804 Cell Reproduction

Cell Reproduction Mitosis & Meiosis

Why Cells Divide Surface Area/ Volume Ratio  As the cell grows, the volume increases at a greater rate than the surface area  Can't take in enough nutrients, or remove wastes  Therefore the cell must grow or divide  Growth and Repair  Replace worn or damaged cells  Frequency of replacement varies: bacteria ~ every 20 minutes human cells ~ every 18-22 hours  Many cells in the body don't divide 

Cell Division

Cellular Reproduction  When

the parent cell divides, it forms new daughter cells  Organisms reproduce in two ways:  Asexual

Reproduction  Sexual Reproduction

Sexual vs. Asexual Reproduction  Asexual

Reproduction

 production

of offspring from one

parent  therefore genetic material is identical to parent  Sexual

Reproduction

 formation

of a new individual from the union of 2 cells  2 parents, therefore offspring have some hereditary material from each

Types of Asexual Reproduction Binary Fission  simplest form; the cell splits in 2  Spore Formation  Begins with replication  Spores can remain inactive until conditions are favorable  molds, fungi  Yeast reproduce by budding  Vegetative Propagation  Some plants, e.g. strawberries  Regeneration  planaria, star fish, etc. 

Examples of mitosis

Amoeba

Onion root tip

Seastar

Hydra

Cell Division in Prokaryotes  Binary

Fission:  The simplest form of cell division  The cell splits in 2

The Process of Binary Fission  First

the single circular chromosome duplicates = Replication  Both chromosomes attach to sites on the cell membrane  As the cell grows, a new membrane forms between attachment sites  Membrane pinches off and the new cells separate

Sexual Reproduction  The

joining of 2 specialized sex cells called gametes  male

= sperm  female = ovum  Process

of combining gametes = fertilization  Fertilization produces a zygote  has

characteristics of both parents

Human Sexual Reproduction 

Male testis produces sperm  Female ovary produces ova  Each has 23 chromosomes  Unite to form a zygote with 46 chromosomes 



23 pair

Develops into a fetus

Cell Division All types of reproduction require cell division  2 processes can be used to divide the cell’s nuclear material:  Mitosis 

Occurs in somatic cells (body cells) in eukaryotes  As a result of mitosis each daughter cell receives an exact copy of the chromosomes present in the parent cell 



Meiosis Occurs in gametes (sex cells)  As a result each daughter cell receives 1 of each pair of chromosomes present in the parent cell 

Mitosis  Cell

division in eukaryotic cells involves nuclear division called mitosis  Occurs in somatic cells  body

 As

cells; not sex cells

a result of mitosis, each daughter cell receives an exact copy of the chromosomes present in the parent cell  Chromosomes contain genetic material  DNA

Chromosomes  During

cell division in eukaryotic cells, the DNA is coiled into chromosomes  Every body cell of the same type of organism has the same number of chromosomes  humans

 2n = 46  goldfish  2n = 94  Mosquito  2n = 6

Chromosome Structure  Each

chromosome is formed from two joined strands called chromatids  Each chromatid is alike  has

a long arm & a short arm  joined at the centromere  Chromosomes

contain DNA and associated proteins

Chromosomal Proteins  Each

chromosome is a single DNA molecule and associated proteins  Histones –  One

type of chromosomal protein  The DNA wraps tightly around the histones  Histones help maintain the shape of the chromosome  Nonhistone  control

DNA

proteins -

the activity of specific regions of

DNA double helix Histones “Beads on a string”

Nucleosome

Tight helical fiber

Supercoil

Centromere Sister chromatids

Picturing Chromosome Structure

Structure of a chromosome

Visualizing Chromosomes

Chromosome Make-up 

Chromosomes of somatic cells are in pairs  One of each pair comes from mother, one from father

 The

2 chromosomes in a pair are homologous  Alike

in appearance and type of genetic information carried  Humans have 23 pairs of chromosomes  22 pairs of autosomes Autosomes are all but the sex  2 sex chromosomes ( X & Y)

Sex Chromosomes Determine the sex of the organism  Also carry other genetic information  In humans, either X or Y  Females are XX, males are XY  Thus in humans, the male chromosome determines the sex of the offspring 

Haploid vs. Diploid  Cells

with two copies of each chromosome = diploid  Autosomal cells are diploid  Gametes (sex cells) have only one of each type of chromosome  Cells with one copy of each chromosome = haploid

Karyotypes A

picture of paired human chromosomes  Used to to detect certain genetic diseases

Mitosis  The

process of dividing the nuclear material in a somatic cell in eukaryotes  Necessary for cell division

Preparation for Mitosis  Interphase  The

time between the formation of a cell through mitosis and the next mitosis

 Most

of the cell cycle is interphase  During this phase cell prepares by:  replicating

genetic material  producing organelles  assembling structures needed for mitosis

Chromosomes & Interphase  During

interphase chromosomes cannot be distinguished under the light microscope  They

 At

appear as chromatin

the start of mitosis, the chromatin thickens, and chromosomes become visible

The Cell Cycle The sequence of cell growth and division  The cell cycle can last several hours to several days  Can be affected by environmental factors, like temperature  Has 4 stages:  mitosis & division of cytoplasm (cytokinesis)  The other 3 are part of interphase: G1 S G2 

Picturing the Cell Cycle

G1 - Growth  After

mitosis, a period of intense cellular activity and growth  The cell doubles in size  Enzyme production is high  Cells that stop growing remain in G1

S- Synthesis   Cells

that divide enter S, or synthesis, phase   The chromosomes replicate

G2 – Further Growth A

second period of growth  Structures used in mitosis are assembled

The Phases of Mitosis  Mitosis

is actually a continuous process  But we divide it into 4 phases: Prophase Metaphase Anaphase Telophase

Prophase 60% of the period of mitosis is prophase  Divided into 3 parts: early, middle, & late  Chromosomes begin to coil into short rods  Nucleoli break down & begin to disappear  2 pairs of dark spots called centrosomes appear outside the nuclear membrane 

In animal cells, the centrosomes contain centrioles, formed from microtubules  Plant cells have no centrioles 



The centrosomes move to opposite sides of the cell

Mid Prophase  At

the beginning of mid-prophase spindle fibers form between the centrioles  Additional fibers radiating out from each centriole form the aster  The nuclear membrane has broken down and disappeared

The Mitotic Spindle Spindle fibers made of microtubules radiate from the centrosomes  This array of spindle fibers = the mitotic spindle  2 types of spindle fibers:  Kinetechore fibers 

Attach to a disk-shaped protein called a kinetechore  Found in the centromere of each chromosome  Extend from the kinetechore of each chromatid to one of the centrosomes 



Polar fibers 

Extend across the dividing cell from one

Late Prophase  The

centrosome pairs are at opposite ends of the cell  The centrosomes are fully formed  Chromosomes are attached to the centrosomes by spindle fibers  Other spindle fibers stretch across the cell from one centriole to the other

Metaphase  The

chromosomes are pushed and pulled by spindle fibers along cell's the midplane  called

the equator

Anaphase  Begins

with the separation of chromatids in each chromosome  Spindle fibers appear to shorten, pulling the chromatids apart at the centomere  Each chromatid is now a chromosome  2 sets of separated chromosomes then move through the cytoplasm to opposite poles of the cell

Telophase The last stage of mitosis  After the individual chromosomes have reached opposite poles of the cell, spindles disappear  A nuclear membrane forms around each set of chromosomes  Chromosomes return to a thread-like mass  Centrioles duplicate 





2 centrioles formed in each daughter cell

Nucleoli re-form within each newly formed nucleus

Cytokinesis  The

division of the cytoplasm  Follows mitosis   Cytokinesis begins during telophase  In animal cells, the cell membrane pinches together  The area that pinches in and separates is called the cleavage furrow  In plants, a cell plate is formed, dividing the two halves

Picturing Cytokinesis

Chromosome Number  Cells

formed thru mitosis have the same number of chromosomes as the parent cells  If combined in sexual reproduction, the offspring would have 2x chromosomes!  Therefore gametes have only half the number of chromosomes of somatic cells  Gametes

= sex cells

Meiosis  Gametes

need another process for nuclear division  Meiosis reduces the number of chromosomes to 1/2 the number in somatic cells

Meiosis I & II 

Forming haploid daughter cells from diploid parent cells requires two successive cell divisions:  First = Meiosis I – homologous chromosomes separate  Second = Meiosis II chromatids of each

Meiosis I  Preceded

by replication of DNA that forms the chromosome  Synapsis = pairing of homologous chromosomes  Each pair of homologous chromosomes twists around each other, forming a structure called a tetrad  Meiosis can be divided into same 4 phases as mitosis:  Prophase,

Metaphase, Anaphase, Telophase

Prophase I Chromatin begins to coil into short rods   Homologous chromosomes are formed   Spindle fibers appear   Nucleoli break down  By the end, the nuclear membrane has dissolved, and tetrads are visible 

Crossing Over  During

synapsis, (prophase I) the chromatids of homologous pairs twist around each other  A portion of one chromatid may break off and reattach, “trading” with the same piece from its homologous partner  The exchange of genes by reciprocal segments of homologous chromosomes during meiosis = “crossing-over”

Crossing over between chromatids of homologous pairs of chromosomes

d a r

t e T

Centromer e Chiasma

A chromosome in prophase of meiosis showing chiasmata (49x)

Crossing over causes exchange of genetic material between maternal & paternal chromosomes  Results in genetic recombination  Genetic recombination is less likely in genes that are closer together. 

Chromosome Mapping The likelihood that recombination will occur due to crossing-over depends on the genes’ distance from each other on the chromosome  Scientists can determine how frequently genes for particular traits occur together in offspring  This can be used to create a map of the chromosome  1% recombination (crossing-over) = 1 map unit 

Metaphase I  Tetrads

line up along the equator of the

cell  Each tetrad becomes attached to spindle fibers

Anaphase I  Homologous

chromosomes that form each tetrad are pulled apart in pairs  One pair goes to one end of the cell, the other to opposite end

Telophase I  Chromosomes

reach ends of the cell  Cell divides into 2 daughter cells

Independent Assortment During Anaphase I, one member of each homologous chromosome pair moves to one end of the cell, the other moves to the opposite end  The separation of homologous chromosomes is random  More, or fewer maternal (or paternal) chromosomes may end up on one side or the other  Each separation is independent of the others  This is the principal of independent assortment of chromosomes 

Meiosis I Summary  Meiosis

I is a Reductive Division  It reduces the number of chromosomes from diploid "2n" to haploid "n"

Meiosis II  Similar

to mitosis but not preceded by replication of DNA  4 Stages: Prophase II Metaphase II Anaphase II Telophase II

Prophase II & Metaphase II  Prophase

II

A

new spindle forms around paired chromatids

 Metaphase

II

 Chromosomes

line up along the equator  They are attached at the centromere to spindle fibers

Anaphase II  Centromeres

duplicate & the chromatids

separate  Resulting single chromatids move to opposite poles  Chromatids are now called chromosomes

Telophase II A nuclear membrane forms around each set of chromosomes   The spindle breaks down and cytokinesis occurs   Result: 4 haploid daughter cells 

Males vs. Females  In

males, during spermatogenesis, all 4 daughter cells differentiate to become sperm   In females, during oogenesis the cytoplasm divides unevenly in Meiosis I  The

smaller cell = first polar body  doesn't survive   In

Meiosis II, the division is again unequal  smaller

 So

half is second polar body

only 1 of 4 daughter cells survives

 rich

in cytoplasm, has many nutrients to

Comparing Mitosis & Meiosis 

# of nuclear divisions:

MITOSIS



MEIOSIS

1

2

2

4

diploid

diploid

Daughter cell type:

diploid

haploid

Genetic likeness to

identical

different

# of daughter cells: Parent cell type:

When Meiosis Goes Awry  What

happens when errors occur in meiosis?

Human female bands

Human female karyotype

Human male bands

Human male karyotype

Down Syndrome: An Extra Chromosome 21  Is

a condition where an individual has an extra chromosome 21  Is also called trisomy 21

 The

incidence of Down Syndrome increases with the age of the mother

How Accidents During Meiosis Can Alter Chromosome Number  In

nondisjunction

 The

members of a chromosome pair fail to separate during anaphase  Gametes with an incorrect number of chromosomes are produced

Meiosis I

Nondisjunction

Meiosis II

Nondisjunction

Gametes

n+1

n+1

n-1

n-1

n+1

n-1

n

n

Number of chromosomes (a) Nondisjunction in meiosis I

(b) Nondisjunction in meiosis II

 The

result of nondisjunction

Egg cell

n+1

Sperm cell

n (normal)

Zygote 2n + 1

Sex determination in humans

Abnormal numbers of sex chromosomes do not usually affect survival  Nondisjunction

can also produce gametes with extra or missing sex chromosomes  Unusual

numbers of sex chromosomes upset the genetic balance less than an unusual number of autosomes

Abnormal Numbers of Sex Chromosomes

 Nondisjunction  Also

affects the sex chromosomes

Klinefelter’s karyotype

A man with Klinefelter syndrome has one or more extra X chromosomes Poor beard growth Breast developme nt

Underdeveloped testes

XYY karyotype

A woman with Turner syndrome lacks an X chromosome Characteristic facial features Web of skin Constriction of aorta Poor breast development

Underdeveloped ovaries

Alterations of chromosome structure can cause birth defects and cancer  Chromosome

breakage can lead to rearrangements that can produce genetic disorders or cancer  Four

types of rearrangement are deletion, duplication, inversion, and translocation

Deletion

Homologous chromosomes Duplication

Inversion

Reciprocal translocation

Nonhomologous chromosomes





Chromosomal changes in a somatic cell can cause cancer A chromosomal translocation in the bone marrow is associated with chronic myelogenous leukemia Chromosome 9

Chromosome 22

Reciprocal translocation

“Philadelphia chromosome” Activated cancer-causing gene

Anchorage, cell density, and chemical growth factors affect cell division  Most

animal cells divide only when stimulated, and others not at all  In laboratory cultures, most normal cells divide only when attached to a surface  They

are anchorage dependent

 Cells

continue dividing until they touch one another  This

is called density-dependent inhibition Cells anchor to dish surface and divide When cells have formed a complete single layer, they stop dividing (density-dependent inhibition)

If some cells are scraped away, the remaining cells divide to fill the dish with a single layer and then stop (density-dependent inhibition)

 Growth

factors are proteins secreted by cells that stimulate other cells to divide After forming a single layer, cells have stopped dividing

Providing an additional supply of growth factors stimulates further cell division

Effect of Growth Factors

Effect of Density  Density

of cells also effects the rate of division  Crowding inhibits cell division

Control of Cell Division  Timing

and rate of cell division varies in different cell types  Control of rate of division is critical  Some cells require regulatory substances to begin division = growth factors

The Restriction Point A

crucial checkpoint occurs late in the G1 phase of the cell cycle  Point of decision to divide = restriction point  Cell cannot turn back after this point  If it is “yes,” cell goes to S phase and copies DNA  If “no,” it goes to non-dividing state (G0)  Most

cells are in G0

Growth factors signal the cell cycle control system  Proteins within the cell control the cell cycle  Signals affecting critical checkpoints determine whether the cell will go through a complete cycle and divide G1 checkpoint

Control system

G2 checkpoint



The binding of growth factors to specific receptors on the plasma membrane is usually necessary for cell division Growth factor

Receptor protein Signal transduction pathway

Plasma membrane

Relay protein G1 checkpoint s Cell cycle control system

MPF  After

S, the cell will enter G2  The “OK” signal that causes the cell to proceed from G2 to mitosis = mitosis promoting factor (MPF) A

complex of proteins

Cancer Cells: Growing Out of Control  Normal

plant and animal cells have a cell cycle control system

 When

the cell cycle control system malfunctions  Cells

may reproduce at the wrong time or

place  A benign tumor may form

What Is Cancer?  Cancer

is caused by a breakdown in control of the cell cycle

Abnormal Cell Division  Cancer

cells do not respond normally to the body’s control mechanisms for cell division  Cancer cells divide excessively  Can invade other body tissues  When a cell divides abnormally = transformed  Abnormal cells are usually destroyed by

Cancer  If

abnormal cells are not destroyed and reproduce, they may form a mass of abnormal cells = tumor  Benign

tumor = abnormal cells remain at the original site  Malignant tumor = cells spread to other parts of the body  Metastasis

the body

= spread of cancer cells in



Cancer cells divide excessively

Cancer cells spread from a malignant tumor  Metastasis is the spread of cancer 

Lymph vessels Tumor

Glandular tissue A tumor grows from a single cancer cell

Cancer cells invade neighboring tissue.

Metastasis Cancer cells spread through lymph and blood vessels to other parts of the body

Normal mammogram

Mammogram of a cancerous breast

Breast Cancer Cell

anning electron micrograph of a breast cancer cell, showin an abnormally uneven surface and cytoplasmic projections

Cancer Treatment  Radiation

therapy disrupts cell division  Chemotherapy involves drugs that disrupt cell division  Surgical removal of tumor

 Cancer

cells are often grown in culture for study

Cancer Prevention and Survival  Cancer

prevention includes changes in lifestyle  Not

smoking  Avoiding exposure to the sun  Eating a high-fiber, low-fat diet  Visiting the doctor regularly  Performing regular self-examinations

EVOLUTION CONNECTION: NEW SPECIES FROM ERRORS IN CELL DIVISION  Errors

in meiosis may have been instrumental in the evolution of many species

 Polyploids  Are

new species  Have more than two sets of homologous chromosomes in each somatic cell Tetraploid red viscacha rat?