Biology concepts and applications

Mitosis vs. Meiosis

Mitosis and meiosis are both processes of cell reproduction.

Mitosis is the process by which eukaryotes reproduce asexually and also how multi-celled eukaryotes reproduce cells for bodily growth. Meiosis is the process by which eukaryotes sexually reproduce. Using humans as the examples, new humans are created by meiosis, but within the human, mitosis reproduces cells. Mitosis results in two identical cells, that is the two cells are clones of each other. Meiosis results in four cells that are not identical, the process of meiosis shuffles the genetic information. By describing the two processes, we will see how this occurs.

Mitosis

In mitosis first the chromosome duplicates, creating two sister chromatids, these sister chromatids are identical. The sister chromatids line up in the center of the cells in what is known as a centromere. A spindle then attaches to each chromatid and the cell divides, with one of the sister chromatids in each of the two new cells. At the end of this process, there are two cells identical to the initial cell.

Meiosis

Meiosis begins just as mitosis did with two chromosomes duplicating each other and creating sister chromatids. At the next stage the sister chromatids pair up and condense, with each swapping segments with its partner. It is this vital step that results in a shuffling of genetic information. As in mitosis the sister chromatids are aligned at the center, a spindle attaches to each and the cell splits, with two nonidentical pairs in each new cell. The centrioles now align again in the new cells, spindles attach and the cell splits again. The final result is four cells with non-identical chromosomes in each and each with half the chromosome number. These are the sex or gamete cells, on fertilization two of these gamete cells will fuse giving the full number of chromosomes. This is another way in which genetic information is shuffled.

The diagram below shows the process:

Source: Access Excellence. Mitosis Labeled Diagram. The National Health Museum, 1999.

As can be seen in mitosis two identical cells are produced but in meiosis four cells are produced with varying genetic information.

2. Mating Pairs

A)

A homozygous dominant has a pair of dominant alleles, TT.

A homozygous recessive has a pair of recessive alleles, tt.

The genotypes for the offspring are shown below:

TT

- t Tt

- t Tt

Result is 4/4 Tt genotypes.

Because T. is the dominant allele, the phenotype is 4/4 T.

B)

A heterozygous has a pair of nonidentical alleles, Tt.

The genotypes for the offspring are shown below:

t T

- t tt

- T Tt

Result is 1/4 tt, 2/4 Tt and 1/4 TT genotypes.

Result is ae T. phenotypes (Tt and TT) and 1/4 t phenotype (tt).

C)

A heterosygous has a pair of nonidentical alleles, Tt.

A homozygous recessive has a pair of nonidentical alleles tt.

A heterozygous has a pair of nonidentical alleles, Tt.

The genotypes for the offspring are shown below:

T t

- t Tt

- t tT

Result is 2/4 tt and 2/4 Tt genotypes.

Result is 2/4 T. phenotypes (Tt) and 2/4 t phenotype (tt).

If the trait is X-linked one dose of the X-linked trait will cause the expression of that characteristic, meaning that the phenotype can be t, even when Tt is the genotype.

A)

A homozygous dominant has a pair of dominant alleles, TT.

A homozygous recessive has a pair of recessive alleles, tt.

The genotypes for the offspring are shown below:

TT

- t Tt

- t Tt

Result is 4/4 Tt genotypes.

Because't is an X-linked trait, t will be expressed, so the phenotype is 4/4 t.

B)

A heterozygous has a pair of nonidentical alleles, Tt.

The genotypes for the offspring are shown below:

t T

- t tt

- T Tt

Result is 1/4 tt, 2/4 Tt and 1/4 TT genotypes.

Result is 1/4 T. phenotype (TT) and 3/4 t phenotypes (tt, Tt).

C)

A heterosygous has a pair of nonidentical alleles, Tt.

A homozygous recessive has a pair of nonidentical alleles tt.

A heterozygous has a pair of nonidentical alleles, Tt.

The genotypes for the offspring are shown below:

T t

- t Tt

- t tT

Result is 2/4 tt and 2/4 Tt genotypes.

Result is 4/4 t phenotypes.

3. Hierarchical Plan for Categorizing Living Organisms

Categorizing living organisms is known as taxonomy and there is a hierarchy of classification.

The scheme is based on differences in physical features such as the number of legs, body size, wings or no wings, spinal column or no spinal column. Initially the system was based on defining things by how closely related they were. It should be noted that many classifications in the system do not represent close relations. The system is however, a way of clearly defining one identified organism with another.

From the broadest level down the classification system is:

Kingdom

Phylum

Class

Order

Family

Genus

Species

The largest is the kingdom. There are six kingdoms: Archaebacteria, Eubacteria, Protista, Fungi, Plantae, and Animalia.

The second is the phylum. For animals, the phylums include Porifera (sponges), Arthropoda and Chordata (which includes the spinal chorded humans.) For plants, one of the phylums is Anthophyta (flowering plants.)

The third is the class. Classes include Mammalia, Reptilia and Amphibia. The Mammalian class is defined as: "having the body covered with hair, a four-chambered heart, and possessing mammary glands producing milk with which the female suckles her young." (Lawrence, Eleanor. Dictionary of Biological Terms. Longman Group Limited: Essex. 1996, p330.)