Cell Reproduction

8.1 – Chromosomes

8.2 – Cell Division

8.3 - Meiosis

What factors limit cell size? Diffusion

DNA Content

Surface area-to-volume ratio

DIFFUSION Certain materials needed for the survival of the

cell can only enter through diffusion.

Ex. waste, carbon dioxide, oxygen

Diffusion is only efficient over a short distance. A 20 cm cell would have to wait several months

to receive the above listed molecules

DNA CONTENT Most cells have only 1 nucleus

The DNA in the nucleus makes RNA and releases it to the cytoplasm where it directs the production of enzymes

Lack of enzymes = Lack of cellular metabolism

Some large cells have compensated by having multiple nuclei.

Surface area-to-volume ratio If you double the size of a cell…

-It has 8x as much volume

-Gives rise to 8x as much waste and need for nutrients

-It becomes impossible for diffusion to keep up with the cellular demands

Chromosomes

Rod-shaped structures made of DNA and protein Coiled chromatin

=chromosomes Histones = proteins that help

maintain the shape of the chromosome

Types of Chromosomes

Sex Chromosomes X or Y chromosomes Determine sex of individual

Autosomes Non-sex chromosomes Determine all other characteristics or

traits Ex. Hair color, eye color, hemophilia

Chromosome Number

Diploid Having 2 sets of chromosomes (2n) Creates genetic diversity

Hapliod Contain one set of chromosomes (n) Sex cells

Two major stages of the cell cycle

Interphase – the growth period of time where cell activities are carried out

-most of the cells life

Mitosis – process of nuclear division, followed by the division of the cytoplasm

INTERPHASE What is going on during this time?

G1 Phase Metabolism – making ATP Cell growth, maintenance, repair

S Phase Chromosomes are replicated (ready for mitosis) Make new proteins Synthesizing new organelles Conducting photosynthesis (autotrophs) Storing excess glucose (starch, glycogen)

G2 Phase Final preparation before cell division (mitosis)

Stages of Mitosis Prophase Metaphase Anaphase Telophase

PROPHASE First stage of mitosis

Longest phase of mitosis Chromatin the nucleus becomes visible

chromosomes Nucleus disappears The chromatin, which was copied during

interphase forms 2 complete sets of chromosomes called sister chromatids

Spindle fibers form between pairs of centrioles

METAPHASE Second stage of mitosis

Chromosomes move to the equator of the cell Spindle fibers attach to each chromatid by its

centromere

ANAPHASE Third stage of mitosis

Chromosomes separate at the centromere Each sister chromatid begins to move to opposite

ends of the cell

TELOPHASE Fourth stage of mitosis

Two daughter cells are formed Each new cell has a complete set of chromosomes The cytoplasm then divides (cytokinesis) Nucleus reappears Chromosomes uncoil to form chromatin Interphase begins

Control of the Cell Cycle Cellular enzymes and genes play a key role in

checking and balancing cell division

Tells cells when to divide/stop

Proto-oncogene – regulates cell growth, division and the ability for cells to adhere to one another

Mutation of the proto-oncogene causes an oncogene ONCOGENE = UNCONTROLLED CELL DIVISION

Control of the Cell Cycle (2)

Contact inhibition – cell to cell communication between neighboring cells…stops the cellular division Eliminates an over-production of certain types of

cells within the body.

CANCER – a loss of control Cancer – an uncontrolled dividing of cells

Caused by a change in one or more of the genes synthesizing enzymes to control the cell cycle (genetic factor)

The cancer genes are often expressed when environmental conditions change (environmental factor)

How can cells regain control Some genes act as brakes to suppress

cancer expression Tumor-suppressor genes – code for

proteins that prevent cell division from occurring too often To get cancer, these tumor suppressor genes

(all 3 of them) must be damaged

Carcinogen Any substance that can induce or promote

cancer Most carcinogens are mutagens

Cause mutation within the cell

Known carcinogens include: Chemicals in tobacco smoke Radiation UV light Certain viruses

MEIOSIS

INTERPHASE

INTERPHASE MAIN EVENTS

Chromatin replicates Just like in mitosis Forms 2 identical chromatids

Centriole pairs replicate (animal cells only)

PROPHASE I

PROPHASE I MAIN EVENTS

Chromatin condenses into chromosomes Tetrad forms by synapsis Crossing over occurs between

homologous chromosomes Centriole pairs move apart Nuclear envelope and nucleoli disappear

METAPHASE I

METAPHASE I MAIN EVENTS

Tetrads align along the metaphase plate Centromeres of homologous

chromosomes point toward opposite poles

Attach to spindle fibers

ANAPHASE I

ANAPHASE I MAIN EVENTS

Homologous chromosomes separate

Sister chromatids move as a unit remain attached at the centromere

TELOPHASE I AND CYTOKINESIS

TELOPHASE I MAIN EVENTS

Chromosomes reach poles Still as sister chromatids

Cytokinesis occurs simultaneously Interkinesis No DNA replication before meiosis II

MEIOSIS II MAIN EVENTS

Very similar to mitosis PII – spindles form MII – chromosomes align AII – centromeres split TII – nuclei reform Cytokinesis – 4 haploid cells

PROPHASE II

METAPHASE II

ANAPHASE II

TELOPHASE II AND CYTOKINESIS

KEY DIFFERENCES BETWEEN MEIOSIS AND MITOSIS…

Meiosis is reduction division 2n to n

Meiosis creates genetic variation Meiosis is 2 successive nuclear

divisions Meiosis I separates pairs of

chromosomes; centromeres do not divide

GAMETE PRODUCTION THROUGH MIEOSIS

Spermatogenesis Creates 4 sperm cells

Oogenesis Creates 1 ootid (egg cell) 3 polar bodies (nonfunctional)

ASEXUAL REPRODUCTION

Production of offspring from one parent without the union of gametes

Occurs only by mitosis

SEXUAL REPRODUCTION

Production of offspring through meiosis and the union of gametes

Offspring genetically different due to genetic recombination Parts of chromatids can be exchanged

(crossing over) Homologous pairs separate