Chapter 8: Cell Division and Mitosis - Study Guide
Required Reading: Introduction, 8.1, 8.2, 8.3, 8.4
Learning Objectives for Chapter 8:
I. Introduction
Definitions: Cell division, Reproduction
Key Concepts: Parents must provide DNA information to their offspring for the next generation to survive and grow. Fertilized eggs that contain the required DNA information must undergo cell division to develop into adult, multi-celled organisms. Cell division, either of a fertilized egg or existing body cells, for growth and development of an organism is the subject of this chapter.
II. Section 8.1
Definitions: Mitosis, Meiosis, Somatic cells, Germ cells, Chromosome, Sister chromatids, Centromere, Chromosome number, Diploid, Haploid.
Key Concepts: The chromosomes within the nucleus of a fertilized egg contain all the information required for a multi-celled organism to develop to adulthood. The process by which cells divide to create additional, identical cells includes a step where the contents of the nucleus are divided in two - a process called mitosis. “Normal” cells (like skin cells, liver cells, muscle cells, etc.), called somatic cells (which means “body cells”), divide via mitosis. There is another type of nuclear division used only for sex cells (germ cells, which produce egg or sperm in humans) – called meiosis. Mitosis is the focus of chapter 8, while meiosis is the focus of chapter 9. Mitosis and meiosis are both carried out onlu by eukaryotes – organisms whose cells contain a nucleus. The DNA enclosed in the nucleus is found in long strands called chromosomes. During most of a cell’s life, the chromosomes are uncoiled so that the DNA information can be easily read and copied. However, during cell division, the DNA gets coiled up into a condensed form, that can take on an X-like appearance. The point where an X-shaped chromosome crosses is called the centromere of the chromosome, and each half of the X is called a sister chromatid. The condensed form is created by coiling-up the DNA, just like you can coil-up a piece of string into a ball by twisting the string many times over. Each coiled-up sister chromatid contains exactly the same information as the other, so that at the end of mitosis, the 2 resulting cells are exact copies of each other. The total number of chromosomes (coiled or uncoiled) in a cell is called the cell's chromosome number. Human somatic cells have a total of 46 chromosomes, and therefore have a chromosome number of 46. Human cells also have 2 copies of each of the 23 types of chromosome (again, resulting in a chromosome number of 46). These duplicate copies of chromosomes come in handy for avoiding genetic disorders (if one piece of DNA doesn’t work, the other is there to be used). We get one copy of each chromosome from each parent (one copy from our mom, the other from our dad). This makes our cells diploid – which means we have 2 copies of each chromosome in our cells. If the number of chromosomes is ever reduced to one copy of each chromosome per cell, the word haploid is used to describe those cells.
III. Section 8.2
Definitions: Interphase, Prophase, Metaphase, Anaphase, Telophase, Cytokinesis, Spindle apparatus.
Key concepts: The division of eukaryotic cells happens in a step-wise fashion that is consistent for almost all eukaryotic cells. Why do our cells divide? Usually to help an organism grow or to replace cells when they die. The process of growth or self-replacement is programmed into the normal life cycle of somatic cells. Somatic cells live a normal, productive existence during the interphase of their lives, where they carry out whatever function they are supposed to carry out (i.e. liver cells do liver cell things, skin cells do skin cell things, etc.). Also during interphase, the cell prepares itself for division. What would a cell need to do to divide? If one cell is going to divide into two cells, the original (parent) cell needs to make 2 copies of everything it has, including its DNA (making sister chromatids), to be able to give a copy to each of its daughter cells (the 2 cells that result from division). This duplicating process occurs during the “S” phase of interphase (“S” stands for synthesis). Once everything has been copied, then the cell is ready for division, and mitosis begins. Mitosis is the process by which only the nucleus of a somatic cell divides into 2 daughter nuclei. Mitosis includes 4 steps, called prophase, metaphase, anaphase and telophase, that result in the nucleus dividing into 2 daughter nuclei. During mitosis, the DNA copies made in S phase of interphase are pulled apart from one another by spindle fibers (strings of microtubles that attach to the chromosomes and pull the sister chromatids apart from one another), and packaged into 2 new nuclear membranes. Finally, one last step is required after mitosis, called cytokinesis, during which the rest of the cell (all the components in the cytoplasm) divide in two. The end result of cell division via mitosis is two separate daughter cells with identical DNA.
IV. Section 8.3
Definitions: Centrioles.
Key concepts: Note: You will want to be able to draw or identify steps in a diagram like Figure 8.7 on a test, and explain what is happening in each step. During interphase, DNA is duplicated during the “S” phase in preparation for division. Since DNA is being copied, the DNA must be uncoiled for easy of reading and copying. But, uncoiled DNA is very hard to separate – it would be like trying to separate a knotted wad of yarn. So, the first part of mitosis involves the winding up of DNA into condensed chromosomes – a step known as prophase. Also during prophase, the nuclear membrane breaks apart. Imagine trying to divide a bag of candy between 2 children – you must open the bag first before you can evenly divide the contents. For this same reason, the nuclear membrane disappears during prophase. Once the DNA is coiled and the nuclear membrane dissolved, metaphase begins. During metaphase, spindle fibers radiating out of the centrioles (in animal cells) connect to the centromeres of each duplicated chromosome, and line up the chromosomes into the center of the cell. An analogy to lining up the chromosomes during metaphase is lining up children to pick kick-ball teams – half the children will go to one team and half to the other team. Once the chromosomes are lined up, anaphase occurs, when the duplicated X-shaped chromosomes get pulled in half. Each “side” of the cell gets half of the “X.” Finally, during telophase, a new membrane is formed around each new set of chromosomes (so, for a brief moment in time, the dividing cell has 2 nuclei, called daughter nuclei). Once telophase is complete, mitosis is complete.
V. Section 8.4
Definitions: Cytokinesis (called cytoplasmic division in your book), Cleavage furrow.
Key concepts: There is one last step required in somatic cell division, which is cytokinesis (called cytoplasmic division in your book). Cytokinesis is the division of the rest of the cell into 2 daughter cells – each daughter cell getting one of the 2 daughter nuclei. Cytokinesis occurs when the cell membrane tightens around the middle of a cell that has undergone nuclear division, and pinches off forming two separate cells (kind of like dividing a ball of bread dough in two). The indentation made from the tightening membrane is called the cleavage furrow.
Suggested Additional Study Resources:
From Textbook:
Review Questions: 1, 2, 6
Self quiz: 1, 2, 3, 4, 5, 6, 7, 8
From Interactive concepts in Biology CD-ROM:
Chapter 8 Quiz Questions (Under “Learning Tools”): 1, 2, 4, 5, 6, 7, 8, 9, 10
Unit 2 (From Main Menu), Chapter 8:
Bridging the Generations: Reading, and Activites 1, 2 & 3
The Cell Cycle: Reading, Activity 1
Stages of Mitosis: Reading, Activities 1 & 3, and Movie 2 (highly recommended)
Cytoplasmic Division: Reading, Activity 1 and Movie 2