HW: DNA, Chromosomes, and Genes

1. You have most likely already learned about deoxyribonucleic acid (DNA), chromosomes, and genes. You know that all three of these substances have something to do with heredity in organisms. You have also learned that all living organisms contain some type of genetic material. Have you made the link that these three components are actually one and the same? Let us start at the beginning.

2. What exactly is DNA? All organisms carry the same basic genetic code on their DNA. However, each organism is slightly different from the next, both within species and between species. These differences are due to specific changes in the nucleotide sequences found on the DNA strand.

3. The DNA molecule is made of two long chains of chemical building blocks. These chains form a double helix structure unique to the DNA molecule. Each link on the chain includes a nucleotide, a sugar, and a phosphate group. It is the sequence of nucleotides along the DNA strand that create an organisms genetic code. These links are ordered in a way that allows the code to be copied and read. Therefore, this information can be duplicated and transferred from one generation to the next. But how is the information on the DNA strand copied and transferred? This is where chromosomes and genes come into play.

4. What exactly is a chromosome? A DNA strand and a chromosome are the same thing, just in different forms. A chromosome is simply a highly coiled version of a DNA strand. Most prokaryotes have one DNA molecule and therefore one chromosome. Eukaryotes may have many DNA molecules. Thus they have many chromosomes. In order for the genetic information carried on the DNA strand to be passed from one generation to the next, the cell must first copy, or replicate, the genetic material. In reproduction, the duplicated genetic material is transferred from parent to offspring. During these processes, the genetic material from the DNA strand is condensed, organized, and packaged into individual chromosomes. The structure of chromosomes makes it easier to copy the genetic material or to transfer it to the next generation during reproduction.

5. What exactly is a gene? The gene is the basic unit of heredity. The information for specifying each trait of an organism, such as hair color, is carried on DNA strands in sequences known as genes. Each genes is found at a specific location, or locus, along a chromosome. (The plural of locus is loci.) Keep in mind that chromosomes are simply condensed forms of DNA. The genetic information of any organism can be found at specific loci. In other words, each coding segment of a chromosome is a gene. Each gene is found at a specific locus on the chromosome.

6. The information encoded on the genes is what gives organisms their specic traits. A slight difference in the nucleotide sequence of a gene can change the trait of the organism. For example, consider the gene for part of a hemoglobin molecule. It is found at a specific locus on the short arm of human chromosome number 11. One nucleotide sequence makes normal hemoglobin. Another nucleotide sequence makes sickle-cell hemoglobin. Both types of hemoglobin are produced by slightly different DNA sequences in the same gene at the same locus on the same chromosome. The nucleotide sequence on the DNA strand is different by one single nucleotide at that locus. This produces a different trait. The person will have either normal blood hemoglobin or sickle cell trait if the person inherits one copy of the faulty gene. If the person inherits two copies of the faulty gene, they will have sickle cell anemia.

7. All three components together (the DNA, the chromosomes, and the genes) make up the entire genome of the organism. The genetic information is carried in the nucleotide sequence of the DNA. Each DNA molecule is condensed into separate chromosomes during reproduction, when the genetic information is passed from parent to offspring. The specific nucleotide sequences on the genes will determine the organisms unique traits.

8. As a final review, a DNA molecule carries the genetic code in a sequence of nucleotides along a strand. During replication and reproduction, the DNA molecule coils and condenses into a form known as a chromosome. The regions of the DNA that code for specific traits are known as genes. The DNA sequence of each gene can be found at a specific locus along the chromosome.