THE CELL CYCLE Chapters 6

Do organisms grow?

How do organisms grow? Generally, do – (1) individual cells keep getting larger and larger? or (2) cells divide to form new cells?

Why don’t individual cells keep growing larger and larger? Cells are limited in size because they get their nutrients by diffusion. If cells become too large, nutrients such as oxygen and glucose (which are necessary for aerobic cell respiration) will not be able to diffuse quickly enough into the cell in order for it to survive.

Another way to look at it … Surface Area = x 2 (the area of the surface of the cell) Volume = x 3 (the volume of cytoplasm) So, if a cell gets larger, what happens to the surface area/volume ratio? If x = 1, SA/V = If x = 2, SA/V = If x = 3, SA/V =

The math way, continued If x = 1, then the surface area/volume ratio (SA/V) is x 2 /x3 = ______________ If x = 2, then the SA/V ratio is _____________ If x = 3, then the SA/V ratio is _____________ As the cell increases in size, the SA/V ratio __________________________. Cells need a large SA/V ratio in order to insure that all parts of the cell can get their nutrients by ____________.

Can a cell simply split in two to form two new cells? No.

Why not? Remember the 4 parts of every cell? What part is the “brain”? Each cell in an organism needs exactly the same DNA in every cell.

Let’s start with some definitions….

Chromatin and Chromosome Remember: Chromatin is long and thin. It is uncoiled DNA. (DNA still keeps it’s double helix shape, however. A chromosome is coiled DNA. The DNA coils around proteins.

Chromatin and Chromosome

Gene A section of DNA that contains the code to make a protein.

Somatic Cells versus Gametes Somatic cell = body cell. Each somatic cell contains 46 chromosomes It contains the diploid number of chromosomes (having 2 sets of chromosomes in the cell) Often written as 2n Gamete = reproductive (sex) cell. Each gamete contains 23 chromosomes. It is haploid (having 1 set of chromosome in the cell). Often written as n

Karyotype ( a chart of chromosomes) showing homologous chromosomes - female

Homologous chromosomes are similar in size, shape and genetic content. Autosomes are pairs The chromosomes of pair 23 are sex chromosomes and determine a person’s gender.

Diploid and Haploid Cells Two haploid cells merge to form one diploid cell. This process is called fertilization. Male gametes are called sperm; female gametes are called eggs Two sex cells (sperm and egg) merge to form a zygote.

Zygotes and Specialization Once a zygote is formed, it undergoes cell division many, many times to form a multicellular organism. The cells specialize to perform specific functions for the organism.

Mitosis Mitosis: the process in cell division in which the nucleus of a cell is divided into two nuclei. Each nucleus has the same number of chromosomes as the original cell.

Prokaryotic Cell Division Prokaryotes (bacteria) have only one chromosome. It is circular in shape and is attached to the cell membrane. When a prokaryote divides, the cell makes a copy of its DNA, then the cell divides. Each cell has one copy of the chromosome. Prokaryotic cell division is called binary fission. Prokaryotes do not go through mitosis, nor do they have gametes or zygotes.

The Cell Cycle Cell Cycle: a repeating sequence of growth and division during the life of a cell. The cell cycle has three parts: 1. Interphase: the part of the cell cycle in which an organism grows and carries out its metabolism Before it is about to divide, the cell will copy its DNA.

The Cell Cycle, Parts 2 and 3 2. Mitosis: Division of the nucleus. There are 4 phases of mitosis 3. Cytokinesis: the splitting of the cytoplasm of the cell.

DNA copies itself at the end of interphase (before mitosis begins)

Early Prophase

Chromatin coils or condenses into visible chromosomes Nuclear membrane begins to break down Centrioles move toward opposite poles of the cell and form the spindle

Late Prophase

Spindle has formed Nuclear membrane has broken down Chromosomes attach to the spindle

Metaphase

Spindle fibers move the chromosomes to the “equator” of the cell so the chromosomes are lined up along the middle of the spindle.

Anaphase

Centromeres split Spindle fibers shorten and pull the chromatids apart toward opposite poles of the cell.

Anaphase

Telophase

Telophase in an animal cell

Telophase Chromatids reach the poles and uncoil Nuclear membranes reform Spindle fibers break down Furrow forms in animal cells while cell plate forms in plant cells.

Cytokinesis comes after mitosis Cytokinesis means cell splitting Final separation of the two daughter cells

Cell Cycle in Animal Cells

Prophase

Metaphase

Anaphase

Telophase

Cytokinesis

INTERPHASE

EARLY PROPHASE

LATE PROPHASE

METAPHASE

EARLY ANAPHASE

LATE ANAPHASE

TELOPHASE

CYTOKINESIS

Cell Cycle - Normal vs. Cancerous Cells Normal Chicken Stomach Cells: Interphase – 120 min Prophase – 60 min Metaphase – 10 min Anaphase – 3 min Telophase – 12 min Cancerous Chicken Stomach Cells: Interphase – 16 min Prophase – 15 min Metaphase – 2 min Anaphase – 1 min Telophase – 3 min

Cancer – the enemy! Cancer – the uncontrolled dividing of cells Video animation: Cancer

Types of Tumors Benign tumor – slow-growing or has stopped growing and does not spread to other tissues Malignant tumor– is actively growing and tends to spread to other tissues Metastasis – the spreading of cancer cells to other parts of the body Types of Tumors