The Nature of the Living State
Macromolecules of Living Systems
Energy Flow in Living Systems
The Instruction Set of Life
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There are two processes used by cells to divide: (1) mitosis (2) meiosis.
Mitosis is used by most cells of the body, called somatic cells, to divide. It is a single division process and yields two daughter cells. Meiosis, however, is used only by reproductive cells which ultimately produce sperm in males or ova (eggs) in females. It is a two division process. In males, four sperm are produced for every cell starting meiosis, but in females, only one mature egg will be produced for every cell starting meiosis. Mitosis: Mitosis is the name applied to the events that occur during M phase of the cell cycle. As above, the dividing cell produces two daughter cells. There are a number of sub-phases involved in mitosis, which in order are called: Prophase Metaphase Anaphase Telophase.
In the non-dividing Interphase cell, we have already learned that the chromosomes are extended so that their DNA can be used by the cells. Because of this, the chromosomes can not be seen in the nucleus. However, as the chromosomes condense by re-coiling during G2, there is a point when they finally become visible in the nucleus as they grow shorter and thicker. This is the beginning of Prophase. During prophase this condensation process continues until the chromosomes reach their maximally condensed state. Also during Prophase, the nuclear membrane is disassembled. The chromosomes are constantly moving around as they condense and all of them line up on a geometric plane called the metaphase plate, in the center of the cell roughly dividing the cell in to equal halves. This is Metaphase. Metaphase is relatively short. Soon after lining up, groups of chromosomes move to opposite sides of the cell (called the poles like in North and South poles) perpendicular to the metaphase plate (which would be the equivalent of the equator). One of each type of chromosome moves to opposite poles. This is Anaphase. The chromosomes ultimately reach the opposite poles where they group together and nuclear membranes re-form around each group of chromosomes. This is Telophase. Shortly thereafter, the chromosomes will start losing their coiling returning back to the Interphase state. In summary then:
Prophase -- first see the chromosomes Metaphase -- chromosomes line up and are maximally condensed Anaphase -- chromosomes move to opposite poles Telophase -- chromosomes re-group
The above are the basic events associated with the chromosomes. In addition, there are other events which take place in the cytoplasm. For instance, during Telophase, the cytoplasm undergoes a pinching process which will ultimately lead to the separating of the cytoplasm forming the two daughter cells. This is known as cytokinesis.
In some cells, notably plant cells, there is also a cytoplasmic structure called a spindle apparatus which forms in the cytoplasm during cell division. In each cell, there is a structure called the centriole which act as an organizing center to produce cytoplasmic fibers called spindle fibers. The centriole is related to cilia and flagella. Usually during Interphase, the centriole duplicates so the cell starts mitosis with two centrioles. During Prophase, the centrioles migrate to the areas of the cell which will becomes the poles. As they move they leave behind spindle fibers made up mostly of microtubules which maintain a connection between the two centrioles. In addition, the centrioles form short fiber projections once they reach the poles called astral rays which seem to anchor them in the cytoplasm. The centrioles with their astral rays and connecting spindle fibers, altogether form the spindle apparatus.
There are specialized areas of chromosomes called centromeres which allow the chromosomes to attach to the spindle fibers. In those cells which form spindle fibers, the chromosomes follow along the spindle fibers almost like guideways to the opposite poles. However, even in those cells which do not form spindles fibers, the chromosomes find their way to their correct positions in ways still not understood.
Chromosome Replication: Each human cell has 46 chromosomes. A problem develops when you think about where the extra chromosomes come from when a cell divides forming the two daughter cells. Each of these cells will require 46 chromosomes apiece.
The answer to this dilemma concerns chromosome replication events. Prior to Interphase, each chromosome has one piece of DNA. But during Interphase, this piece of DNA makes a copy of itself by the process of DNA replication mentioned in the previous key word list. Therefore, we can distinguish two different states that chromosomes can exist in: an un-replicated state with one piece of DNA per chromosome and a replicated state following Interphase with two pieces of DNA per chromosome. In the replicated chromosome, the two pieces of DNA remain connected together at the centromere so it is counted as only one chromosome. The two halves of a replicated chromosome are termed chromatids. They remain together until Anaphase when the centromeric connection breaks, and what were two chromatids, separate and go to opposite poles of the dividing cell. Once separated, they are no longer called chromatids but are now complete, unreplicated chromosomes with one piece of DNA each. So, after Interphase, human cells have 92 chromatids which remain together till Anaphase when they separate forming the two groups of 46 chromosomes which move to each of the daughter cells. Meiosis: Meiosis will be dealt with separately in the next key concept list.