Cellular Basis of Reproduction and Inheritance

Chapter 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

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

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 50,000-100,000 genes

organized into separate, linear chromosomes

DNA complexed with proteins Just prior to division, chromosomes become

visible remain visible during division process

Somatic (body) cells contain 2x chromosomes (diploid) compared to sex cells (haploid) human cells:

• somatic cells-46 chromosomes (2n=46)

• sex cells-23 chromosomes (n=23)

Prior to cell division, chromosomes are duplicated visible chromosomes consist of two identical

sister chromatids attached at centromere sister chromatids are divided among daughter

cells (now chromosomes) each cell gets identical set of chromosomes

Cell cycle results in cell multiplication most cells in organism divide on regular basis dividing cells undergo cycle-sequence of steps

repeated during each division

Cell cycle divided into several steps 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

mitotic (division) phase divided into two steps mitosis-nuclear division cytokinesis-cytoplasmic division result is two daughter cells with identical

chromosmes

Mitosis

While continuum, several established dividing points for cell cycle phases Interphase: duplication of genetic material,

ends with visible chromosomes Prophase: 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

Anaphase: chromosomes separate; migrate to spindle poles

Telophase: reverse of prophase Cytokinesis: division of cytoplasm movement of chromosomes driven by addition or

subtraction of protein subunits to kinetichore 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

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

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

Growth Factors

Three major check points in cell cycle G1 of interphase

G2 of interphase

M phase Release of growth factor at each of these

checkpoints allows cell cycle to continue

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

Common treatments for cancer radiation-disrupts normal processes of cell

division; cancer cells more susceptible chemotherapy-disrupt cell division

Meiosis

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

same traits each homologue 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

Gametes have single set of chromosomes somatic cells have two sets of homologues

diploid (2n)

sex cells 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

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

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 starting cells are haploid

mitosis results in two daughter cells with same number of chromosomes as parent 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

Independent orientation of chromosomes in meiosis and random fertilization lead to varied offspring during prophase I each homologue pairs up

with its “other” 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 half 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