Ch 10- Cell Growth

• What problems does growth cause for the cell?– Larger a cell becomes, the more demands that cell places on

its DNA– The cell has more trouble moving enough nutrients and

wasted across the cell membrane• Where is the information that tells cells what to do

stored at?– DNA

• Understanding the relationship between a cell’s volume and surface are is key to understanding why cells must divide as they grow

Division of the Cell

• If cell got to large, it would be more difficult to get sufficient amounts of oxygen and nutrients in and waste products out

• Cell division- process by which a cell divides into two new daughter cells– Copies DNA, each daughter cell gets one complete set of

genetic information- solves the problem of information storage

– Solves the problem of increasing size by reducing cell volume- each daughter cell has an increased ratio of surface area to volume

Sec 2- Cell Division

• Prokaryotes- process of cell division is simply separating the contents of the cell into two parts

• Eukaryotes- process is more complex and occurs in 2 stages

• Mitosis- division of cell nucleus– Asexual, cells produced are identical to parent cell– Source of new cells as multicellular organism grows and

develops– Begins shortly after egg is fertilized

• Cytokinesis- division of cytoplasm

Chromosomes

• What are they?– Carries the genetic information passed from one generation to

the next, made of DNA and proteins• How many chromosomes do human cells have?

– 46• Fruit flies have 8 chromosomes• Not visible except during cell division• Before cell division, chromosomes are replicated• Each chromosomes consist of 2 identical “sister”

chromatids- when cell divides one chromatid goes to each of two new cells

• Centromeres- area where chromatids are attached, usually located near middle

Cell Cycle

• During the cell cycle, a cell grows, prepares for division, and divides to form two daughter cells, each of which then begins the cycle again

• Interphase- the in between time between cell division• Cell cycle- series of events that cells go through as

they grow and divide• Cell cycle consists of 4 phases– Mitosis and cytokinesis take place during M phase– Chromosome replication takes place during S phase– G phases are periods of intense growth and activity

Events of the Cell Cycle

• Interphase- phase is quite long, made up of G₁, S, and G₂ phases

• G₁ phase- where cells do most of their growing– Cells increase in size and synthesize new proteins and

organelles• S phase- chromosomes are replicated and synthesis of

DNA molecules takes place• G₂ phase- many of organelles and molecules required for

cell division are produced• Cell is ready for the M phase and begin process of cell

division

Mitosis

• Divided into 4 phases- prophase, metaphase, anaphase, and telophase

• May last anywhere from few minutes to several days- depends on type of cell

• Prophase- chromatin condenses into chromosomes, centrioles separate and a spindle begins to form, nuclear envelope breaks down– Centrioles- two tiny structures located in cytoplasm near nuclear

envelope, separate and take position on opposite sides of nucleus– Spindle- fan-like microtubule structure that helps separate the

chromosomes

• Metaphase- chromosomes line up across the center of cell, each chromosome is connected to a spindle fiber at its centromere

• Anaphase- sister chromatids separate into individual chromosomes and are moved apart

• Telophase- chromosomes gather at opposite ends of cell and lose their distinct shapes, two new nuclear envelopes will form

• Mitosis is complete

Cytokinesis

• Result of mitosis= two nuclei each with duplicate set of chromosomes formed within cytoplasm of a single cell

• Cytokinesis- division of cytoplasm, usually occurs at same time of telophase– Cytoplasm pinches in half, each daughter cell has

an identical set of duplicate chromosomes• Mitosis and Cytokinesis

Sec 3- Regulating the Cell Cycle

• Cyclin- proteins that regulate the timing of the cell cycle in eukaryotic cells

• Discovered by Tim Hunt and Mark Kirschner• Many more proteins have been discovered since- 2 types of

proteins- regulatory proteins in and outside of cell• Internal Regulators- allow cell cycle to proceed only when

certain processes have happened inside cell– Make sure cell does not enter mitosis until all chromosomes have

been replicated• External Regulators- direct cells to speed up or slow down cell

cycle– Growth factors among most important external regulators

Uncontrolled Cell Growth

• Why is cell growth regulated so carefully?• Cancer- disorder in which some of body’s own cells

lose the ability to control growth– Do not respond to the signals that regulate the growth of

most cells– Divide uncontrollably and form masses of cells called

tumors– May break free from tumor and spread to rest of body

• What causes the loss of growth control that characterize cancer?– Smoking tobacco, radiation exposure, viral infection