Cell Division

Structure of chromosomes

Our genetic material or DNA (deoxyribonucleic acid) is packaged in our chromosomes. The DNA holds a very specialized code that gives our body instructions about how to grow, function and develop. These instructions are organized into sections called genes.

Most of the human body’s sells have 23 pairs of chromosomes so 46 in total. One chromosome is inherited from our mother while its counterpart comes from our father. Because of this, no two people are exactly alike (except for identical twins). Scientists are able to study chromosomes and can organize them by their size and shading and this is called a karyotype.

There are twenty two autosomes in a karyotype ranging from numbers 1 to 22. Number 1 is the largest and twenty two is the smallest of the autosomes. The twenty third set is called a sex chromosome and this is the one that determines whether a person will be male or female. One X and one Y chromosome will make a male while two X chromosomes makes a female.

Essentially, there are three parts to a chromosome; the shorter topmost arm is called the P arm. You can remember this easily because P stands for petit (meaning small) and the longer lower arm is referred to as the Q arm because it is the letter that follows P in the alphabet. The middle part where the two arms connect is called the centromere.

Mitosis, Meiosis and Cytokinesis in Plants and Animals.

As we grow, our bodies need to rejuvenate or remake cells. When the cells divide to remake the dying cells it is known as mitosis. When this takes place, the new cells have the exact same amount of chromosomes and genetic information.

There are seven phases to mitosis; interphase, prophase, prometaphase, metaphase, anaphase, telophase and cytokinesis. Then the cycle begins anew.

In the interphase, which is the longest period of the cycle, DNA is replicated, the centrolies are divided and proteins are made.

During the prophase, chromatin (new DNA and proteins) condenses and turns into chromosomes. In this stage, the mitotic spindle grows from the area of the centrosomes.

In the prometaphase, the nucleus can no longer be recognized as the nuclear envelope has broken down. Some of the mitotic spindle fibers become longer and fuse with the kinetochores and proteins gather in the centromere region.

The metaphase is when the spindle fibers create tension which makes all the chromosomes line up on one plane in the middle of the cell.

Durring the anaphase, the chromatids pull apart and migrate to the ends of the cell while the spindle fibers shorten.

The telophase marks the time in the cycle where the daughter chromosomes complete their migration and now rest at the poles of the cell. In this phase the spindle fibers disappear.

The final phase, cytokinesis, is when the last of the spindle fibers that aren’t fused to the chromosomes start to break down and the microtubules rearrange themselves so that the interphase can begin anew.

Meiosis is a different from mitosis in that it makes sure that sexual reproduction is genetically diverse. First, fertilization takes place when a female’s egg is combined with a male’s sperm. Both of these things; egg and sperm, are referred to as gametes meaning that they only include half of the genetic information that is needed to make a person. After the two gametes fuse together it is called a zygote. Meiosis is the part of the process where a diploid cell converts to a haploid (half genetic set) gamete so that genetic information will never be identical. This is important so that offspring can be unique.




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