THE CELLULAR BASIS OF

REPRODUCTION AND INHERITANCE

Chapter 8

When demands are too high The Result: Cell Division

Cell Division

Ratio of surface area to volume cannot get too large because…Demands on DNA = “info crisis”Nutrient and waste movement = “traffic

jams”

Cell Division in Our Bodies Brain cell - doesn’t divide in a lifetime

RBC’s - 120 days

Skin cells - 35 days

Stomach cell - 2 days

Bacteria cell - 20 to 30 minutes

DNA PackagingSimple to complex Double helix Chromatin: DNA

and histones (protein)

Supercoils Chromosomes:

tightest packaged DNA

Chromosomes Genetic information in eukaryotic cells is

organized into chromosomes. Chromosomes are only visible (with a light

microscope) during cell division, it is then that they pack tightly into a dense visible structure.

Chromosomes are copied before division Copied chromosomes consist of two

identical “sister” chromatids held together by an area called the centromere.

Parts of a Homologous Pair of Chromosomes

Centromere

Chromosome

Chromatid

Homologous Chromosomes

Cell Division “All cells come from pre-existing cells” Prokaryotic cell division: binary fission

Eukaryotic cell division: mitosis and meiosis

Cell Division Why do cells divide?

To repair damaged or old cellsSo the organism can get larger (grow)

The Cell Cycle Describes the Life Cycle of a Cell Every cell

follows a specific sequence of steps, just like a human goes through stages of life.

THE MAJOR AND MINOR STAGES OF THE CELL CYCLE

Interphase: Cell growth and DNA replicationGap 1SynthesisGap 2

The Mitotic Phase:Mitosis: the division of the nucleus

○ Prophase○ Metaphase○ Anaphase○ Telophase

Cytokinesis: the division of the cytoplasm.

Interphase (Stage 1)

Gap 1 (G1) – Cell grows, preps for DNA replication

Synthesis phase (S-phase) – DNA is replicated

Gap 2 (G2) – Cell grows, preps for mitosis

3 Sub-Stages of Interphase

Mitosis (Stage 2 – part of the mitotic phase)

Prophase Metaphase Anaphase Telophase PMAT!

4 Sub-Stages of Mitosis

Mitosis: The division of the nucleus

ProphaseChromosomes

become visible Centrioles separate

and move to opposite ends of the cell

The spindle (a structure that helps to separate the chromosomes) forms

Nucleolus disappears and nuclear envelope breaks down

MetaphaseMicrotubles attach

to the centromeres to move the chromosomes

Chromosomes line up in the center of the cell

Mitosis: The division of the nucleus

AnaphaseCentromeres that join

sister chromatids split in half

Sister chromatids split and become individual chromosomes

Chromosomes move to opposite poles of the cell

TelophaseChromosomes

begin to disperse and are less visible

Nuclear envelopes begin to reform around each cluster of chromosomes

The spindle fibers dissolve

MitosisAt the end of

InterphaseProphase Metaphase Anaphase Telophase

Blood lilly

Mitosis

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Pro

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Meta

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An

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Telo

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Cytokinesis (Stage 2 – part of the mitotic phase)

The result of Mitosis is the formation of two nuclei each with duplicate sets of chromosomes formed within the cytoplasm of a single cell.

Cytokinesis separates the cytoplasm of the cell into two.During cytokinesis in plant cell a cell

plate forms between the nucleiDuring cytokinesis in animals a cleavage

furrow forms pinching the two cells apart

Do You Know the Stages of Mitosis?

CYTOKIN

ESIS

PROPHASE

ANAPHASE

ANAPHASE

INTE

RPHASE

MET

APHASE

TELO

PHASE

Controls on cell division

Anchorage dependence – cells must be in contact with a solid surface to divide.

Density dependent inhibition – when cells are touching other cells they don’t usually divide

Growth factors are proteins that tell cells to divide

Growth factors control the cell cycle

Cells do not usually divide unless they get a chemical signal.

The cell cycle control system is a set of molecules that help to trigger and coordinate important steps in the cell cycle.

Checkpoints are critical control points in the cycle when cells will either stop or go forward in the cycle.

Proteins that control the steps in the cell cycle are called cyclins.

Controls on Cell Division Most cells are in interphase Checkpoints for division:

Is DNA fully replicated?Is the DNA damaged?Are there enough nutrients to support cell

growth Proto-oncogenes – start cell division Tumor-Suppressor Genes – stop cell

division

Mutations in both types of

genes can lead to cancer

Controls on Cell Division

Example Tumor suppressor gene:p53 stops cell division and induces

apoptosis in abnormal cells (cells with damaged DNA)○ Apoptosis – cell suicide (pre-programmed

death)

Example proto-oncogene:Ki-ras – when mutated can lead to

increased risk of lung, ovarian, colon and pancreatic cancer

Apoptosis Video

Uncontrolled Cell Growth Cancer is a disease caused by cells that

do not respond to the signals that regulate growth.

According to the American Cancer Society:As of 2008 in the USA a man has a 1 in 2

chance of developing cancer in his lifetime and a woman has a 1 in 3 chance of developing cancer in her lifetime

Cancer accounts for 1 in 4 deaths in the U.S. each year.

Cancer occurs when extra cells form a mass or a tumorBenign tumor remain within the mass (non-

cancerous)Malignant tumor cells invade and destroy healthy

tissues elsewhere in the body (cancerous)

Types of Cancer Carcinoma - affects skin cells

(most common) Sarcoma - affects muscle,

bone, cartilage, fat or connective tissues.

Leukemia - affects white blood cells or their precursors (begins in bone marrow)

Lymphoma - affects bone marrow cells/lymphatic system (solid tumors)

Malignant Tumor

Normal Cells

Metastasized Tumor

What Causes Cancer?

Gene changes due to: diet – lack of antioxidants tobacco use exposure to radiation exposure to chemicals Inherited genes (p53 or Ki-ras don’t work)

Immune System Failure

Carcinogens

Allergic reactions, swelling, itching also flu like symptoms, greatest concern is extreme allergic reactions.

Biological Therapy:Elicits a response from your immune system (helps immune system fight cancerous cells)

Weakening of the bones can also effect sex characteristics

Hormone Therapy:Blocks hormones that cancer needs to grow

Similar to radiation therapyChemotherapy: Chemicals target cancer cells given 2 or more at a time

Can injure or kill healthy cellsRadiation Therapy:Damages cancer cell’s DNA, cells won’t reproduce

Restricted to a few types of cancer, may not get all cells, damages organs.

Surgery: physically remove tumor

Side EffectsTreatment

Cancer Among Men

The three most common cancers among men include:Prostate cancerLung cancerColorectal cancer

The leading causes of cancer death among men are:Lung cancerProstate cancerLiver cancer

Cancer Among Women

The three most common cancers among women include:Breast cancerLung cancerColorectal cancer

The leading causes of cancer death among women are:Lung cancerBreast cancerColorectal cancer

Meiosis and Crossing Over

Mitosis is cell division to replace dead cells or allow an organism to grow.

Meiosis as a specialized type of cell division that occurs only in the reproductive organs, to create reproductive cells.

Chromosomes in Somatic Cells are Matched in Homologous Pairs

Somatic cell: a body cellEx. skin cell, bone cell, etc.Contain 46 chromosomes

(diploid meaning 2 copies)○ Pairs 1-22 are called autosomes○ Pair 23 (X and Y) are sex

chromosomesThere are 23 types of

chromosomes which exist as pairs called homologus chromosomes

Homologous chromosomes

Carry the same genes at the same locations (called loci)

Genes may be different versionsEx. gene is eye color

versions of those genes include: brown, blue, grey, hazel, green

Gametes have a single set of chromosomes

Gametes are sex cellsSperm cell and egg cellGametes have only 1 copy

of each type of chromosome (called haploid – 1n)

When two gamete cells combine a diploid cell (2n) called a zygote is created.

Meiosis reduces the chromosome number from diploid to haploid

Meiosis Occurs only in sex cellsTwo divisions

○ Meiosis I○ Meiosis II

Creates haploid cells from diploid cellsCreates genetic diversity through:

○Crossing over (Prophase I)

○ Independent assortment (Metaphase I)

PROPHASE I

METAPHASE I

ANAPHASE I TELOPHASE I

STEPS OF MEIOSISMEIOSIS I

STEPS OF MEIOSIS: MEIOSIS I

Homologous chromosomes

sister chromatids sister chromatidsTetrad

Maternal Chromosom

e

Paternal Chromosom

e

PROPHASE I: Grouping of tetrads and crossing over

Non-sister chromatids Crossing

over

Crossing over: homologous chromosomes exchange genetic material

STEPS OF MEIOSIS: MEIOSIS I

PROPHASE I: Grouping of tetrads and crossing over

Crossing over contributes to genetic diversity

centriolesspindle fiber

STEPS OF MEIOSIS: MEIOSIS IPROPHASE I: Grouping of tetrads and

crossing over

OR

Independent Assortment: tetrads line up randomly during Metaphase I

Contributes to genetic diversity

STEPS OF MEIOSIS: MEIOSIS I

METAPHASE I: Independent assortment

ANAPHASE I: Homologous chromosomes separate

Tetrads split Homologous chromosomes move to opposite ends

of cell Sister chromatids stay together (not like mitosis)

STEPS OF MEIOSIS: MEIOSIS I

2 new cells each with haploid set of chromosomes

In human sex cells 23 chromosomes

STEPS OF MEIOSIS: MEIOSIS I

TELOPHASE I/CYTOKINESIS I: Homologous chromosomes separate

MEIOSIS I – recap!

PROPHASE I

METAPHASE I

ANAPHASE I TELOPHASE I

MEIOSIS I – recap!Q: What is the purpose of Meiosis I?

A: To decrease the # of chromosomes by 1/2

Q: What 2 features of Meiosis I increase the diversity of the daughter cells?

A: Crossing Over & Independent Assortment

PROPHASE II METAPHASE II ANAPHASE IITELOPHASE II

MEIOSIS II

STEPS OF MEIOSIS: MEIOSIS II

Happens immediately after cytokinesis I – interphase does not happen!

Meiosis II stages: prophase IImetaphase IIanaphase II telophase II

Meiosis II is nearly identical to Mitosis

PROPHASE II METAPHASE IIANAPHASE II TELOPHASE II

MEIOSIS II

Summary of MEIOSIS

The entire process of meiosis results in

four haploid daughter cells.

gametes = sperm or egg

Karyotypes

A karyotype is a photographic inventory of an individual’s chromosomes

White blood cells are made to go through mitosis. Cells are treated to stop cells during metaphase of mitosis and a picture is taken of the cell.

Karyotypes are used to determine gender and also identify chromosomal abnormalities.

Development of a Karyotype

Is it a boy or girl?

Chromosomal Abnormalities

Occasionally chromosomes do not separate properly during meiosis, these mistakes are called nondisjunction

This can result in too many or too few chromosomes

Most cases result in miscarriage. Some types of nondisjunction result in specific

syndromes

Chromosomal Abnormalities Down syndrome =

trisomy 21 (three copies of chromosome 21)Effects 1 out of every

700 childrenSymptoms:

○ Round face○ Short stature○ Heart defects

Chromosomal Abnormalities Klinefelter

syndrome = XXYMales born

are sterileUsually

normal intelligence

Chromosomal Abnormalities Turner

syndromeFemales lack a

second X chromosome.

Short statureWebbed neckSterile

Alterations of chromosomes can cause birth defects and cancer

Deletion – section of chromosome is deleted

Duplication – section of chromosome is copied twice

Inversion – section of chromosome is out of order

Translocation – non-homologous chromosomes cross over