Biology 3201Unit 2 – Reproduction &

DevelopmentChapter 14 & 6

Cell Division & Reproductive Systems: Strategies

Ms. K. Morris 2010-2011

Chapter 14 – Cellular Reproduction

Section 14.1: How Body Cells Reproduce

p. 460-469

The cell cycle is divided into two parts:A) growth stage (G1, S, G2)B) division stage (M & C)

Another name for the growth stage is interphase, and here the cell:

-makes new molecules which increases volume and mass

-DNA is copied

• The first part of interphase is called the gap 1 or G1 and in this part cells are carrying out metabolic activities to prepare for cell division.

• The second part of interphase called the S phase is when DNA synthesis and replication occur.

• Cells that progress through the S phase enter the gap 2 or G2 phase as the cell prepares to divide. During this stage centrioles replicate to prepare for mitosis and cytokinesis.

• The 2 parts of the division stage are:–Mitosis: division of nucleus–Cytokinesis: division of cytoplasm

• Mitosis and cytokinesis are the two shortest events in the cell cycle.

See figure 14.2 p. 460

Why is mitosis important ?• new cells are needed for growth

maintenance and repair• cells can regenerate damaged

tissues(cuts)• cells that do not function properly must

be replaced• cells die ( blood cells)• chromosome number must be maintained

e.g. humans 46 chromosomes in somatic cells

• When human somatic cells undergo mitosis, a parent cell replicates to produce a daughter cell with the same number of chromosomes.

• Mitosis and cell division occur in many somatic (body) cells. New cells are exact copies of previously existing cells. This occurs because of DNA stored in the nucleus.

• Why is it important to maintain a constant number of chromosomes through cell and organism reproduction?– This is the function of mitosis. During cell division, the

original parent cell divides to produce two new daughter cells (identical). Mitosis ensures that each daughter cell contains the same number of chromosomes and same genetic information as the parent cell. A body cell has two copies of each chromosome. Each human somatic cell has 46 chromosomes (two copies of each). Parent cells and daughter cells have 46 chromosomes each.

– This is important because each new cell must have a complete set of genetic instructions to maintain itself and produce new cells.

The Stages of Mitosis are:

1. Prophase (p. 462)- chromatin coils and thickens forming

chromosomes- chromosomes are composed of two sister

chromatids held together by a centromere. Each sister chromatid is a genetic copy of the other (identical DNA)

- because of this each daughter cell receives a full set of parent genes

- also the nuclear membrane disappears- centrioles made up of microtubules divide and

migrate to opposite ends of the cell - spindle fibers (also made of microtubules) form

between the two centrioles

See figure 14.6 p. 462

A Chromosome: contains 2 sister chromatids

2. Metaphase (p. 463)- follows prophase- chromosomes migrate to the equator of the

spindle

- spindle fibers attach to the centromere of the replicated chromosome

- spindles attach to the kinetochore fibers - chromatids are guided to the cell’s equator- spindle fibers from one pole attach to one

chromatid and spindle fibers from the other pole attach to the other chromatid

3. Anaphase (p. 465)

• the centromere splits apart (or separates) and chromatids are pulled to opposite poles of the cell by spindle fibers

• chromatids are pulled apart as a result of a shortening of microtubules that make the spindle fibers

4. Telophase (p. 465)

- is when the chromosomes reach the poles of their respective spindles (opposite poles)

- the nuclear envelope reforms - the nucleolus (which had disappeared during Prophase)

reform- chromosomes uncoil into chromatin form and become

less visible- Where there was one cell there are now two smaller

cells each with exactly the same genetic information- chromatids are now a single, non-replicated

chromosome- spindle fibres are not needed so they break down and

disappear

Cytokinesis (p. 465)

• this is defined as the separation of the cytoplasm and the formation of two new daughter cells

• cytoplasm and all its contents divides between the two halves of the cell

• in animal cells an indentation of the membrane between two daughter cells forms and deepens

• in plant cells a new cell wall and membrane form and separate the newly formed nuclei

Radiation and Chemotherapy:• Cancer cells divide more rapidly than any

other type of body cells. Therefore , anything that interferes with cell division will affect cancer cells more than healthy cells.

• This is the basis for radiation and chemotherapy.

• Mutated genes that cause cancer are called oncogenes.

Radiation Therapy

• direct radiation such as x-rays and gamma rays at the affected part of the body

• usually treated two to three times per week• internal radiation therapy involves placing

radioactive material next to the cancerous growth (inside the body)

• generally radiation therapy works by damaging the chromosomes in a cell; then it cannot divide

• healthy cells are also damaged but many are able to repair themselves

• goal of radiation therapy is to focus the radiation on the diseased part of the body and avoid affecting healthy tissue

• usually used on localized cancerous tumours such as on the skin, breast, larynx, and cervix

Chemotherapy• may include one or more types of drugs

depending on the patient and the cancer• may be used in conjunction with radiation or

on its own• some drugs attack dividing cells as they divide

or prevent cells from dividing• chemotherapy affects the entire body and is

usually used to treat cancers that are spread throughout the body such as leukemia

• unfortunately, healthy cells are affected• is chemicals that stop cell division

Side effects of radiation and chemotherapy:Radiation:• skin inflammation/ swelling of skin• fatigue (tiredness)• specific side effects depending on location of

treatmente.g. brain - hair loss

testicular cancer - sterility

Chemotherapy:• hair loss, nausea, diarrhea

• For both treatments , side effects usually last only for the duration of the treatment.

• However, sterility can be permanent.• Treatments are particularly harmful to body cells that

divide quickly, such as bone marrow cells, skin cells, hair cells, cells in the GI tract and cells of the reproductive system.

• Most people feel the benefits of treatment outweigh the risks.

What is the goal of cancer research ?• To find treatment that affects cancerous cells only, and

leaves healthy cells unharmed.• Research is expensive, but since cancer is so common and

devastating many people contribute to research efforts (i.e. fundraising).

Section 14.2: How Reproductive Cells are

Produced

p. 470-480

• Life Cycle –> sperm + egg = zygote • Meiosis to produce sperm or egg• Chromosome Count -> – Haploid n, 23 or one set (sperm or egg)– Diploid 2n, 46 or 2 sets (somatic cells)

Meiosis • Sexual reproduction occurs only in eukaryotes.

During the formation of gametes, the number of chromosomes is reduced by half, and returned to the full amount when the two gametes fuse during fertilization. Gametes are the reproductive cells.

• A zygote ( fertilized egg) contains chromosomes from both parents but does not contain double the number of chromosomes found in each body cell.

• This happens because of meiosis which produces haploid cells called gametes.

• Gametes are haploid (h) which means that they contain one copy of each type of chromosome that the diploid (2n) contains.

• The first part of meiosis reduces the number of chromosomes from diploid to haploid. This is referred to as reduction division.

• Human sperm or egg cells contain 22 autosomes and 1 sex chromosome.

• Autosomes - not directly involved in sex determination.

Phases of meiosis:Meiosis is similar to mitosis, but there is an extra set of phases for each stage.

Prophase I• each pair of homologous chromosomes ( carry genes for same

information) become aligned.• replicated homologous pairs are called tetrads.• one copy of a homologous chromosome came from one

parent ( egg) and one from the other parent (sperm).• Mothers chromosome - maternal origin• Fathers chromosome- paternal• homologous chromosomes are like a pair of shoes; same

characteristics but not identical.• Although homologous chromosomes contain the same genes,

they may have different forms of these genes called alleles. Alleles determine how a gene is expressed.

• During the pairing process , crossing over of chromatids can occur and non-sister chromatids can exchange segments of chromosomes.

• Each segment contains hundreds or thousands of genes and this contributes greatly to genetic variation.

• Result - some chromosomes will have genes from paternal origin and some from maternal.

Metaphase I• spindle fiber attaches to the centromere of each chromosome• spindle fiber pulls each tetrad to center of cell• unlike mitosis, chromosomes do not line up in a single line.

Instead they line up in homologous pairs so that one of each homologous pair is on a different side of the equator.

• chromosomes from one parent are not on one side of the cell. They are positioned randomly.

Anaphase I

- Homologous chromosomes (or tetrads) separate and move to poles (by the spindle fibers)

- Note: centromere does not split like in mitosis (it remains intact)

- sister chromatids are held together- only one chromosome from each pair will move to each pole

of the cell

Telophase I• Telophase I does not occur in all cells. If it does not occur the

cell goes to meiosis II.

If telophase does occur :• cytoplasm is divided• nuclear membrane forms around new homologous

chromosomes• second replication does not occur because each chromosome

already contains two chromatids• each cell contains some maternal and paternal chromosomes

and some paternal and maternal alleles. (crossing over during prophase I)

• In females , meiosis II occurs after the egg has been fertilized.

Meiosis II• phases are identical to mitosis• each cell beginning meiosis II is haploid• at the end of meiosis II each daughter cell is still haploid but

contains a single unreplicated chromosome (not two attached chromatids)

• in animals the daughter cells develop into gametes and in plants they turn into spores or gametes

• NOTE: It is important for sex cells to be reduced to haploid so when the egg and sperm meet the zygote will be diploid.

Meiosis 1- pro, meta, ana, tetrads, lineup, separate Meiosis 2- pro, meta, ana, coil up, line up, split up

Crossing Over- p. 471 & diagram p. 473 & diagram p. 475- Occurs between non-sister chromatids

whereby they exchange segments of chromosomes.

- Chromosomes contribute to genetic variations.

- Its role is to help randomize the gene combinations for sex cells.

- The end result of meiosis is production of gametes (sperm and egg)

- Process is called gametogenesis

Spermatogeneis:• is the process of male gamete production in

animals• meiosis in mature males takes place in the

testes, the male reproductive organs• the production of sperm starts with a diploid

germ cell called a spermatogonium• cell enlarges and undergoes meiosis I and II• final product is four haploid sperm cells each

with the same amount of cytoplasm and number of chromosomes

• after meiosis II the sperm cells develop into a mature sperm. This happens when each cell loses cytoplasm and the nucleus forms a head. A flagellum is formed for locomotion.

• humans produce sperm all year round but some organisms only produce sperm in breeding season

Oogenesis:• in females occurs in the ovaries• process begins with a diploid cell called the oogonium

which enlarges and undergoes meiosis I and II.• at the end of meiosis I the cytoplasm is not equally

divided between the daughter cells• the cell that receives most of the cytoplasm is called

the primary oocyte. The other cell is called a polar body and is not a viable sex cell.

• as the primary oocyte undergoes meiosis II, the cytoplasm is again unequally divided.

• Only one cell becomes an egg or an ovum and contains most of the cytoplasm.

• the purpose of unequal division is to provide the ovum with sufficient nutrients to support the zygote in the first few days following fertilization.

• meiosis I begins in ovarian tissue before birth and does not continue past prophase I

• continuation of meiosis I occurs after puberty and usually in one oogonium per month

• meiosis II takes place after fertilization by a sperm cell• production of ova (two or more egg cells) in females

continues from the start of puberty until menopause which usually occurs between 40 and 50

– Egg cells - contain X chromosome, Sperm cells - contain X or Y

– See table 14.2 p. 478 for differences in Sperm & Egg cell structure (important outcome #10).

CloningA clone is an identical copy of an organism.• meiosis stage is bypassed• nucleus of an egg cell from a surrogate mother

is removed• diploid nucleus of the animal to be cloned is

placed in the empty egg cell• egg cell is implanted into the mother’s uterus• cell divides and forms an embryo, which

develops into an exact copy of the animal which donated the nucleus of one of its cells

• Stem cells are blank slates of the human body - undifferentiated (non-specialized) cells that can give rise to any type of cell, from a nerve cell to a white blood cell.

• Stem cells replace worn out or damaged cells (i.e. bone marrow, blood, muscle tissue, brain, retina, digestive tract lining)

• Can also be used to treat leukemia (bone marrow transplants)

• Potential to heal: cancers, strokes, hepatitis, spinal cord injuries, AIDS, diabetes, heart disease, Alzheimer’s, muscular dystrophy

• Sources of Stem Cells:– Aborted fetuses– Unused embryos (in vitro)– Cord blood (placenta)

• Therapeutic cloning - culturing of human cells for use in treatment of medical disorders

• Reproductive cloning -the development of a cloned human embryo for the purpose of developing a cloned human being

• Cell transplant- transplanting stem cells to replace damaged cells (e.g. Pancreatic islet cells)

Reproduction Systems: StrategiesCh. 6 Info (Plants)

Modes of Reproduction• Asexual - one parent cell divides by mitosis to produce

2 identical cells which are clones of the parent.• Budding - an outgrowth on the parent organism;

develops into a new organism that separates from the parent.– ex. yeast and hydra

• Binary Fission - parent DNA is copied mitotically and original cell splits into two smaller, genetically identical cells.– ex. bacteria

• Spore Production - spores are produced mitotically and released from a single structure that is the remains of the original parent cell from which the spores came.– Fungi: ex. Rhizopus

• Fragmentation - Piece of the parent organism breaks off and is dispersed. Each section is able to form a new organism.– E.g. House plants formed from cuttings

• Parthenogenesis -offspring are produced from unfertilized eggs.– Some insects E.g. Balsam woody aphid

• Sexual Reproduction - new offspring are the result of the fusion of egg and sperm nuclei. The offspring resemble but are not identical to the parents.

Sexual Reproduction in Flowering PlantsFlower Parts:

• Pistil (carpel)- Female reproductive organ and consists of the stigma, style ovary and ovules.

• Stamen- Male reproductive organ, it consists of the anther, filament and pollen.

• Sepels- surround and protect the flower bud.• Petals- colourful structures that attract pollinators.

– See figure 6.13 p. 176

Female Flower Parts:

• Stigma- sticky lip of the carpel that captures pollen grains

• Style- stalk that supports the stigma• Ovary- swollen base of the carpel that

contains the ovules• Ovules- sacs that contain female gametes

Male Flower parts:

• Anther- the place where pollen is produced and stored

• Pollen- cases that contain male gametes• Filament- stalk that supports the anther

• Angiosperms are the class of plants that protect their seeds inside the body of fruit and have the flower as the organ of reproduction.

• They are divided into two large classes depending on the number of seed leaves or cotyledons on the embryo within the seed.– One leaf seed- monocot– Two leaf seed- dicot

• For seeds to develop, pollen grains from the anther must reach the stigma of the pistil (pollination).

Fertilization • Haploid spores are produced by meiosis within

the anthers. The spores undergo mitosis once developed into pollen grains.

• Therefore two haploid cells are found inside each pollen grain.

• One cell is called a tube cell and the other a generative cell (which will contain two sperm nuclei)

• Every ovule in the ovary has a micropyle (small opening for the pollen tube).

• Also every ovule is connected to the ovary by a short stalk.

• In each ovule, meiosis of a single cell results in four haploid spores.

• Three of these spores die and the remaining spore undergoes mitosis three times.

Fertilization Steps:

• (1) Pollen grain reaches the stigma of a flower it germinates and the protective coat of the pollen grain breaks open.

• (2) Chemicals in the stigma cause an extension of the cytoplasm and becomes a structure called a pollen tube. The tube grows through the cells of the style towards the ovary.

• (3) As the tube grows, the generative cell divides by mitosis forming two haploid nuclei.

• (4) When the pollen tube reaches the opening to the ovule the end of the tube pushes through the ovule wall and it breaks open.

• (5) The tube cell nucleus disintegrates and the two sperm nuclei fertilize the nuclei in the ovule.

• (6) One sperm nuclei fertilizes the egg forming a diploid zygote which will eventually form an embryo.

The process of sexual reproduction in flowering plants is a complex process. It starts with the production of pollination and ends with the production of a seed.

Pollen contains:1. a generative nucleus producing 2 sperm nuclei.2. the tube nucleus producing the pollen tube.

One sperm nucleus (n) unites with the egg (n) to produce a diploid zygote (2n), and the other sperm nucleus (n) uniting with two haploid polar nuclei (n each) to produce a triploid endosperm (3n).

Mitosis & Meiosis in Plant Reproduction

• Plants grow through mitosis• Plants produce pollen and ovules through

meiosis

• Remember:– Asexual: 1 parent, identical offspring– Sexual: 2 parents, offspring resemble parent but

not identical

Some more terminology:• Seed- Structure formed from the ovule of a

flower. It contains the zygote (developing plant) and endosperm.

• Fruit- A swollen ovary of a plant. It surrounds and helps to protect developing seeds.

• Zygote- A fertilized egg. A diploid (2n) structure.• Polar Nuclei- Two haploid (n) structures within

the center of an ovule.• Endosperm- Triploid (3n) structure formed

when a sperm nucleus fuses with the two polar nuclei within an ovule. This is a food source within a seed for a developing plant embryo.

• Ovule- Area inside an ovary where an egg and two polar nuclei are found. The ovule is the site of fertilization within a flower.

• Tube Nucleus- Nucleus within a pollen grain that forms the pollen tube in a flower.

• Generative Nucleus- Nucleus within a pollen grain that produces two sperm nuclei or sperm.

• Pollen Tube- Hollow tube leading from the stigma to an ovule within the ovary of the flower.

• Also review handouts regarding sexual reproduction in flowering plants.

• Review text Ch. 6, pages:– 175*-181– Also review diagrams: 6.13* p. 176

– End of Notes for Test #3 (Thurs. Dec. 9th, 2010)– Please review all diagram handouts on Ch. 14.