Chromosomes

A chromosome is a long strand of DNA. DNA is the molecule that carries the instructions for every protein made in a cell. During much of the life of the cell, the chromosomes exist in the nucleus as long tangled threads that cannot be distinguished even with a microscope. From there, the DNA directs all of the activities of the cell. 

a micrograph of a chrmosome

New cells can be formed only by the division of existing cells. Just before the cell divides, the DNA of each chromosome replicates (makes a copy of itself.) Following this, the chromosomes coil up tightly which allows them to be sorted and moved to the new cells without tangling and breaking apart.  This sorting and moving process occurs before the cell divides and is called mitosis.

During this period of cell division, chromosomes appear as dense, bulky objects when the cell is viewed through a microscope. At very high  magnifications, they have the shape of a fuzzy, bulky X.  Each half of the X comprises one chromatid—an exact copy of the original chromosome.  The two chromatids (often referred to as "sister chromatids") are joined  together at a specific small region called the centromere.

Using a microscope, it is possible to count and characterize the individual chromosomes during the time they are coiled and condensed.  A photograph of the entire set of chromosomes can  be made. Then the images of the individual chromosome can be cut out and arranged by shape and size in an orderly arrangement called a karyotype (see below).  (This is a nightmare project to the untrained eye, a simulation of which is often assigned to 10th grade students as a "learning exercise"!) 

In most higher plants and animals, including humans, chromosomes from the body cells can be matched up in pairs.  The two chromosomes of a pair are called homologous chromosomes.  The members of most homologous pairs of chromosomes look alike.  They are the same length, their centromeres are in the same position, they show the same pattern of light and dark bands when stained. Genes for the same inherited characteristics are lined up on each homologous chromosome in the same order.   

unsorted human karyotype

Unsorted human karyotype

sorted human male karyotype

Sorted human male karyotype

sorted human female karyotype

Sorted human female karyotype

unsorted human karyotype

Unsorted human karyotype

sorted human male karyotype

Sorted human male karyotype

sorted human female karyotype

Sorted human female karyotype

The occurrence of paired chromosomes in our karyotype is a result of our sexual origins.   We inherit one member of each chromosome pair from each parent.  So the 46 chromosomes in our somatic cells are actually two sets of 23 chromosomes—a maternal set (from our mother) and a paternal set (from our father.) A cell with two of each kind of chromosome is called a diploid cell and is said to contain a diploid, or 2n, number of chromosomes. 

In humans, the homologous pairs are defined and numbered and carry the genes for the same trait in each person.  For example, human chromosome #1 contains, along with many others, the genes for the Rh blood protein and for a starch-digesting enzyme in the saliva.  However, the corresponding genes on the two homologous chromosomes are not necessarily identical. For instance, some chromosomes have a gene for the protein that makes a person's blood type Rh-positive, and some have a gene coding for a different version of this protein (Rh-negative) at the Rh location.  Different versions of the same gene are referred to as alleles.  An individual with two genes that are the same for a trait is said to be homozygous for that trait.  A person with two different alleles for the same trait is heterozygous for that trait.

In human males, the partners of 22 of the pairs of chromosomes look similar, but the twenty-third pair in males is mismatched with two unlike chromosomes, called X and Y (see the far right chromosome pair in the bottom row of the male karyotype above.)  In the cells of a female, both members of homologous pair #23 are X chromosomes (far right pair of chromosomes in the bottom row in the female karyotype.) The X and Y chromosomes are called the sex chromosomes because they differ between the sexes and because they carry the genes that determine the sex of the individual.  The other 22 chromosomes are called autosomal chromosmes or simply autosomes.

Where Can I Go From Here?

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©️2002 - 2017 Context.info

Where Can I Go From Here?

©️2002 - 2017 Context.info

Contexo.info is a not for profit, educational website.