Chapters 12 and 13

Objectives Describe binary fission in bacteria Describe the structures that play roles in the mitotic

phase of the cell cycle: the centrioles, spindle microtubules and chromosomes

Outline the phases of the cell cycle Describe the factors that control cell growth and

how cancer results from a breakdown of this control Outline the general progression and overall results

of meiosis, contrasting them with mitosis

Explain how meiosis provides possibilities for genetic recombination

Introduction Ch12/13

Life cycle is sequence of life forms from one generation to next

Sexual reproduction involves passing traits from two parents to next generation

Asexual reproduction involves passing traits from one parent to next generation

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)

True only for organisms that reproduce asexually single-celled organisms reproduce asexually by

dividing in two called binary fission daughter cells receive identical copy of parent’s

genes

offspring of multi-cellular organisms not genetically identical to parents unique combination of parents traits breeders of domestic plants and animals manipulate

sexual reproduction by selecting offspring that exhibit desired traits

Cells arise from preexisting cells cell reproduction called cell division two roles

enables fertilized egg to develop through various stages to adult organism

ensures continuity from generation to generation

Binary Fission

Bacterial chromosomes genes carried on single circular DNA molecule

up to 500x cell length

minimal packaging complexed with few proteins and attached to plasma

membrane at one point

Binary fission prior to cell division, genome copied

copies attached to adjacent parts of membrane

cell elongation and new plasma membrane separates two genomes

plasma membrane pinches through cell

Eukaryotic Cell Division

Eukaryotes have large, complex, multiple chromosomes human cells contain about 30,000-35,000 genes

organized into separate, linear chromosomes

DNA complexed with proteins Just prior to division, chromosomes become

visible remain visible during division process

Somatic Cells

Somatic cells are body cells (not sex cells) 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

This complete set of chromosomes (46) is known as the Diploid Number in Humans

Sex Cells (Gametes)

Sex cells are known as gametes

These cells have half of the number of chromosomes that a body cell would have.

In humans this number is 23

Sooooo..Somatic (body) cells contain the diploid number of chromosomes compared to sex cells (haploid number)

human cells: somatic cells-46 chromosomes (2n=46)

sex cells-23 chromosomes (n=23)

What is a chromosome?????? Prior to cell division, chromosomes are duplicated

visible chromosomes consist of two identical sister chromatids attached at centromere

sister chromatids are able to be separated…

Once sister chromatids separate they are again called chromosomes

I know you are all thinking: WHAAAAAAATTTTTT????????

Lets tie it all together

Humans have 23 pairs of chromosomes They get numbers 1-23 from Mom and 1-23

from Dad = 46 These 46 chromosomes are found in somatic

cells Sex cells ( gametes) have only 23 Each species has a specific diploid number

Cell Cycle

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.

It is regulated by many chemicals inside the cell.

Cell Cycle

Cell cycle results in cell division

many cells in an organism divide on regular basis

dividing cells undergo cycle: sequence of steps repeated during each division

Cell Cycle Cont. Cell cycle divided into several steps (phases)

interphase represents 90% or more of cycle time

G1-cell increases in size and increases supply of proteins and organelles

S-DNA synthesis occurs G2-cell prepares for division, increases supply of proteins

necessary for division, checks for DNA damage

G0 – cell stops progressing through cycle- will not divide

Cell Cycle

G0 = This is a very important phase of cellular activity

The cell has the opportunity to stop progressing towards division, or DNA synthesis

Why would this be important for a cell??????

Cells can phase into and out of G0 from several other cell cycle phases, its like an escape hatch

Cell Cycle Cont.

Different cells are in various phases of cycle even in same tissue

Also Different Tissues May Regulate Cycle Differently

Ex. Hair Divides Constantly

Nerve Tissue Never Divides In Adults

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 ? It works a bit like turning on a light switch….

A PO4 is taken off ATP:

AT-PO4 - PO4 - PO4 AT-PO4 - PO4 + PO4

The PO4 is placed onto an enzyme, which activates the enzyme

The enzyme ( and many other chemicals) now tell the cell to move to the next phase of its cell cycle

Soooooooooooooooooooooooooooo

If you are thinking…. Who cares????

How is this relevant to my life?????

Get ready to write down the ways!

Cyclins

Cyclins are special chemicals that make the cell cycle go around

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

mitotic (division) phase divided into two steps:

mitosis-nuclear division

cytokinesis-cytoplasmic division

result is two daughter cells with identical

chromosmes

Mitosis

Somatic cells in humans have 46 chromosomes At the end of mitosis will they be diploid or

haploid and why?????

Mitosis

Interphase: not part of division; Cell does other work

Prophase (division beginning): mitotic spindle forms from MTOC’s; ends when chromatin coiled into chromosomes; nucleoli and nuclear membrane dissolved

Metaphase: spindle formed; chromosomes aligned single file with centromeres on metaphase plate; MAD

Anaphase: sister chromatids separate; migrate to poles

Telophase: reverse of prophase

Cytokinesis: division of cytoplasm

movement of chromosomes driven by addition or subtraction of protein subunits to kinetochore end of spindle microtubules

Cytokinesis differs in plants and animals in animals, ring of microfilaments contracts

around periphery of cell forms cleavage furrow that eventually divides

cytoplasm

in plants, vesicles containing cell wall material collect on spindle equator vesicles fuse from inside out forming cell plate cell plate gradually develops into new cell wall

between new cells membranes surrounding vesicles fuse to form new

parts of plasma membranes

In Normal Cells

In mitotic normal mammal cells division only occurs 20-50 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

Control of cell division important for proper growth, development and repair of organisms growth factors regulate cell division

product of dividing cell

most plant and animal cells will not divide unless in contact with solid surface-anchorage dependence

Density Dependent Inhibition

division usually stops when single layer of cells formed and cells touch= density-dependent inhibition

due to depletion of growth factor proteins in cell mass

Three Cell Cycle Checkpoints

Three major check points in cell cycle G1 of interphase

G2 of interphase

M phase Release of growth factor/ chemical signals at each

of these checkpoints allows cell cycle to continue

The cell will ultimately divide if not halted at a checkpoint

Cancer

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

dividing

Benign tumor becomes malignant when cancerous cells from tumor mass spread to new sites and continue to proliferate 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

Common treatments for cancer: radiation-disrupts normal processes of cell division;

cancer cells more susceptible

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

Chromosomes are matched in homologous pairs share shape, genetic loci; carry genes controlling same

traits- alleles each homolog inherited from separate parent in humans, 22 pairs are autosomes, remaining pair sex

chromosomes female-two X chromosomes male-one X and one Y chromosome

Question

Are X AND Y Homologous?

Gametes

Normal Gametes have single set of chromosomes- No Pairs somatic cells have two sets of homologues

diploid (2n) sex cells(gametes) have one set of homologues

haploid (n) produced by meiosis

sexual life cycle involves alternation between diploid and haploid

fusion of haploid gametes at fertilization results in diploid zygote ( embryo)

Meiosis

Meiosis reduces chromosome number from diploid to haploid occurs only in diploid cells destined to become

gametes preceded by single duplication of chromosomes results in four haploid daughter cells consists of two consecutive phases:

meiosis I-halving of chromosome number meiosis II-separation of sister chromatids

PROPHASE I

2n- diploid

nuc. memberane breakdown homologs pair; synapsis,

chiasmataDNA 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

Comparison of mitosis and meiosis

all unique events in meiosis occur in meiosis I crossing over during prophase I separation of homologous pairs during anaphase I meiosis II virtually identical to mitosis Except starting cells are haploid mitosis results in two daughter cells with same

number of chromosomes as parent cells but meiosis results in 4 haploid cells

can occur in either diploid or haploid cells

:

meiosis results in four daughter cells with half number of chromosomes as parent cells

only occurs in diploid cells that will become gametes

Cells only run thru meiosis I and II ONCE Why?

Independent assortment of chromosomes in meiosis and random fertilization lead to varied offspring

during prophase I each homologue pairs up with its “partner of the same number”

during anaphase I maternally and paternally inherited homologues move to one pole or other independently of other pairs

for n chromosomes, there are 2n different combinations of haploid pairs for humans, 223 different combinations there are 223x223 combinations possible at

fertilization (64 billion)

Homologous chromosomes carry different versions of genes

Crossing over increases genetic variability exchange of corresponding segments between

two homologues site of crossing over called chiasma

occurs between chromatids within tetrads as homologues pair up during synapsis

produces new combinations of genes-genetic recombination

can occur several times in variable locations variability much greater than calculated two individual parents can never produce identical

offspring from separate fertilizations

Visual Comparison of Mitosis and Meiosis