Chapters 12 and 13 n Objectives F Describe binary fission in bacteria F Describe the structures that play roles in the mitotic phase of the cell cycle: the centrioles, spindle microtubules and chromosomes F Outline the phases of the cell cycle F Describe the factors that control cell growth and how cancer results from a breakdown of this control F Outline the general progression and overall results of meiosis, contrasting them with mitosis

F Explain how meiosis provides possibilities for genetic recombination

Introduction Ch12/13 n Life cycle is sequence of life forms from one generation to next n Sexual reproduction involves passing traits from two parents to next generation n Asexual reproduction involves passing traits from one parent to next generation n Cell division is basis of all processes that link phases of life cycle

Cellular Basis of Reproduction and Inheritance Chapter 12 and 13

Like beget like (more or less) n True only for organisms that reproduce asexually u single-celled organisms reproduce asexually by dividing in two F called binary fission F daughter cells receive identical copy of parent’s genes

u offspring of multi-cellular organisms not genetically identical to parents F unique combination of parents traits F breeders of domestic plants and animals manipulate sexual reproduction by selecting offspring that exhibit desired traits

n Cells arise from preexisting cells u cell reproduction called cell division u two roles F enables fertilized egg to develop through various stages to adult organism F ensures continuity from generation to generation

Binary Fission n Bacterial chromosomes u genes carried on single circular DNA molecule F up to 500x cell length u minimal packaging F complexed with few proteins and attached to plasma membrane at one point

n Binary fission u prior to cell division, genome copied F copies attached to adjacent parts of membrane u cell elongation and new plasma membrane separates two genomes u plasma membrane pinches through cell

Eukaryotic Cell Division n Eukaryotes have large, complex, multiple chromosomes u human cells contain about 30,000-35,000 genes F organized into separate, linear chromosomes u DNA complexed with proteins u Just prior to division, chromosomes become visible F remain visible during division process

Somatic Cells n Somatic cells are body cells (not sex cells) n Ex. Hair cells These cells need to contain the full set of chromosomes so that all the directions for functions and activities of the cell can be carried out. Normally you inherit 23 chromosomes from each of your parents n This complete set of chromosomes (46) is known as the Diploid Number in Humans

Sex Cells (Gametes) n Sex cells are known as gametes n These cells have half of the number of chromosomes that a body cell would have. n In humans this number is 23

u Sooooo..Somatic (body) cells contain the diploid number of chromosomes compared to sex cells (haploid number) u human cells: u somatic cells-46 chromosomes (2n=46) u sex cells-23 chromosomes (n=23)

What is a chromosome?????? n Prior to cell division, chromosomes are duplicated u visible chromosomes consist of two identical sister chromatids attached at centromere u sister chromatids are able to be separated… u Once sister chromatids separate they are again called chromosomes u I know you are all thinking: WHAAAAAAATTTTTT????????

Lets tie it all together n Humans have 23 pairs of chromosomes n They get numbers 1-23 from Mom and 1-23 from Dad = 46 n These 46 chromosomes are found in somatic cells n Sex cells ( gametes) have only 23 n Each species has a specific diploid number

Cell Cycle n The cell cycle is like a “ alarm clock” that tells the cell when it is time to do some essential activities and when to divide. n It is regulated by many chemicals inside the cell.

Cell Cycle n Cell cycle results in cell division u many cells in an organism divide on regular basis u dividing cells undergo cycle: sequence of steps repeated during each division

Cell Cycle Cont. n Cell cycle divided into several steps (phases) u interphase represents 90% or more of cycle time u G 1 -cell increases in size and increases supply of proteins and organelles u S-DNA synthesis occurs u G 2 -cell prepares for division, increases supply of proteins necessary for division, checks for DNA damage u G 0 – cell stops progressing through cycle- will not divide

Cell Cycle n G 0 = This is a very important phase of cellular activity n The cell has the opportunity to stop progressing towards division, or DNA synthesis n Why would this be important for a cell?????? n Cells can phase into and out of G 0 from several other cell cycle phases, its like an escape hatch

Cell Cycle Cont. n Different cells are in various phases of cycle even in same tissue n Also Different Tissues May Regulate Cycle Differently n Ex. Hair Divides Constantly n Nerve Tissue Never Divides In Adults n Adult Liver Tissue Does Not Divide, Except For Repair

Cell Cycle cont.  How does a cell progress through the cell cycle?  Many biochemicals stimulate the transition  One of them is a Kinase  A Kinase is an enzyme that catalyzes the transfer of a phosphate group from ATP to another molecule.

How does a Kinase work ? n It works a bit like turning on a light switch…. n A PO 4 is taken off ATP: n AT-PO 4 - PO 4 - PO 4  AT-PO 4 - PO 4 + PO 4 n The PO 4 is placed onto an enzyme, which activates the enzyme n The enzyme ( and many other chemicals) now tell the cell to move to the next phase of its cell cycle

Soooooooooooooooooooooooooooo n If you are thinking…. Who cares???? n How is this relevant to my life????? n Get ready to write down the ways!

Cyclins n Cyclins are special chemicals that make the cell cycle go around n There are many different types

Cell Cycle cont.  A Cdk is a cyclin dependent kinase  MPF is a co- chemical that is attached to Cdk  These chemicals stimulate the transition to cell division. When they are HIGH, the cell will divide  Why do we care about this?  BECAUSE CYCLIN AND CDK LEVELS ARE ALTERED IN CANCER CELLS……..

Mitosis: Somatic Cell Division u mitotic (division) phase divided into two steps: F mitosis-nuclear division F cytokinesis-cytoplasmic division F result is two daughter cells with identical chromosmes

Mitosis n Somatic cells in humans have 46 chromosomes n At the end of mitosis will they be diploid or haploid and why?????

Mitosis n Interphase: not part of division; Cell does other work n Prophase (division beginning): mitotic spindle forms from MTOC’s; ends when chromatin coiled into chromosomes; nucleoli and nuclear membrane dissolved

u Metaphase: spindle formed; chromosomes aligned single file with centromeres on metaphase plate; MAD u Anaphase: sister chromatids separate; migrate to poles u Telophase: reverse of prophase u Cytokinesis: division of cytoplasm u movement of chromosomes driven by addition or subtraction of protein subunits to kinetochore end of spindle microtubules

n Cytokinesis differs in plants and animals u in animals, ring of microfilaments contracts around periphery of cell F forms cleavage furrow that eventually divides cytoplasm

u in plants, vesicles containing cell wall material collect on spindle equator F vesicles fuse from inside out forming cell plate F cell plate gradually develops into new cell wall between new cells F membranes surrounding vesicles fuse to form new parts of plasma membranes

In Normal Cells  In mitotic normal mammal cells division only occurs times prior to cell death.  Telomeres are the “cell clocks” that govern cell longevity  Telomeres shorten with each division; after about fifty times they reach a critical length and a division cessation signal is given

Factors Affecting Cell Division n Control of cell division important for proper growth, development and repair of organisms u growth factors regulate cell division F product of dividing cell u most plant and animal cells will not divide unless in contact with solid surface-anchorage dependence

Density Dependent Inhibition u division usually stops when single layer of cells formed and cells touch= density-dependent inhibition u due to depletion of growth factor proteins in cell mass

Three Cell Cycle Checkpoints n Three major check points in cell cycle u G 1 of interphase u G 2 of interphase u M phase n Release of growth factor/ chemical signals at each of these checkpoints allows cell cycle to continue n The cell will ultimately divide if not halted at a checkpoint

Cancer n Cancer cells not affected by growth factors that regulate density-dependent inhibition u malignant tumor-metastasize u benign-no metastasis u named for organ or tissue of origin u some cancer cells produce factors that keep them dividing

u Benign tumor becomes malignant when cancerous cells from tumor mass spread to new sites and continue to proliferate F movement mediated by either blood or lymph systems

Cancer cells and telomerase  Keeps telomeres lengthened  Cells keep dividing; cells with short telomeres should stop manufacturing this enzyme  Not so simple cells in mice lacking telomerase also became cancerous

n Common treatments for cancer: n radiation-disrupts normal processes of cell division; cancer cells more susceptible n chemotherapy-disrupt cell division

Cell Death  Cells die two ways:  Necrosis- from damage, poisons,starvation, hypoxia, ATP depletion  Apoptosis- genetically programmed cell death; often normal in developmental pathways

Apoptosis sunburned cells  Also extends damage after a stroke  Cancer cells loose ability to carry out apoptosis  become a problem

Meiosis CH 13 n Chromosomes are matched in homologous pairs u share shape, genetic loci; carry genes controlling same traits- alleles u each homolog inherited from separate parent u in humans, 22 pairs are autosomes, remaining pair sex chromosomes F female-two X chromosomes F male-one X and one Y chromosome

Question n Are X AND Y Homologous?

Gametes n Normal Gametes have single set of chromosomes- No Pairs u somatic cells have two sets of homologues F diploid (2n) u sex cells(gametes) have one set of homologues F haploid (n) F produced by meiosis u sexual life cycle involves alternation between diploid and haploid u fusion of haploid gametes at fertilization results in diploid zygote ( embryo)

Meiosis n Meiosis reduces chromosome number from diploid to haploid u occurs only in diploid cells destined to become gametes u preceded by single duplication of chromosomes u results in four haploid daughter cells u consists of two consecutive phases: F meiosis I-halving of chromosome number F meiosis II-separation of sister chromatids

PROPHASE I 2n- diploid nuc. memberane breakdown homologs pair; synapsis, chiasmata DNA condenses Spindle app. forms METAPHASE I 2n- diploid Homologs aligned in cell center =equatorial plate (MAD genes)

ANAPHASE I 2n- diploid Homologs pulled apart TELOPHASE I n- haploid (end) Each cell new haploid Short interphase no S phase-No DNA synthesis

PROPHASE II n- haploid nuc. membrane breakdown DNA condenses Spindle app. forms METAPHASE II n- haploid Chromosomes aligned in cell center = equatorial plate

ANAPHASE II n- haploid TELOPHASE II n- haploid The other cell from Telophase I also divides into 2 cells so 4 cells total :each haploid

n Comparison of mitosis and meiosis u all unique events in meiosis occur in meiosis I u crossing over during prophase I u separation of homologous pairs during anaphase I u meiosis II virtually identical to mitosis u Except starting cells are haploid u mitosis results in two daughter cells with same number of chromosomes as parent cells but meiosis results in 4 haploid cells u can occur in either diploid or haploid cells :

n meiosis results in four daughter cells with half number of chromosomes as parent cells n only occurs in diploid cells that will become gametes n Cells only run thru meiosis I and II ONCE Why?

n Independent assortment of chromosomes in meiosis and random fertilization lead to varied offspring u during prophase I each homologue pairs up with its “partner of the same number” u during anaphase I maternally and paternally inherited homologues move to one pole or other independently of other pairs

u for n chromosomes, there are 2 n different combinations of haploid pairs F for humans, 2 23 different combinations F there are 2 23 x2 23 combinations possible at fertilization (64 billion)

n Homologous chromosomes carry different versions of genes n Crossing over increases genetic variability u exchange of corresponding segments between two homologues F site of crossing over called chiasma u occurs between chromatids within tetrads as homologues pair up during synapsis

u produces new combinations of genes-genetic recombination u can occur several times in variable locations F variability much greater than calculated F two individual parents can never produce identical offspring from separate fertilizations

Visual Comparison of Mitosis and Meiosis