2. DNA Overview

All living things have genetic material that they pass on to their offspring.  This genetic material, deoxyribonucleic acid, or DNA, contains a code that serves as the blueprint for building each unique organism.  In humans, and many other organisms such as plants and fungi, DNA is housed in a membrane-bound organelle called a nucleus. Every cell in your body contains a complete copy of the DNA instructions for building all of the parts of your body.  This complete copy is referred to as your genome. Unless you have an identical twin, your genome is unique.

Despite their identical DNA, your body cells can be very different from each other.  For example, the cells lining the inner surface of your cheek have a different structure and function than do the cells that make up the cheek muscles that allow you to smile.  This is because each cell uses only a subset of the DNA instructions to make the specific proteins that it needs.

DNA structure

DNA is a macromolecule made up of a string of smaller units called nucleotides. Each nucleotide is made up of a sugar, a phosphate group and a base (Figure 1).

 

A single nucleotide is shown, containing three chemical groups linked together with covalent bonds. The groups are a phosphate, a sugar, and a nitrogenous base. Next two pairs of nucleotides are shown. Within each pair, the nitrogenous bases are linked with non-covalent bonds so that they are in the center, and the two sugars are on either side of them, with the two phosphate groups are on the outside of those. The two pairs are stacked on top of each other like the rungs of a ladder, and the pairs are connected to each other through non covalent bonds between the phosphate group of one pair and the sugar of the next pair, on either side. A double helix of DNA is also shown, where the rungs of the ladder, which are the base pairs, are in the center of a spiraling ladder.
Figure 1. Basic DNA structure. Each base pair is composed of a phosphate group, a sugar, and a nitrogenous base. Two bases are connected to form a base pair. Base pairs are connected through phosphate bonds, and this forms the DNA double helix.

 

There are four different possible bases, adenine, thymine, cytosine and guanine.  It is the order of these bases (often referred to simply as As, Ts, Cs and Gs) which make up the DNA “code.” DNA is found in the nuclei of all human cells, and serves as the blueprint for the production of the proteins necessary to maintain life.

 

 

A double helix of DNA, which looks like a spiraling ladder, is shown. These double helixes are shown coiling up on themselves and around small proteins to form a tight coil of DNA which is larger in diameter than the double helix This tight coil coils up on its self even more in to a structure with a larger diameter, and and is packed into a larger X-shaped structure. This is the chromosome. Each side of the chromosome is labelled chromatid. The ends of each chromatid are labelled telomere. The center of the chromosome, where the two chromatids meet in the center of the X, is labelled centromere. Several chromosomes are depicted in the nucleus of the cell, a round bubble in the middle of a cell.
Figure 2. DNA organized into chromosomes. Double helical DNA is packed tightly together into coils, which are packed into chromosomes. The chromosomes are found in the nucleus of human cells.

 

Human DNA is organized into 23 pairs of chromosomes (Figure 2). Before a cell divides, it replicates its DNA so that both daughter cells have a complete copy of the DNA blueprint. Occasionally, errors are made in DNA replication. Because the order of base pairs determine the proteins a cell makes, these errors, if left uncorrected, can lead to the production of different proteins. Errors in DNA replication that result in changes in the DNA molecule are called mutations. Mutations can happen in the DNA of any cell in the body.  If a mutation happens in a sex cell, which is responsible for makings gametes (sperm or eggs), that mutation could be passed on to the organism’s offspring.  This videoclip offers a brief overview of the organization of human DNA and how small variations in human DNA caused by mutations can result in the variation we see in human populations.

 

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License: CC-BY 4.0By: Kristina Prescott

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Human Biology 2nd edition Copyright © by Sarah Malmquist and Kristina Prescott is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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