Cell Division

Prokaryotes -Have a Simple Cell Cycle Cell division in prokaryotes takes place in two stages, which together make up a simple cell cycle 1.copy the DNA  this process is called replication 2.split the cell in two to form daughter cells  this process is called binary fission

Prokaryotes Heredity information in a prokaryote - stored in DNA –the prokaryotic chromosome -single circle of DNA –DNA replication begins with the unzipping of the double-stranded DNA at a point called the origin of replication a new double helix is formed by adding complementary nucleotides to the exposed DNA strands that have been unzipped the end result of replication is that the cell possess two complete copies of the hereditary information

Prokaryotes After replication, the cell grows in order to partition the replicated DNA molecules –cell splits into two equal halves –new plasma membrane and cell wall are added at a point between the partitioned DNA –cell constricts in two to form two daughter cells each daughter cell is a complete, living cell with its own DNA

Figure 9.1 Cell division in prokaryotes

Eukaryotes - Have a Complex Cell Cycle Eukaryotic cells contain more DNA than prokaryotic cells and the DNA is also packaged differently –cell division is more complex –DNA in eukaryotic cells is linear and packaged into a compact chromosome there is more than one chromosome in a eukaryotic cell

Eukaryotes Eukaryotic cells undergo two different mechanisms to divide up the DNA –mitosis is a cell division mechanism that occurs in nonreproductive cells these cells are called somatic cells –meiosis is a cell division mechanism that occurs in cells that participate in sexual reproduction these cells are called germ cells

Eukaryotes The eukaryotic cell cycle is divided into distinct phases –Interphase (G 1,S, and G 2 phases) –Mitosis (M phase) –Cytokinesis (C phase)

Eukaryotic Cell Cycle

Chromosomes Chromosome number varies among organisms –most eukaryotes have between 10 and 50 chromosomes in their somatic cells Chromosomes are paired in somatic cells – pairs-homologous chromosomes, or homologues –homologues -contain information about the same traits but the information may vary –diploid cells- cells that have two of each type of chromosome one chromosome of each pair is inherited from the mother and the other is inherited from the father

Chromosomes Sister chromatids- form when homologous chromosomes replicate –the sister chromatids are joined together by a structure called a centromere –humans have 23 pairs of homologous chromosomes when each chromosome in the pair is replicated, this makes for a total of 92 chromatids

Figure 9.3 The difference between homologous chromosomes and sister chromatids

9.3 Chromosomes A karyotype is an arrangement of chromosomes Chromosomes can be compared based on size, shape, and centromere location The karyotype at right shows the 23 pairs of human chromosomes Figure 9.4 The 46 chromosomes of a human

Chromosomes – are comprised of chromatin a complex of DNA and protein –the DNA in a chromosome is one very long double-stranded fiber that extends unbroken for the length of the chromosome –the DNA is coiled in order to allow it to fit into a small space despite being very long

Chromosomes DNA is coiled around proteins called histones –the histones have positive charges to counteract the negative charges associated with the phosphate groups of the DNA The DNA coils around a core of eight histone proteins to form a complex called a nucleosome –the nucleosomes in turn can be coiled together further to ultimately form a compact chromosome

Figure 9.5 Levels of eukaryotic chromosomal organization

Figure 9.2 How the cell cycle works

Interphase 1. The time preparing for cell division 2. Cells spend most of their time in this phase 3. DNA is uncoiled in the form of chromatin 4. 3 Parts –A. G1: Growth 1 –B. S: Synthesizing DNA –C. G2: Growth 2

Cell Nucleus

Mitosis Prophase –Metaphase Anaphase –Telophase »Cytokinesis

Prophase 1. Chromatin condenses and now exist as chromosomes Chromosomes are made of two sister chromatids 2. Nuclear membrane and nucleolous break down 3. Spindle forms 4. Centrioles move to opposite poles in animal cells.

Chromosome Sister Chromatids Centromere Centriole Spindle

Metaphase 1.Spindle attaches to centromere (center of the chromosome) 2.Chromosomes line up (in the middle) on the metaphase plate M= Middle

Metaphase Plate

Anaphase 1.Sister Chromatids separate and move to opposite poles A = Away

Telophase 1.Chromosomes uncoil 2.Nuclear membrane and nucleolus reform 3.Cytokinesis begins 4.Looks like start of prophase (with 2 joined cells)

Cytokinesis 1.Cytoplasm and its contents divide into 2 new cells (daughter cells) A.Animal Cell- Pinching in of cell membrane forming a cleavage furrow B.Plant cells- Cell Plate forms between 2 cells and turns into a cell wall

2 New Daughter Cells

Figure 9.7 Cytokinesis

Cell Cycle Control System

External Regulations 1.Anchorage dependence – cells must be anchored (attached) to something before they will divide (extracellular matrix) 2.Contact inhibition – cells stop dividing when they touch each other A. Caused by growth factors – proteins secreted by some cells that signal nearby cells to begin dividing

Contact Inhibition

Internal Regulations Checkpoint – critical point where stop and go-ahead signal can regulate the cell cycle Signals to checkpoints come from cellular surveillance mechanisms and from signals outside the cell G 1 checkpoint –this checkpoint makes the decision about whether the cell should divide and enter S –some cells never pass this point and are said to be in G 0 G 2 checkpoint –this checkpoint leads to mitosis M checkpoint –this checkpoint occurs during metaphase and triggers the exit process of the M phase and entry to the G 1 phase Cyclins – proteins that help regulate the timing of the cell cycle in eukaryotic cells Called cyclins because their concentration rises and falls with the cell cycle

Figure 9.9 Control of the cell cycle

CANCER A breakdown in the control of the cell cycle leads to CANCER!!!!! The cells: Do not exhibit contact inhibition –Make more cells even when no more are needed “Immortal” – divide indefinitely if supplied with proper nutrients (HeLa cells ) Caused by transformation of single cell

What Is Cancer? Cancer is a growth disorder of cells –begins when apparently normal cells grow uncontrollably and spread to other parts of the body –growing cluster of cells called a tumor benign tumors are surrounded by a healthy layer of cells (also known as encapsulated) and do not spread to other areas malignant tumors are not encapsulated and are invasive –cells from malignant tumors leave and spread to different areas of the body to form new tumors »these cells are called metastases

Lung Cancer Figure 9.11 Lung cancer cells (300x)

What Is Cancer? Cancer is caused by a gene disorder in somatic tissue in which damaged genes fail to properly control the cell cycle –mutations cause damage to genes may result from chemical or environmental exposure, such as UV rays –viral exposure may also alter DNA

What Is Cancer? There are two general classes of genes that are usually involved in cancer –proto-oncogenes these genes encode proteins that stimulate cell division mutations to these genes can cause cells to divide excessively –when mutated, these genes become oncogenes –tumor-suppressor genes these genes normally turn off cell division in healthy cells when mutated, these genes allow uncontrolled cell division

Cancer and Control of the Cell Cycle Cancer results from damaged genes failing to control cell division –one such gene, p53, affects the G 1 checkpoint its normal action is to detect abnormal DNA –it prevents cell division of a cell with damaged DNA until the DNA is repaired or directs the cell to be destroyed if the damage cannot be fixed if this gene itself becomes damaged, it will allow damaged cells to divide unchecked

Figure 9.12 Cell division and p53 protein

What causes loss of cell cycle control and leads to cancer?