Cell Reproduction 8.1 – Chromosomes 8.2 – Cell Division 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?  G 1 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)  G 2 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