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The Cellular Basis of Reproduction and Inheritance. Chapter 8. Asexual Reproduction. Parent cell divides and two ‘daughter cells’ are created Chromosomes and DNA are duplicated 2 daughter cells are identical to each other and to the parent . Sexual Reproduction.

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The Cellular Basis of Reproduction and Inheritance


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asexual reproduction
Asexual Reproduction
  • Parent cell divides and two ‘daughter cells’ are created
  • Chromosomes and DNA are duplicated
  • 2 daughter cells are identical to each other and to the parent
sexual reproduction
Sexual Reproduction
  • Offspring produced generally resemble the parent but are not identical to the parents or to each other
  • Each offspring inherits a unique set of genes from the parent
  • Highly varied
cells arise only from preexisting cells
Cells arise only from preexisting cells
  • Roles of cell division
    • Asexual reproduction
      • Reproduction of an entire single-celled organism
      • Growth of a multicellular organism
      • Growth from a fertilized egg into an adult
      • Repair and replacement of cells in an adult
    • Sexual reproduction
      • Sperm and egg production
binary fission

Plasmamembrane

Prokaryoticchromosome

Cell wall

Duplication of chromosomeand separation of copies

Continued growth of the cell and movement of copies

Division intotwo cells

Binary Fission
  • Prokaryotes reproduce by binary fission, or ‘dividing in half’
    • These cells possess a single chromosome, containing genes
    • The chromosome is replicated
    • The cell then divides into two cells, a process called binary fission
eukaryotic cell division
Eukaryotic Cell Division
  • A eukaryotic cell has many more genes than a prokaryotic cell
    • The genes are grouped into multiple chromosomes, found in the nucleus
  • Chromosomes contain a very long DNA molecule with thousands of genes

http://www.botany.org/PlantImages/ImageData.asp?IDN=15-002h&IS=700

chromosomes

Sister chromatids

Centromere

Chromosomes
  • Individual chromosomes are only visibleduring cell division
  • They are packaged as chromatin
  • Before a cell starts dividing, the chromosomes are duplicated
    • This process produces sister chromatids
sister chromatids

Chromosomeduplication

Sister chromatids

Centromere

Chromosomedistributiontodaughtercells

Sister Chromatids
  • When the cell divides, the sister chromatids separate
    • Two daughter cells are produced
    • Each has a complete and identical set of chromosomes

http://faculty.uca.edu/~benw/biol1400/notes14.htm

the cell cycle
The Cell Cycle
  • Interphase, where chromosomes duplicate and cell parts are made
  • The mitotic phase, when cell division occurs
  • Orderly sequence of events which consists of two major phases
interphase
Interphase
  • Majority of the cells time is spent in interphase
  • Cells activity is very high
    • Various metabolic activities
    • Duplicates chromosomes
    • Cell parts made, proteins, organelles
    • Preparation for mitotic division
    • 3 phases: G1(Gap 1), S (DNA Synthesis), and G2
mitotic phase
Mitotic Phase
  • 2 processes:
    • Mitosis- nucleus and its contents divide and are evenly distributed to form two daughter nuclei
    • Cytokinesis- division of the cytoplasm into two daughter cells
the four stages of mitosis
The Four Stages of Mitosis
  • Prophase:
    • Chromatin coils into distinct chromosomes
    • Sister chromosomes pair and move towards center of cell
    • Nucleolus disappears, nuclear envelope fragments
    • Mitotic spindle begins to form

Mitotic

Spindle

Nuclear

membrane

Sister

Chromosomes

prometaphase
Prometaphase

PROMETAPHASE

  • Spindle microtubules reach chromosomes and attach
  • Move chromosomes to center
  • Nuclear envelope disappears

Fragments

of nuclear

envelope

Kinetochore

Spindle

microtubules

PROMETAPHASE

the four stages of mitosis14

Mitotic Spindle

Centromere

Sister

Chromosomes

The Four Stages of Mitosis
  • Metaphase-
    • The sister chromatids line up in the center of the cell
    • The spindle fibers form and attach in the center of the chromatids in the centromere
the four stages of mitosis15

Mitotic Spindle

Sister

Chromosomes

The Four Stages of Mitosis
  • Anaphase-
    • The sister chromatids then separate and move to opposite poles of the cell
    • The spindle fibers from the mitotic spindle pull them apart
the four stages of mitosis16

Nucleus reforming

Cytokinesis

The Four Stages of Mitosis
  • Telophase-
    • Spindle fibers disintegrate
    • Chromosomes unwind
    • Nuclear envelope reforms, nucleus reforms
    • Cytokinesis splits the cytoplasm
    • In plants, new cell wall is formed
cytokinesis

Cleavagefurrow

Cleavagefurrow

Contracting ring ofmicrofilaments

Daughter cells

Cytokinesis
  • In animals, cytokinesis occurs by cleavage
    • Ring of microfiliments forms around the circumference of the cell
    • The ring then contracts
    • This process pinches the cell apart
cytokinesis18

Wall of

parent cell

Cell plate

forming

Daughter

nucleus

Cytokinesis
  • In plants, a membranous cell plate splits the cell in two
  • Vesicles from the golgi deposit cell wall material into the center
  • The vesicles then fuse into a cell plate which spans the cell

New cell wall

Cell wall

Vesicles containing

cell wall material

Daughter cells

Cell plate

cell growth factors
Cell Growth Factors
  • Cells must be able to control growth and development in order for an organism to grow normally
  • In laboratory cultures, most normal cells divide only when attached to a surface
    • They are anchorage dependent, this keeps cells from dividing in the body while detached
  • Cells continue dividing until they touch one another
    • This is called density-dependent inhibition, it keeps cells from overgrowing their organs
slide20

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
Growth Factors
  • Inadequate supplies of certain growth factor proteins may be the cause of density-dependant inhibition
  • A growth factor is a protein secreted by certain body cells that stimulate cells in the vicinity to divide
    • These signals affect critical checkpoints determine whether the cell will go through a complete cycle and divide
growth factors22

Growth factor

Plasma membrane

Relayproteins

G1 checkpoint

G1 checkpoint

Receptor

protein

Signal transduction pathway

Cell cyclecontrolsystem

Controlsystem

M checkpoint

G2 checkpoint

Growth Factors
  • The binding of growth factors to specific receptors on the plasma membrane is usually necessary for cell division
cancer cells
Cancer Cells
  • Cancer cells have abnormal cell cycles
    • They divide excessively and can form abnormal masses called tumors
  • Radiation and chemotherapy are effective as cancer treatments because they interfere with cell division
  • Malignant tumors can invade other tissues and may kill the organism
functions of mitosis
Functions of Mitosis
  • Growth-
    • Roots continue to grow in soil
    • Hair continues to grow on your head
    • New leaves develop on trees in the fall
    • Seeds and embryos develop into mature beings
functions of mitosis25

Deadcells

Epidermis, the outer layer of the skin

Dividingcells

Dermis

Functions of Mitosis
  • Cell replacement
    • Skin replacement
    • Healing and scarring
    • Starfish
  • Asexual Reproduction
    • Cuttings
    • Runners
  • Amoebas
  • Hydras
homologous chromosomes

Chromosomes

Centromere

Sister chromatids

Homologous Chromosomes
  • In humans a typical body cell, somatic cell, has 46 chromosomes
  • 23 matched pairs (4 chromosomes all together), each set of chromosomes has a twin nearly identical in length and centromere position
    • These matched pairs are called homolgous chromosomes
homologous chromosomes27
Homologous Chromosomes
  • Both carry the genes controlling the same inherited characteristics
  • Both have the gene controlling the characteristic but they may have a different version of that gene
    • One has the blue eye version, the other the brown eye version
sex chromosomes
Sex Chromosomes
  • Of the 23 pairs:
    • 22 pairs are autosomes-found in both males and females
    • The other pair are sex chromosomes that determine gender
  • Females have a pair of X chromosomes
  • Males have an X chromosome and a Y chromosome
  • X and Y chromosomes differ in size and shape
gametes

Haploid gametes (n = 23)

Egg cell

Sperm cell

MEIOSIS

FERTILIZATION

Diploidzygote (2n = 46)

Multicellulardiploid adults (2n = 46)

Mitosis anddevelopment

Gametes
  • Cells with two sets of chromosomes are said to be diploid
  • Gametes are the sex cells: sperm and eggs
  • Gametes are haploid, with only one set of chromosomes
  • Gametes are formed by a process called meiosis
meiosis
Meiosis
  • Meiosis, like mitosis, is preceded by chromosome duplication
    • However, in meiosis the cell divides twice to form four daughter cells
  • In the first division, meiosis I, homologous chromosomes are paired
    • While they are paired, they cross over and exchange genetic information
    • The homologous pairs are then separated, and two daughter cells are produced
meiosis reduces the chromosome number from diploid to haploid
Meiosis reduces the chromosome number from diploid to haploid
  • Events in the nucleus during meiosis I
    • Prophase I
      • Chromosomes coil and become compact
      • Homologous chromosomes come together as pairs by synapsis
      • Each pair, with four chromatids, is called a tetrad
      • Nonsister chromatids exchange genetic material by crossing over
meiosis reduces the chromosome number from diploid to haploid32
Meiosis reduces the chromosome number from diploid to haploid
  • Metaphase I
    • Tetrads align at the cell equator
  • Anaphase I
    • Homologous pairs separate and move toward opposite
    • poles of the cell
  • Telophase I
    • Duplicated chromosomes have reached the poles
    • A nuclear envelope forms around chromosomes in some species
    • Each nucleus has the haploid number of chromosomes
meiosis reduces the chromosome number from diploid to haploid33
Meiosis reduces the chromosome number from diploid to haploid
  • Meiosis II follows meiosis I without chromosome duplication
  • Each of the two haploid products enters meiosis II
  • Events in the nucleus during meiosis II
    • Prophase II
      • Chromosomes coil and become compact
    • Metaphase II
      • Duplicated chromosomes align at the cell equator
meiosis reduces the chromosome number from diploid to haploid34
Meiosis reduces the chromosome number from diploid to haploid
  • Anaphase II
    • Sister chromatids separate and chromosomes move toward opposite poles
  • Telophase II
    • Chromosomes have reached the poles of the cell
    • A nuclear envelope forms around each set of chromosomes
    • With cytokinesis, four haploid cells are produced
meiosis i

MEIOSIS I: Homologous chromosomes separate

INTERPHASE

PROPHASE I

METAPHASE I

ANAPHASE I

Centrosomes(withcentriolepairs)

Microtubules attached tokinetochore

Metaphaseplate

Sister chromatidsremain attached

Sites of crossing over

Spindle

Nuclearenvelope

Sisterchromatids

Tetrad

Centromere(with kinetochore)

Homologouschromosomes separate

Chromatin

Meiosis I
meiosis ii

MEIOSIS II: Sister chromatids separate

TELOPHASE IAND CYTOKINESIS

TELOPHASE IIAND CYTOKINESIS

PROPHASE II

METAPHASE II

ANAPHASE II

Cleavagefurrow

Sister chromatidsseparate

Haploiddaughter cellsforming

Meiosis II
  • Meiosis II is essentially the same as mitosis
    • The sister chromatids of each chromosome separate
    • The result is four haploid daughter cells
mitosis vs meiosis

MITOSIS

MEIOSIS

MEIOSIS I

PARENT CELL(before chromosome replication)

Site ofcrossing over

PROPHASE I

Tetrad formedby synapsis of

homologous chromosomes

PROPHASE

Chromosomereplication

Chromosomereplication

Duplicatedchromosome(two sister chromatids)

2n = 4

Chromosomes align at the metaphase plate

Tetradsalign at themetaphase plate

METAPHASE I

METAPHASE

ANAPHASE I

TELOPHASE I

Homologouschromosomesseparateduringanaphase I;sisterchromatids remain together

ANAPHASETELOPHASE

Sister chromatidsseparate duringanaphase

Haploidn = 2

Daughtercells of meiosis I

2n

2n

MEIOSIS II

No further chromosomal replication; sister chromatids separate during anaphase II

Daughter cellsof mitosis

n

n

n

n

Daughter cells of meiosis II

Mitosis vs Meiosis
causes of genetic variation
Causes of Genetic Variation
  • 1. Different homologous chromosomes
    • Each chromosome of a homologous pair comes from a different parent
    • Each chromosome thus differs at many points from the other member of the pair
    • The large number of possible arrangements of chromosome pairs at metaphase I of meiosis leads to many different combinations of chromosomes in gametes
    • Random fertilization also increases variation in offspring
causes of genetic variation39

POSSIBILITY 1

POSSIBILITY 2

Two equally probable arrangements of chromosomes at metaphase I

Metaphase II

Gametes

Combination 1

Combination 2

Combination 3

Combination 4

Causes of Genetic Variation
causes of genetic variation40
Causes of Genetic Variation
  • 2. Different versions of the same gene:
    • The differences between homologous chromosomes are based on the fact that they can carry different versions of a gene at corresponding loci
    • One chromosome carries one version of a gene, the other carries another
causes of genetic variation41

Coat-color genes

Eye-color genes

C

E

Brown

Black

C

E

C

E

c

e

c

e

c

e

White

Pink

Tetrad in parent cell(homologous pair ofduplicated chromosomes)

Chromosomes ofthe four gametes

Causes of Genetic Variation
3 crossing over
3. Crossing Over
  • Crossing over- the exchange of corresponding segments between two homologous chromosomes
  • Chiasma- sites of crossing over
    • During synapsis (when the homologous chromosomes are lined up together) the chromosomes may overlap
    • When these segments overlap, the overlapping segments may be detached and re-attached to the opposite chromosome
crossing over

Coat-colorgenes

Eye-colorgenes

Tetrad(homologous pair ofchromosomes in synapsis)

1

Breakage of homologous chromatids

2

Joining of homologous chromatids

Chiasma

Separation of homologouschromosomes at anaphase I

3

Tetrad

Chaisma

Separation of chromatids atanaphase II and completion of meiosis

4

Parental type of chromosome

Recombinant chromosome

Recombinant chromosome

Parental type of chromosome

Centromere

Crossing Over