Copendium Review # 2

For Chapters 18-21:

--Cell division: Mitosis

Cell division is an elegant process that enables organisms to grow and reproduce. Through a sequence of steps, the replicated genetic material in a parent cell is equally distributed to two daughter cells. While there are some subtle differences, mitosis is remarkably similar across organisms. Before a dividing cell enters mitosis, it undergoes a period of growth called interphase. Interphase is the "holding" stage or the stage between two successive cell divisions. In this stage, the cell replicates its genetic material and organelles in preparation for division. Mitosis is composed of several stages:

Prophase

-In prophase, the chromatin condenses into discrete chromosomes. The nuclear envelope breaks down and spindles form at opposite "poles" of the cell.

Metaphase

-In metaphase, the chromosomes are aligned at the metaphase plate.

Anaphase

-In anaphase, the paired chromosomes move to opposite ends of the cell.

Telophase

-In this last stage, the chromosomes are cordoned off in distinct new nuclei in the emerging daughter cells. Cytokinesis is also occurring at this time.At the end of mitosis, two distinct cells with identical genetic material are produced.

--Mitosis vs. Meiosis

Mitosis-

~Produces body cells(Somatic cells)

~Daughter cells diploid(2N)

~Two daughter cells produced

~In metaphase chromosomes line up singley

~One nuclear division

~Produces cells for growth and repair

~Daughter cells have two sets of chromosomes(pairs)

~Daughter cells are genetically identical to the parent cell

~Insures that all daughter cells are genetically identical

Meiosis:

~Produces sex cells(Gametes)

~Daughter cells haploid(N)

~Four daughter cells produced

~In metaphase I chromosomes line up as homologous pairs(synapsis)The two double chromosomes are called a tetrad when they are lined up side-by-side.Crossing over occurs during the formation of the tetrad.

~Two nuclear divisions

~Produces cells for sexual reproduction

~Daughter cells have only one member of each pair of chromosomes

~Daughter cells have one-half of the genes from the parent cell

~Generates genetic diversity through crossing over and ramdom seperation of homologous pairs of chromosomes

--Cancer The cancer cell
Cancer cells are different to normal cells in several ways. These are some features of cancer cells
They carry on reproducing
They don't obey signals from other neighbouring cells
They don't stick together
They don't become specialized, but stay immature
They don't die if they move to another part of the body
Cancer cells don't stop reproducingUnlike normal cells, cancer cells do not stop reproducing after they have doubled 50 or 60 times. This means that a cancer cell will go on and on and on doubling. So one cell becomes 2, then 4, then 8, then 16.... The cancer cells may be able to stop themselves self destructing. Or they may self destruct more slowly than they reproduce, so that their numbers continue to increase. Eventually a tumour is formed that is made up of billions of copies of the original cancerous cell. Scientists describe cancer cells as being 'immortal'.
Cancer cells do not obey signals. Something in the cancer cells overrides the normal signalling systen.This may be because the genes that tell the cell to reproduce keep on and on firing. Or because the genes that normally tell the cell to stop reproducing have been damaged or lost. So the cancer cell keeps on doubling, regardless of the damage the extra cells cause to the part of the body where the cancer is growing.
Cancer cells do not stick together

Cancer cells can lose the molecules on their surface that keep normal cells in the right place. So they can become detached from their neighbours. This partly explains how cancer cells spread to other parts of the body.
Cancer cells do not become specialised, Unlike normal cells, cancer cells do not carry on maturing once they have been made. In fact, the cells in a cancer can become even less mature over time. With all the reproducing, it is not surprising that more of the genetic information in the cell can become lost. So the cells become more and more primitive and tend to reproduce more quickly and even more haphazardly. A doctor will mention about the 'grade' of your cancer. This means how well developed or mature the cell looks under a microscope. The more the cancer cell looks like a normal cell, the more it will behave like one
The more normal a cancer cell looks, the lower its grade
The more abnormal or less well developed a cancer cell is, the higher its grade. A doctor may call this 'differentiation'. Cells can be well differentiated, moderately differentiated, or poorly differentiated. This is the same as low, medium or high grade. Other doctors may talk about grades 1, 2, or 3, where grade 1 is low grade.Although there are many different ways of talking about this, it all comes down to the same thing. A low grade cancer is likely to be less aggressive in its behaviour than a high grade one. Doctors cannot be certain how the cells will behave. But grade is a useful indicator. Grade is one of the factors doctors use to decide on treatment with some types of cancer.

--Genetic Inheritance:

Researchers have identified over 15,000 genetically inherited human traits. More than 5,000 of them are diseases or other abnormalities. As we learn more about the inheritance patterns for these traits, it is becoming clear that at least some of the twelve exceptions to the simple Mendelian rules of inheritance described here are, in fact, relatively common. Genes that follow simple rules of dominance increasingly seem to be rare. It would not be surprising if other "exceptions" to Mendelian genetics were discovered in the future.

Some traits are determined by the combined effect of more than one pair of genes. These are referred to as polygenic , or continuous, traits. An example of this is human stature. The combined size of all of the body parts from head to foot determines the height of an individual. There is an additive effect. The sizes of all of these body parts are, in turn, determined by numerous genes. Human skin, hair, and eye color are also polygenic traits because they are influenced by more than one allele at different loci. The result is the perception of continuous gradation in the expression of these traits.

Apparent blending can occur in the phenotype when there is incomplete dominance resulting in an intermediate expression of a trait in heterozygous individuals. For instance, in primroses, snapdragons, and four-o'clocks, red or white flowers are homozygous while pink ones are heterozygous. The pink flowers result because the single "red" allele is unable to code for the production of enough red pigment to make the petals dark red.

For some traits, two alleles can be codominant. That is to say, both are expressed in heterozygous individuals. An example of this is people who have an AB blood type for the ABO blood system. When they are tested, these individuals actually have the characteristics of both type A and type B blood. Their phenotype is not intermediate between the two.

There are three categories of genes that may have different effects depending on an individual's gender. These are referred to as:
1. sex-limited genes
2.sex-controlled genes
3.genome imprinting
Sex-limited genes are ones that are inherited by both men and women but are normally only expressed in the phenotype of one of them. The heavy male beard is an example. While women have facial hair it is most often very fine and comparatively sparse.

--Sex-Linked Inheritance

alleles on sex chromosomes are inherited in predictable patterns. For example, in Drosophila the locus for eye color is located on the X chromosome. The allele for red eye color, which is normal in wild flies, is dominant to the mutant allele for white eyes.
As females have two chromosomes X (with a locus for eye color), they might be homozygous or heterozygous for either allele.Males, who carry only one X chromosome, are always hemizygous. They carry only the one X chromosome inherited from their mother, and it determines their eye color.

For Exaple-Homozygous red eyed females (RR) mate with hemizygous white eyed males (w-). In the offspring, all the daughters are red eyed heterozygotes (Rw) and all sons are red eyed hemizygotes (R-). In the right hand, homozygous white eyed females (ww) mate with hemizygous red eyed males (R-). In the offspring, all the daughters are red eyed heterozygotes (Rw) and all sons are white eyed hemizygotes (w-).