Cell Division

Ch 10 & 11

Sections 10-2, 10-3 & 11-4

Reproduction of Prokaryotes

Prokaryotes reproduce through a process called binary fission

The single, circular chromosome (which is attached to the plasma membrane) replicates

The cell grows & elongates, separating the two chromosomes

The plasma membrane and cell wall pinch inward, eventually forming two daughter cells

Eukaryotic Chromosomes and Cell Division Almost all of a eukaryotic cell’s genes are

found in the cell’s nucleus, and are grouped into multiple chromosomes

Chromosomes are only visible when a cell is in the process of dividing

The rest of the time, the DNA + protein threads exist as a diffuse mass called chromatin

Chromosomes

Chromosomes: Found in eukaryotic cells Pass genetic information from one generation

to the next Made up of DNA The cells of every organism have a specific #

of chromosomes EX: Fruit flies= 8 chromosomes, Humans= 46 chromosomes, carrot cells= 18 chromosomes

Each chromosome is composed of two chromatids

Eukaryotic Cell Division

Before dividing, the eukaryotic cell replicates its chromosomes

The two copies of the chromosome are called chromatids, and they are connected to each other at a region called the centromere

The pair of chromatids is still only one chromosome, and the chromatids are not considered chromosomes until they separate from one another

Chromatids

Chromatids: 2 identical strands that make up 1 chromosome During cell division, sister chromatids separate

from each other One chromatid then goes to each of the

2 new cells

The Cell Cycle

The Cell cycle= A series of events that cells go through as the grow and divide Cells grow Prepare for division Divide to form 2 daughter cells Each daughter cell then begins the cycle

again

Events of the Cell Cycle

Events of the cell cycle can be divided into two major stages:1. Interphase= Can be quite long. Divided into 3

parts G1= Cells grow and increase their size;

producing proteins and organelles S= Chromosomes replicate, DNA is synthesized G2= A time of metabolic activity; proteins needed

for cell division are produced

Events of the Cell Cycle

Mitosis Phase (M Phase) Consists of two processes: mitosis and cytokinesis During mitosis, the chromosomes divide and are

distributed into two daughter nuclei During cytokinesis, the cytoplasm is divided into two These two processes result in the production of two

genetically identical daughter cells

Mitosis

Mitosis= Process of cellular division. Divided into 4 phases:

1. Prophase

2. Metaphase

3. Anaphase

4. Telophase

Prophase

Prophase= longest phase of mitosis (50-60%) of total time required to complete mitosis Chromosomes become visible Centrioles (2 tiny structures located in the cytoplasm)

separate and take opposite sides of nucleus Condensed chromosomes attach to the spindle Each chromosome has two chromatids joined at the

centromere Chromosomes coil more tightly Nuclear envelope breaks down & nucleolus disappears The chromosomes begin moving toward the center of the

cell

Metaphase

Metaphase= Lasts only a few minutes The spindle is fully formed All the centromeres of chromosomes line up in

center of the cell Microtubules connect centromere of each

chromosome to the 2 poles of the spindle

Anaphase

Anaphase- The centromeres of the chromosome come

apart and the sister chromatids split & separate becoming individual chromosomes (called daughter chromosomes)

Chromosomes move into two groups near poles of the spindle

The cell elongates Anaphase ends when chromosomes stop

moving

Telophase

Telophase- (basically the reverse of Prophase) Cell elongation that began in anaphase continues Chromosomes begin to disappear into tangle of

dense material (Chromatin fibers uncoil) Nuclear envelope reforms around each cluster of

chromosomes Spindles break apart and disappear Nucleolus becomes visible in each daughter

nucleus Mitosis is complete

**CELL DIVISION HOWEVER IS NOT COMPLETE**

Cytokinesis

Cytokinesis- Division of the cytoplasm itself and is the final phase of cellular division Usually occurs during the same time as

telophase In animal cells= cytoplasm is drawn inward

and pinched into two equal parts In plant cells= Cell plate forms causing a cell

wall to form between the 2 divided nuclei

Regulating the Cell Cycle

Controls on Cell Division: Cell growth can be turned on and off depending on the situation EX: if cells growing on a Petri dish are

removed, the cells surrounding that now empty space will start dividing, filling in the area where the cells were removed

Uncontrolled Cell Growth

The consequences of uncontrolled cell growth in multicellular organisms can be very severe. Cancer cells DO NOT respond to the signals

that regulate the growth of most cells They divide uncontrollably forming tumors Some fail to respond to internal regulator proteins Some fail to respond to external regulator

proteins Causes of cancer can be: smoking, radiation

exposure, and even some viral infections

The Cell Cycle & Cancer

Cancer is a disease of the cell cycle Systems that control the cell cycle do not

work in cancer cells As a result, cancer cells divide excessively

and can invade other body tissues The excessive growth can result in an

abnormal mass called a tumor

Tumors

Benign tumor-abnormal mass of normal cells; can cause problems if they grow in an organ; remain in their original site; can usually be completely removed by surgery

Malignant tumor-cancerous; capable of spreading to other body areas; displaces normal tissue; can form new tumors by breaking apart and spreading through the circulatory system (called metastasis)

Types of Cancers

Carcinomas-originate in the external or internal coverings of the body (skin)

Sarcomas-originate in tissues that support the body (bone and muscle)

Leukemias and Lymphomas-cancers of blood forming tissues (bone marrow, spleen, and lymph nodes)

Treatments for Cancer

Radiation- parts of the body with cancerous tumors are exposed to high energy radiation, which disrupts cell division (damage to ovaries or testes may cause sterility)

Chemotherapy-administration of drugs that disrupt cell division; some interfere with the formation or functioning of the spindle

Meiosis

Essential for the formation of gametes (ova and sperm) that will be used for sexual reproduction

Occurs in two consecutive cell divisions Reduces the chromosome number by ½ (for

humans, this means creating gametes that have 23 chromosomes)

Meiosis Vocabulary

Chromosomes – the number in each cell is cut in half and are different from each other and the parent cell.

Gametes – are made during meiosis and are specialized cells involved in sexual reproduction.

Tetrad – a structure containing 4 chromatids. (key to Meiosis) Pg. 276

Crossing-over – the exchange of portions chromatids during the pairing up of chromosomes. Pg. 276

Meiosis Vocabulary

Chromosome Number P. 275 Homologous – chromosomes have a

corresponding chromosome from the opposite-sex parent. Ex. Adult fruit fly has 8 chromosomes- 4 from male parent & 4 from female parent

Diploid - “two sets”; a cell that contains both sets of homologous chromosomes & genes. 2(N) Ex. Fruit fly has a diploid number of 8 so 2N=8.

Haploid – “one set”; a cell that contains one set of chromosomes & genes. (N) Ex. N =4

Human Chromosomes

Occur in pairs (there are 23 pairs) in somatic cells (typical body cells); these are called diploid cells (di=2)

Occur unpaired (a single set of 23) in the gametes; these are called haploid cells

The paired chromosomes in somatic cells are called homologous chromosomes, because they have genes for the same traits

Human Chromosomes

22 of the chromosome pairs are called autosomes

The 23rd pair consists of the sex chromosomes

Human females have a pair of sex chromosomes called X chromosomes (XX)

Human males have one X chromosome and one Y chromosome (XY)

Inheritance of Human Chromosomes One chromosome from each pair is inherited

from our mothers, and the other chromosome in the pair is inherited from our fathers

At the time of fertilization, the two haploid gametes (sperm and ovum) unite to form a diploid cell called the zygote

Fertilization results in the formation of a diploid cell, thus restoring the normal diploid number of human chromosomes

Meiosis occurs in reproductive organs

Meiosis I

DNA that is in special cells in the reproductive organs is copied.

The cells divide and 2 cells are formed which are different from each other and the parent cell.

Meiosis I Phases Pg. 276

Interphase I – DNA replication, forming duplicated chromosomes.

Prophase I – Each chromosome pairs with its corresponding homologous chromosome to form a tetrad.

Metaphase I – Spindle fibers attach to the chromosomes.

Anaphase I – Fibers pull the homologous chromosomes toward opposite ends of the cell.

Telophase I & Cytokinesis – Nuclear membranes form and the cell separates into 2 cells.

Meiosis II

Cells divide again, but DNA is not copied first. 4 daughter cells are produced. Each cell contains half of the number of

chromosomes as the original parent cell.

Meiosis II

Prophase II – Meiosis I results in 2 (N) haploid daughter cells, each with half the number of chromosomes as the original cell.

Metaphase II – Chromosomes line up in a similar way to metaphase stage in mitosis.

Anaphase II – sister chromatids separate & move toward opposite ends of the cell.

Telophase II & Cytokinesis – Meiosis II results in 4 (N) haploid daughter cells.

Comparing Mitosis & Meiosis

Mitosis - 2 cells that are exactly alike & like parent.

Meiosis – 4 cell with each having only half the # of chromosomes as the parent cell and they are genetically different form one another.