




Centromeres holding sister chromatids together
simultaneously break
Sister chromatids separate
Each becomes a daughter chromosome
Sisters of each type distributed to opposite daughter
nuclei
10
Mitosis in Animal Cells: Prophase

Prophase

Chromatin has condensed

Chromosomes distinguishable with microscope

Visible double (two sister chromatids attached at centromere)

Nucleolus disappears

Nuclear envelope disintegrates

Spindle begins to take shape

Two centrosomes (region where centrioles are located)
move away from each other

Form microtubules in star-like arrays – asters
11
Mitosis in Animal Cells: Prometaphase

Prometaphase

Centromere of each chromosome develops
two kinetochores

Specialized protein complex

One over each sister chromatid

Physically hook sister chromatids up with specialized
microtubules (kinetochore fibers)

These connect sisters to opposite poles of mother cell
12
Mitosis in Animal Cells: Metaphase &
Anaphase

Metaphase


Chromosomes are pulled around by kinetochore fibers
Forced to align across equatorial plane of cell




Appear to be spread out on a piece of glass
Metaphase plate
Represents plane through which mother cell will be divided
Anaphase

Sister chromatids separate



Now called daughter chromosomes
Pulled to opposite poles along kinetochore fibers
Centromere dissolves, releasing sister chromatids
13
Mitosis in Animal Cells: Telophase

Telophase

Spindle disappears

Now two clusters of daughter chromosomes

Nuclear envelopes form around the two incipient
daughter nuclei

Chromosomes uncoil and become diffuse chromatin again

Nucleolus reappears in each daughter nucleus
14
Cytokinesis: Animal Cells

Division of cytoplasm

Allocates mother cell’s cytoplasm equally to
daughter nucleus

Encloses each in it’s own plasma membrane

Often begins in anaphase

Animal cytokinesis:




A cleavage furrow appears between daughter nuclei
Formed by a contractile ring of actin filaments
Like pulling on a draw string
Eventually pinches mother cell in two
15
Function of Mitosis

Permits growth and repair.


In plants it retains the ability to divide
throughout the life of the plant
In mammals, mitosis is necessary:



Fertilized egg becomes an embryo
Embryo becomes a fetus
Allows a cut to heal or a broken bone to mend
16
Prokaryotic Cell Division

Prokaryotic chromosome a ring of DNA

Folded up in an area called the nucleoid

Replicated into two rings prior to division

Replicate rings attach to plasma membrane
17
Asexual Reproduction

Binary fission

Splitting in two between the two replicate
chromosomes

Produces two daughter cells identical to original cell –
Asexual Reproduction
Budding
 Regeneration
 Oarthenogenesis

18
Meiosis: Halves the Chromosome Number

Special type of cell division

Used only for sexual reproduction

Halves the chromosome number prior to
fertilization

Parents diploid

Meiosis produces haploid gametes

Gametes fuse in fertilization to form diploid zygote

Becomes the next diploid generation
19
Homologous Pairs of Chromosomes




In diploid body cells chromosomes occur in pairs
Humans have 23 different types of chromosomes
Diploid cells have two of each type
Chromosomes of the same type are said to be
homologous





They have the same length
Their centromeres are positioned in the same place
One came from the father (the paternal homolog) the other from
the mother (the maternal homolog)
When stained, they show similar banding patterns
A location on one homologue contains gene for the same trait that
occurs at this locus on the other homologue


Although the genes may code for different variations of that trait
Alternate forms of a gene are called alleles
20
Homologous Pairs of Chromosomes

Homologous chromosomes have genes controlling the
same trait at the same position







Each gene occurs in duplicate
A maternal copy from the mother
A paternal copy from the father
Many genes exist in several variant forms in a large
population
Homologous copies of a gene may encode identical or
differing genetic information
The variants that exist for a gene are called alleles
An individual may have:


Identical alleles for a specific gene on both homologs
(homozygous for the trait), or
A maternal allele that differs from the corresponding paternal
allele (heterozygous for the trait)
21
Phases of Meiosis I: Prophase I &
Metaphase I

Meiosis I (reductional division):

Prophase I
Each chromosome internally duplicated (consists of
two identical sister chromatids)
 Homologous chromosomes pair up – synapsis

Physically align themselves against each other end
to end
 End view would show four chromatids – Tetrad


Metaphase I

Homologous pairs arranged onto the metaphase
plate
22
Phases of Meiosis I: Anaphase I &
Telophase I

Anaphase I
Synapsis breaks up
 Homologous chromosomes separate from one
another
 Homologues move towards opposite poles
 Each is still an internally duplicate chromosome with
two chromatids


Telophase I
Daughter cells have one internally duplicate
chromosome from each homologous pair
 One (internally duplicate) chromosome of each type
(1n, haploid)

23
Phases of Meiosis I: Cytokinesis I &
Interkinesis

Cytokinesis I
Two daughter cells
 Both with one internally duplicate chromosome of
each type
 Haploid
 Meiosis I is reductional (halves chromosome
number)


Interkinesis
Similar to mitotic interphase
 Usually shorter
 No replication of DNA

24
Genetic Variation: Crossing Over

Meiosis brings about genetic variation in two key
ways:



Crossing-over between homologous chromosomes, and
Independent assortment of homologous chromosomes
Crossing Over:


Exchange of genetic material between nonsister chromatids
during meiosis I
At synapsis, a nucleoprotein lattice (called the
synaptonemal complex) appears between homologues




Holds homologues together
Aligns DNA of nonsister chromatids
Allows crossing-over to occur
Then homologues separate and are distributed to different
daughter cells
25
Genetic Variation: Significance

Asexual reproduction produces genetically
identical clones

Sexual reproduction cause novel genetic
recombinations

Asexual reproduction is advantageous when
environment is stable

However, if environment changes, genetic
variability introduced by sexual reproduction may
be advantageous

Offspring adapt to that environment
26
Phases of Meiosis II: Similar to Mitosis

Metaphase II





Prophase II – Chromosomes condense
Metaphase II – Chromosomes align at metaphase
plate
Anaphase II



Unremarkable
Virtually indistinguishable from mitosis of two haploid cells
Centromere dissolves
Sister chromatids separate and become daughter
chromosomes
Telophase II and cytokinesis II


Four haploid cells
All genetically unique
27
Meiosis vs. Mitosis

Meiosis







Requires two nuclear
divisions
Chromosomes synapse and
cross over
Centromeres survive
Anaphase I
Halves chromosome
number
Produces four daughter
nuclei
Produces daughter cells
genetically different from
parent and each other
Used only for sexual
reproduction

Mitosis







Requires one nuclear
division
Chromosomes do not
synapse nor cross over
Centromeres dissolve in
mitotic anaphase
Preserves chromosome
number
Produces two daughter
nuclei
Produces daughter cells
genetically identical to
parent and to each other
Used for asexual
reproduction and growth
28
Meiosis I Compared to Mitosis
29
Meiosis II Compared to Mitosis
30
The Human Life Cycle



Sperm and egg are produced by meiosis
A sperm and egg fuse at fertilization
Results in a zygote


The one-celled stage of an individual of the next generation
Undergoes mitosis

Results in multicellular embryo that gradually takes on
features determined when zygote was formed
 All growth occurs as mitotic division
 As a result of mitosis, each somatic cell in body


Has same number of chromosomes as zygote
Has genetic makeup determined when zygote was formed
31
The Human Life Cycle
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
MITOSIS
2n
2n
2n
MITOSIS
2n
zygote
2n = 46
diploid (2n)
MEIOSIS
FERTILIZATION
haploid (n)
n = 23
n
n
egg
sperm
32
Gametogenesis in Mammals
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
SPERMATOGENESIS
Primary
spermatocyte
2n
Meiosis I
Secondary
spermatocytes
n
Meiosis II
spermatids
n
Metamorphosis
and maturation
sperm
n
OOGENESIS
Primary
oocyte
2n
Meiosis I
First
polar body
n
Secondary
oocyte
n
Meiosis II
Second
polar body
n
Fertilization
cont'd
sperm nucleus
n
fusion of sperm
nucleus and
egg nucleus
Meiosis II is completed
after entry of sperm
egg
n
zygote
2n
33
Changes in Chromosome Number
Euploid is the correct number of
chromosomes in a species.
 Aneuploid is change in the chromosome
number


Results from nondisjunction
Monosomy - only one of a particular type of
chromosome,
 Trisomy - three of a particular type of chromosome

34
Changes in Chromosome
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
pair of
homologous
chromosomes
pair of
homologous
chromosomes
Meiosis I
nondisjunction
normal
nondisjunction
Meiosis II
normal
Fertilization
Zygote
2n
a.
2n
2n + 1
2n - 1
2n + 1
2n + 1
2n - 1
2n - 1
b.
35
Trisomy

Trisome 21




Occurs when an individual has three of a particular
type of chromosome
The most common autosomal trisomy seen among
humans
Also called Down syndrome
Recognized by these characteristics:





short stature
eyelid fold
flat face
stubby finger
wide gap between first and second toes
36
Changes in Sex Chromosome


Result from inheriting too many or too few X or Y
chromosomes
Nondisjunction during oogenesis or spermatogenesis

Turner syndrome (XO)




Female with single X chromosome
Short, with broad chest and widely spaced nipples
Can be of normal intelligence and function with hormone therapy
Klinefelter syndrome (XXY) – a male

Male with underdeveloped testes and prostate; some breast
overdevelopment

Long arms and legs; large hands

Near normal intelligence unless XXXY, XXXXY, etc.

No matter how many X chromosomes, presence of Y renders
individual male
37
Changes in Chromosome Structure

Changes in chromosome structure include:

Deletions



Duplications


One or both ends of a chromosome breaks off
Two simultaneous breaks lead to loss of an internal segment
Presence of a chromosomal segment more than once in the
same chromosome
Translocations

A segment from one chromosome moves to a nonhomologous chromosome

Follows breakage of two nonhomologous chromosomes and
improper re-assembly
38
Changes in Chromosome Structure

Duplication


A segment of a chromosome is repeated in the
same chromosome
Inversion

Occurs as a result of two breaks in a
chromosome
The internal segment is reversed before re-insertion
 Genes occur in reverse order in inverted segment

39
Types of Chromosomal Mutation
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
a
s
a
s
b
t
b
t
c
c
u
a.
d
v
e
u
translocation
d
v
w
e
w
f
x
f
x
g
y
y
g
h
z
z
h
b.
b: American Journal of Human Genetics by N. B. Spinner. Copyright 1994 by Elsevier Science & Technology Journals. Reproduced
with permission of Elsevier Science & Technology Journals in the format Textbook via Copyright Clearance Center
40
Reproductive System


The only system not essential for life, but ensures continued
human existence
Functions of Reproductive system include:
Production of gametes
 Storage gametes
 Nourishment gametes
 Transport gametes
 In female additional functions such as provide nutrient
and support to the developing embryo, fetus and infant
Fertilization
 Fusion of male and female gametes to form a zygote


Reproductive system includes:

Organs of reproductive system include:

1) Gonads (testes, ovaries) – produce sperm/egg



2) Ducts – a passageway that opens to the exterior and
transport gamete




Testes produce gametes (spermatozoa/ sperm) 1.5 billion each
day and secrete sex hormones (testosterone)
Ovaries release one immature gamete (oocyte) per month
Sperm is mixed with secretion of accessory glands along ducts
and converted to semen
Oocyte travels along uterine tube to uterus
3) Accessory glands and organs – these organs secrete
fluid e.g. seminal vesicle secrete seminal fluid
4) External genitalia
Male Reproductive System: Sperm
Passageway
1.
Spermatozoa is produced in testes in the
seminiferous tubules

Seminiferous tubules are tightly coiled tubes
found in lobules of testes
From seminiferous tubules sperms are
passed to afferent ductules
3. Afferent ductules pass the sperm to rete
stestes
4. From rete testes spearm leave the testes by
efferent tuctules
5. Efferent tuctules deliver the sperm to the
epididymis
2.
The Structure of the Testes
Male Reproductive System: Sperm
Passageway
Epididymis pass the sperms to the duct deference
7. From ductus deferens, sperms are passed to the
ejaculatory duct
8. Ejaculatory duct is connected to the urethra
Accessory organs of male reproductive system
1.
Seminal vesicles
2.
Prostate gland
3.
Bulbourethral glands
4.
Scrotal sac encloses testes
5.
Penis
6.
The Male Reproductive System
Sperm production: Spermatogenesis
• Sperm production takes place in seminiferous
tubules
• Seminiferous tubules contain sustencular cells
and stem cells called spermatogonia
• Stem cells involved in spermatogenesis
• Sustencular cells sustain and promote
development of sperm
• Interstitial cells between seminiferous tubules
secrete sex hormones (testosterone)
Sperm production: Spermatogenesis

Spermatogenesis involves three processes

1) Mitosis


2) Meiosis


In this process spermatocytes are produced from
spermatogonium
In this process spermatocytes go through meiosis I
and meiosis II and produce 4 spermatids
3) Spermiogenesis

In this process spermatids differentiate into
spermatozoa
Anatomy of spermatozoon

Each spermatozoon is divided into 3 part:

1) Head that contains:


2) Middle piece that contains


Nucleus and densely packed chromosomes
Mitochondria that produce the ATP needed to move
the tail
3) Tail
The only cell with flagellum in the human body
 It enables the spermatozoa to swim

Male reproductive tract: Epididymus


Epididymus is elongated tubule with head, body and tail
regions
Functions of epididymus are to:




1) Monitor and adjust fluid in seminiferous tubules
2) Store and protect spermatozoa
3) act as a recycling center for damaged spermatozoa
4) Facilitates functional maturation of spermatozoa
Ductus deferens AKA vas deferens






Begins at epididymus
Passes through inguinal canal
Enlarges to form ampulla
Peristaltic contractions propel spermatozoa and fluid along the
duct
Functions of Duct Deferens:
 Transport spermatozoa,
 Store spermatozoa (for several months)
The junction of the ampulla with the duct of the seminal vesicle
marks the start of the ejaculatory duct
 Ejaculatory duct is a short passageway that penetrates the
muscular wall of the prostate gland and empties into the
urethra
Urethra
This passageway is used by both urinary
and reproductive systems
 This passageway begins at the urinary
bladder and ends at the tip of the penis
 Urethra divides into three regions

Prostatic
 Membranous
 Penile

Accessory glands

Important glands include the seminal vesicles, the prostate
gland, and the bulbourethral glands
 Seminal vesicles
 Contributes about 60% of the total volume of semen
 Secretions of seminal vesicle contain fructose,
prostaglandins, fibrinogen
Fructose is metabolized by spermatozoa and used as a source of
energy
 Prostaglandins stimulates smooth muscle contractions along the
reproductive tracts (e.g. vagina), thus it helps spermatozoa to move
 Fibrinogen forms a temporary clot within the vagina to prevent
damage to the spermatozoa by the acidic environment of vagina.
Spermatozoa become mobile after mixing with seminal vesicles
secretion


Accessory glands

Prostate gland
Secretes an alkaline prostatic fluid that accounts
for about 20-30 % of semen volume
 Contains seminalplasmin (an antibiotic that
prevents urinary tract infection)
 The alkaline secretion helps to neutralizes acid
along the urethra and in the vagina


Bulbourethral glands

Secrete alkaline mucus with lubricating
properties
Contents of Semen

Typical ejaculate = 2-5 ml fluid that contain:
 1) Contains between 20 – 100 million spermatozoa per
ml
 2) Seminal Fluid about 60%
 3) Prostatic Fluid about 20-30 %
 4) Enzymes such as:




Protease – helps dissolve mucous secretion in the vagina
Seminalplasmin – antibiotic enzyme that kills bacteria
Enzymes that convert fibrinogen to fibrin that will clot the semen
Fibrinolysis – liquefies the clotted semen after the virginal
environment is neutralized
Hormones of male and female
reproductive system




At puberty Hypothalamus produces GnRH (Gonadotropin releasing
hormone)
GnRH stimulates production of FHS and LH by the pituitary gland
FSH (Follicle stimulating hormone)
 In male FSH targets sustentacular cells of testes and stimulates
spermatogenesis (production of sperm)
 In female FSH stimulates development of follicle (egg)
 Developing follicle produces estrogen
LH (leutinizing hormone)
 In male LH causes secretion of testosterone and other androgens
by the interstitial cells
 In female LH stimulates secretion of progesterone by corpus luteum
 In female LH surge leads to ovulation
Hormones of male and female
reproductive system



Testosterone
 1) stimulating spermatogenesis
 2) affect sexual drive(libido)
 3) stimulate metabolism
 4) stimulate the male secondary sexual characteristics
 5) maintaining accessory glands and organs of male reproductive
tract
Progesterone

Stimulate endometrial growth and secretion
Estrogen
 1) stimulating bone and muscle growth
 2) stimulate female secondary sexual characteristics
 3) initiating the repair and growth of the endometrium
The Reproductive System of the Female
Principle organs of the female reproductive
system
Ovaries
 Uterine tubes
 Uterus
 Vagina

Oogenesis
• Oogenesis is the process of ovum production
• It occurs monthly in ovarian follicles
• Part of ovarian cycle
• Follicular phase (preovulatory)
• Luteal phase (postovulatory)
The ovarian cycle

Steps in the ovarian cycle
1) Formation of primary, secondary, and tertiary
follicles
 2) Ovulation
 3) Formation and degeneration of the corpus
luteum



Corpus luteum begins to degenerate roughly 12 days
after ovulation if fertilization does not occur
4) Degradation of the corpus luteum
The uterus


The uterus is a muscular organ
Its functions are:
 1) Mechanical protection
 2) Nutritional support
 3) Waste removal for the developing embryo and fetus
The Uterine Wall
• Uterine wall consists
of 3 layer:
• Myometrium –
outer muscular
layer
• Endometrium – a
thin, inner,
glandular mucosa
• Perimetrium – an
incomplete serosa
continuous with
the peritoneum
Uterine cycle

Repeating series of changes in the
endometrium
 Uterine cycle continues from menarche (the first
minstruation) to menopause (the last
minstruation)
 Uterine cycle is divided into 3 phases:



1) Menses
 Degeneration of the endometrium
 Menstruation
2) Proliferative phase
 Restoration of the endometrium
3) Secretory phase
 Endometrial glands enlarge and accelerate their rates of
secretion
Hormones of the female
reproductive cycle
Control the reproductive cycle
 Coordinate the ovarian and uterine cycles

The Hormonal Regulation of the Female
Reproductive Cycle
The Hormonal Regulation of the Female
Reproductive Cycle
Animation: Regulation of the Female Reproductive Cycle (see tutorial)