In order to communicate, human beings have developed verbal language. However, we recognized that it was easier and more accurate to pass on important information by writing things down. So, we developed written language and created books to write them in. In order to organize and store our books we invented bookcases. Further on, in order to share information with each other we developed libraries where we could store a large catalog of books which others could access and use.
The body has developed a similar system. Cells in the body must communicate so they have a language that they use. That language is written in ‘books’ called genes. These genes are stacked on ‘book shelves’ called the DNA (deoxyribonucleic acid). These book shelves are part of ‘bookcases’ called the chromosome. The chromosomes are then neatly organized within the nucleus of the cell which you could look at as the library of the cell.
Packaging the DNA in the form of a chromosome is necessary, since it is so long. The DNA is about 6 feet long if you should stretch it out from end to end. If you connected all the ends of all the DNA in your body it would stretch 10 billion miles. That is over 1000 wraps around the earth!
DNA, Chromosomes and Genes
Below is a great image that gives a more realistic illustration of the actual relationship between the nucleus, chromosomes, DNA and genes.
Each human cell contains a nucleus. Each nucleus has 23 pairs of chromosomes. Each chromosome consists of DNA which is made up of sections called genes. The gene is just a segment of the DNA. Hence, along the DNA you will have many genes (1000 to 1100). The information in each gene is written in a language consisting of only 4 letters called bases, i.e., guanine, thymine, adenine and cytosine (G-T-A-C). Notice in the above diagram below that these letters are organized on the DNA in a specific format. That is, A always attaches to T (remember apple under the tree) and C always attaches to G (remember car in the garage). The bases along the rung of the DNA latter is connected by a phosphate deoxyribose backbone.
Genes hold the code to make proteins. Proteins determine the structure and function of each cell, and hence they dictate our physical features, i.e., our phenotype. When the cell wants to make a certain protein, it utilizes the gene that has the code for that specific protein, by “photocopying” it and sending the copy to the cytoplasm of the cell in the form of mRNA (messenger ribonucleic acid). This process is called transcription. Once in the cytoplasm, mRNA gets attached to ribosomes where the genetic code is read and proteins corresponding to the code are made. That process is called translation.
We have 23 pairs of chromosomes in the nucleus. We get one from our mother and one from our father. Therefore we have features of both our parents. All of our chromosomes are referred to as our karyotype. Twenty (22) pairs determine everything about us (hair color, hair texture, size and shape of nose etc.) except our gender. They may also determine intelligence and personality although these psychological traits are also affected by external factors. These 22 chromosomes are called autosomes. The 23rd pair determines characteristics that are gender specific and are therefore called sex chromosomes. The sex chromosomes are either X or Y. If you are a male you have an X and Y chromosome and if you are a female you have two X chromosomes.
Along the pair of chromosomes are specific positions we call loci where you will find genes coding for the same characteristic e.g. the color of your hair. These genes are called alleles. If one allele codes for black hair and the other for blond, then the color of your hair will depend on which allele is dominant. If black hair is dominant and blond hair is recessive, then you will have black hair. In some cases you may have incomplete dominance resulting in the offspring having features that are a blend of the two parents rather than having one feature or the other.
If you have two dominant or two recessive alleles, they are called homozygous alleles. If you have a recessive and a dominant gene, they are called heterozygous alleles. If only one allele is present, then the allele is called a hemizygous allele.
Centromeres and Chromatids
When cells are undergoing division, the chromosomes appear like an X held together at a point known as the centromere. The two sides of the chromosome are called chromatids.
When body cells (called somatic cells) divide, they produce an exact replica of the DNA with all 23 pairs of chromosomes in the daughter cells. This type of cell division is called mitosis and the daughter cells are called diploid cells since they have all 23 pairs of chromosomes (one set from each parent). Mitosis is important for growth, and repair or damaged or worn-out cells.
Reproductive cells called primary gametocytes are diploid. They replicate by mitosis to form more gametocytes or by a process called meiosis to form gametes (sperm and eggs cells). Gametes are haploid cells since they have only half the number of chromosomes, i.e., only 23 chromosomes instead of 23 pairs. Gametes from your father and mother combine to form a diploid cell (you) called a zygote, during sexual reproduction.
Unlike mitosis that produces two genetically identical cells, meiosis produce four cells with genetic variation from the parent cell. This is due to gene recombination, or gene-swapping during cell division of meiosis. As a result, siblings look different although they are coming from the same two parents.