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.  We have already studied how Somatic Cells make new cells through the Cell Cycle and Mitosis.  Sex Cells (Gametes) need to produce cells that have only half of the genetic instructions, so that when an egg is fertilized, the full compliment of chromosomes is once again present.  If the cell doesn't reduce the number of chromosomes, the resulting Zygote (fertilized egg) will have double the chromosomal number of the parents.


We will limit our investigation to cells - cells with a nucleus.  

Eukaryotic cells can be divided into two types - 

 

Sex Cells & Meiosis

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).

Somatic Cells   

 

 

 

 


Gamete Cells   

 

 

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

 

  • Diploid Human cells (SOMATIC) have 2(n) = 23 PAIRS of chromosomes (46 chromosomes).

  • Haploid Human cells (GAMETE) have 1(n) = 23 SINGLE chromosomes.

So let's get to it:

We have reviewed the Cell Cycle in a previous section, and we know the 2 stages (+ 4 steps of Mitosis) that the cell goes through to divide into two Somatic Cells (Interphase, Prophase, Metaphase, Anaphase, Telophase & Cytokinesis).  The diagram above compares the steps of Mitosis (the top diagram) with the steps of Meiosis (the lower diagram).  We can see that Meiosis 1 goes through the same steps as Mitosis with few differences, and then proceeds through another cell division without duplicating the DNA.  This second splitting of the cells causes the number of chromosomes to be reduced from 2(n) to 1(n) in the final cells produced. 


 

Meiosis follows similar steps as mitosis, with a few major differences.  

  • The cell starts with Interphase 1.  

  • Interphase 1

    • Interphase 1 starts with a resting period - Gap 1, during which the cell grows to its full size, and stores energy to prepare to replicate the DNA.  

    • Gap 1 is followed by DNA Synthesis, when the genetic material is replicated.  At the end of synthesis, the cell contains TWO IDENTICAL COPIES of each chromosome, joined with a centromere. 

    • DNA Synthesis is followed by Gap 2, when the cell stores up energy to complete all of the stages of Meiosis.

Meiosis:

  • Prophase 1 in Meiosis is similar to Prophase in Mitosis, in that the DNA coils tightly around the Histone proteins to form Chromosomes, and the Nuclear Membrane will start to dissolve, as the Centrosomes start to migrate to the poles of the cell.  

    • There is a major difference between Mitosis and Meiosis that happens here:  The duplicated homologous chromosomes join, forming a structure that consists of all copies of a specific chromosome, both maternal and paternal, which is visible under a light microscope as a Tetrad. 

    • The Tetrad allows the chromosomes to exchange genes, and crossing-over (a physical exchange of homologus genes of chromosoms) occurs. Crossing-over is a process that causes genetic recombination. At this point, each homologous chromosome pair is visible as a tetrad, a tight grouping of the homologus chromosomes, each consisting of two sister chromatids. 

  • Metaphase 1 proceeds as in Mitosis, with the chromosomes aligning on the cells equator, and spindle fibers attaching to the Centromeres.

  • Anaphase 1 continues the process of separating the chromosomes. 

    • The chromosomes can align to either side of the cell randomly - this is the concept of "Independent Assortment".  The maternal chromosomes and paternal chromosomes are randomly assigned together into the germ cells.

  • Telophase 1 has the chromosomes uncoiling, and the nuclear membranes reforming as the cell splits into two separate cells. It should be noted that the cells are no longer identical due to the crossing over that took place back in Prophase 1, and the segregation of chromosomes in Anaphase 1.

  • Cytokinesis finalizes the separation of the cells, but not Meiosis.

Meiosis continues directly from this point into Prophase again, skipping interphase.

  • There is no need for another Interphase during Meiosis II, as DNA Syntheses is unnecessary. 

  • Prophase 2 in Meiosis continues with the DNA coiling tightly around the Histone proteins to form Chromosomes.  The nuclear membrane breaks down, and the centrosomes migrate to the poles of the cell.  

  • Metaphase 2 has the chromosomes lining up on the cellular equator, connected to spindle fibers witch are also connected at the poles of the cell with the centrosomes. 

  • Anaphase 2 separates the chromosomes, but this time there are only 46 chromosomes to separate.  This is the DIPLOID number, because the cell did not replicate the chromosomes in before entering Prophase 2.  As the Diploid chromosomes are split, we will end up with cells that are HAPLOID.

  • Telophase 2 has the Chromosomes uncoiling, and the Nuclear membranes reforming.

  • The final Cytokinesis results in the formation of a total four gametes - either sperm or egg depending on the sex of the individual.  These cells are Haploid because they do not contain chromosome PAIRS, but only individual chromosomes.

Gametes combine during fertilization to generate the needed diploid (2n) number of chromosomes.

Let's start with a video:

Meiosis:

Independant Assortment Video:

Crossing Over

A Diploid (2n) cell has a pair of paternal and maternal chromosomes (homologous chromosomes). Each chromosome is doubled by DNA replication and become two sister chromatids. During the somatic cell division process, each homologous chromosome moves independently, and two sister chromatids of the single chromosome are divided into two cells during cell division.

In meiosis, on the other hand, the homologous chromosomes form pairs. (This pairing also occurs between sex chromosomes (in humans, X and Y), andgenetic crossover takes place between paternal and maternal homologous chromosomes.) The diagram to the left shows formation of multiple crossover points between homologous chromosomes and chromatids; chromosome transfer takes place at crossover points.  This genetic recombination changes gene combinations. This crossing over process occurs randomly between homologous

 chromosomes and chromatids; chromosome transfer takes place at crossover points.  This genetic recombination changes gene combinations. This crossing over process occurs randomly between homologous chromosomes, and involves the creation of unique chromosomes with a mixture of paternal and maternal genes. The point at which paternal and maternal chromosomes cross and attach is called chiasma, and these paired chromosomes are lined up in the center of a pair of mitotic spindles - the chromosome segregation apparatus. The chromosomes are pulled apart and distributed to two cells (representing the first division). The second division then occurs, in which the sister chromatids that constitute the homologous chromosomes are segregated, and each is distributed to one cell.

The microtubules that constitute the mitotic spindle bind to chromosomes with their kinetochores, pushing and pulling them. The directions of kinetochores are different in somatic cell division and in meiosis. 

 

In somatic cell division (where paired chromatids are carried in opposite directions), kinetochores are positioned facing opposite directions, and in meiosis I (in which paired chromatids are carried in the same direction), kinetochores are positioned facing the same way - Reference diagram to the right.

Independant Assortment:

The Principle of Independent Assortment describes how different genes independently separate from one another when reproductive cells develop. 

 

 

We know that independent assortment of genes occurs during meiosis in eukaryotes (cells with a nucleus). Remember, Meiosis is the type of cell division that reduces the number of chromosomes in a parent cell by half to produce four reproductive cells called gametes. 

 

In humans, diploid cells contain 46 chromosomes, with 23 chromosomes inherited from the mother and a second similar set of 23 chromosomes inherited from the father. Pairs of similar chromosomes are called homologous chromosomes. During meiosis, the pairs

 

of homologous chromosome are divided in half to form haploid cells, and this separation, or assortment, of homologous chromosomes is random. This means that all of the maternal chromosomes will not be separated into one cell, while the all paternal chromosomes are separated into another. Instead, after meiosis occurs, each haploid cell contains a mixture of genes from the organism's mother and father.

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).