In order to make a new cell, an existing cell has to split from one cell into two cells. This is called (appropriately enough) Cell Division. Cell Division is used to duplicate a cell. Multicellular organisms (living things with more than one cell) increase in size and complexity by making more cells. Cell division is also used to repair damaged parts of the organism and to replace dead cells.
We will investigate the cell cycle of Eukaryote cells - cells with a nucleus.
Eukaryotic cells can be divided into two types -
The Cell Cycle & Mitosis
The general body cells of an organism. Somatic cells contain two copies of each chromosome - one chromosome inherited from the mother, and one chromosome inherited from the father. Somatic cells are diploid (2n) cells, and contain pairs of each chromosome. Somatic Cells are produced by Mitosis. Mitosis is the way that cells produce exact copies of the original cell. (This will be explained later on this web page).
The sex cells of an organism (sperm cells and egg cells). Gamete cells contain one copy of each chromosome (half the number that Somatic cells contain). Gamete cells are haploid (1n) cells and contain single copies (not pairs) of each chromosome. The cell that results when sperm and egg combine will have two copies of each chromosome (1n from sperm + 1n from egg = 2n, or diploid). The full compliment of chromosomes is present only after sperm and egg combine Gamete Cells are produced by Meiosis. Meiosis is the way that cells produce copies of themselves with only half of the genetic material as the original cell. (This will be explained later on a different web page - and we'll learn more about Meiosis later in this marking period).
Chromosomes are paired in the body of each organism. Two copies of the same chromosome are homologes. Humans have 23 pair of chromosomes (half inherited from the father, and half inherited from the mother). This gives a total of 46 chromosomes, or 23 homologous pairs of chromosomes.
So let's get to it:
You could learn all you need to know about the cell cycle by studying the above model, but that would be really difficult. The model is good, but it doesn't provide enough detail to really understand what is happening during the cell sysle. To make it better, we need to add more detail. We'll do that by looking at each major part of the cycle in more detail.
Interphase is the first step in Cell Division (Mitosis).
During Interphase, the cell performs its normal functions.
The cell grows to its optimal size, and produces the proteins that are coded in the DNA for the type of cell it is (Pancreas cells produce Insulin protein, Liver cells produce bile protein, etc). The DNA is in the form of Chromatin; when the DNA is coiled as Chromatin it can be transcribed into m-RNA.
The cell will only be triggered to divide when specific proteins are present in the cell. When this happens, the cell makes a copy of all of its DNA in a process called DNA Replication.
DNA Replication is when the cell makes a duplicate copy of its DNA.
The cell will then have TWO identical copies of DNA in its nucleus. When there are two copis of DNA present, the cell is ready to start dividing.
Let's start with a video:
We know that DNA carries all the information for making the cell's proteins.
Proteins determine the organism's characteristics and traits. Every time a cell reproduces (makes a copy of itself), it has to pass all of this information on to the daughter cells.
Before a cell can reproduce, it must first replicate, or make a copy of, its DNA. This happens during the end of Interphase.
The DNA molecule is double-stranded, with each strand in opposite (anti-parallel) directions. The elegance of this design is that DNA can, like a zipper, 'unzip' down the center of the molecule, and each side can serve as a template for the side to be created. This process is called called semi-conservative replication, since half of each of the new strands is an original version. However, all of the DNA does not unzip at once. A small section of DNA will unzip (in the area called a replication fork), and that section then moves down the entire length of the DNA molecule.
Different types of cells replicated their DNA at different rates. Some cells are constantly dividing, like those in your hair and fingernails and bone marrow cells. Other cells go through several rounds of cell division and stop (including specialized cells, like those in our brains, muscles and heart). Some cells stop dividing, but can be induced to divide to repair injury (such as skin cells and liver cells). In cells that are not constantly dividing, the signal for DNA replication/cell division come in the form of chemicals. These chemicals can come from other parts of the body (hormones) or from the environment.
Chromosomes and Histones
DNA is a negatively charged molecule. the DNA molecule is wrapped around positively charged proteins (Histones), to form the chromatin, and when coiled tighter, the DNA forms chromosomes. The coiling of the DNA molecule helps reinforce it and prevent damage, and to control gene expression. Only the portions of DNA that are being replicated are uncoiled.
The structure of chromatin depends on several factors. The overall structure depends on the stage of the cell cycle. During interphase, the chromatin is structurally loose to allow access to transcribe and replicate the DNA. The local structure of chromatin during interphase depends on the genes present on the DNA: DNA for genes that are actively transcribed ("turned on") are more loosely packaged, while DNA for inactive genes ("turned off") are found associated with structural proteins and are more tightly packaged. As a cell prepares to divide, the chromatin packages more tightly to assist seperation of the chromosomes.
Chromatin is not visible with a microscope in a cell during Interphase. Loosely coiled Chromatin is the normal form that DNA takes while the cell is working normally (not during cell division).
Prophase is the step in Cell division when the Chromatin (lightly coiled DNA) in the cell further condenses into Chromosomes.
DNA in the form of a Chromosome is tightly coiled to prevent damage to the delicate DNA strand. As Chromosomes form, the nuclear membrane will start to break down, and the centrioles will start to migrate to the poles of the cell. The Centrioles are organelles that spindle fibers will connect to later in the cell cycle, and will control the separating of the chromosomes into the proper daughter cells. Centrioles migrate towards the cellular poles during this phase.
Chromosomes are a form DNA takes when it is tightly coiled around proteins to protect it from damage. DNA cannot function (be transcribed into m-RNA) when in this form.
Chromosomes are visible in the nucleus of a cell that is entering Prophase.
In Metaphase, the Chromosomes are brought to the center plate of the cell (sometimes called the cell equator).
Chromosomes line up here and attach to spindle fibers, which will seperate the Chromosomes to opposite poles of the cell. This ensures that each of the two resulting cells (the "daughter" cells) will have one copy of each of the chromosomes - in other words, giving them identical versions of genetic material (DNA).
Sister Chromatids are identical copies of the genetic material. A sister chromatid refers to either of the two identical copies (Chromatids) formed when a single Chromosome is replicated. Both copies of the Sister Chromatids are joined by a Centromere. A full set of sister chromatids is created during Interphase when all the chromosomes in a cell are replicated. The two sister chromatids are separated into two different cells during Mitosis.
Here are some details:
An enzyme called DNA gyrase makes a nick in the double helix and each side separates.
An enzyme called helicase unwinds the double-stranded DNA.
Several small proteins called single strand binding proteins (SSB) temporarily bind to each side and keep them separated.
An enzyme complex called DNA polymerase "walks" down the DNA strands and adds new nucleotides to each strand. The nucleotides pair with the complementary nucleotides on the existing stand (A with T, G with C).
A subunit of the DNA polymerase proofreads the new DNA to ensure it doesn't contain errors.
An enzyme called DNA ligase seals up the fragments into one long continuous strand.
The new copies automatically wind up again.
Watch a short video of DNA Replication:
The Spindle Fibers that formed between the Centrosomes
(at the poles of the cell) and the centromeres
(that join the Sister Chromatids) begin to contract,
pulling the Sister Chromatids apart and towards the ends of the cell.
As the Sister Chromatids are pulled, it will resut in each of the cells being formed to have the same exact genetic information.
The Chromosomes have completed the migration to the far ends of the cell. Nuclear membranes form around each of the two groupings of Chromosomes. The Chromosomes start to uncoil (back into chromatids) as the nuclear membranes reform.
The cell changes shape - and stats to pinch in at the center, forming new cells.
The original cell splits into identical two daughter cells. These cells enter into Interphase where they start performing their jobs and grow to full size, in preparation of undergoing cell division once again.
Comparison Cytokinesis of Animal and Plant Cells
Remembering the stages of Mitosis can be difficult. This song may help you remember the order of Mitosis Stages, and the basics of what happens in each stage.
How do I remember all this?
A Video Representation of the Cell Cycle: