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When a cell is getting ready to divide, genetic information in the form of DNA will condense into structures called chromosomes. This is how genetic information is passed from parent to offspring.
Homologous chromosomes are chromosomes that contain the same set of genes and are the same length and shape. One is from the mother of the offspring, and other is from the father.
There are two types of chromosomes within our body:
The chromosome number is the number of chromosomes in a species' cells. Each species has its own number of chromosomes.
EXAMPLE
For humans, the chromosome number is 46. This means that we have 46 chromosomes, or 23 pairs of homologous chromosomes, in our cells. Of those 46 chromosomes, most of them are autosomes. Only two of those chromosomes are sex chromosomes. A mouse has a total of 40 chromosomes.Chromosomes are only visible in this form when the cell is preparing to divide. The rest of the time, our genetic information can be found in the form of chromatin, which has a balled-up, thread-like form and is found within the cell's nucleus.
This makes sense; when the cell isn't dividing, the DNA is more stretched out so its information is physically accessible. When the cell is dividing, the DNA has to move all the way across the cell. It's not being accessed for much information, so it's better to be wound up tight with a bunch of protective proteins. It's like packing a suitcase: It's easier to take everything you need if all the clothes are rolled up tightly (just as the DNA is condensed into visible chromosomes) than if you just throw clothes in a pile in your suitcase (like when the DNA is stretched out).
DNA is said to be in the structure of a double helix, or a "twisting ladder". The outside parts of the ladder (the "side rails") are made up of a phosphate-sugar backbone—that is, phosphate and deoxyribose sugar molecules. In DNA, the sugar within its nucleotides is called deoxyribose; in RNA, the sugar within its nucleotides is called ribose.
The "rungs" of the ladder are made up of four nitrogenous bases:
The phosphate, the sugar, and the nitrogenous base together make a nucleotide, and each "rung" of the double helix (and the rung's small portion of "side rail") is made of two nucleotides facing each other. In DNA, the two nucleotides that make up a particular rung of the twisted ladder are called a base pair. Adenine always pairs with thymine, and cytosine always pairs with guanine.
If you follow one of the "rails" of the DNA's "twisting ladder", you will see the nucleotides' order (A, T, C, etc.). This is called a nucleotide sequence. The order of letters (nucleotides) in the nucleotide sequence is very important because the sequence contains instructions or "recipes" for all our thousands of proteins. These "recipes" for our proteins are called genes. Any change in the nucleotide sequence is a mutation and can have a negative impact on a protein's structure or production.
For example, one of the genes for making hemoglobin is 1,605 nucleotides long. Within that stretch of DNA, there is a sequence of three nucleotides that reads "GAG", but in some people, the nucleotide sequence at that location reads "GTG". It's like a typo; instead of saying, "Shall I compare thee to a summer's day" the gene says, "Shawl I compare thee to a summer's day".
Hemoglobin produced from this mutated gene is more likely to clump. If only one the two copies of chromosome 11 (one of its two homologous pairs) has this mutation, it means only half of the hemoglobin the person produces is clumpy, and the person is less vulnerable to malaria. But if both of the homologous chromosomes have the mutated genes, all of the hemoglobin produced is clumpy, and the person will suffer from sickle cell anemia.
Source: SOURCE: THIS WORK IS ADAPTED FROM SOPHIA AUTHOR AMANDA SODERLIND
