What makes up base pairs

And those nucleotides always pair. We also count DNA and the amount of DNA, or the length of DNA by using units of base pairs, so if we're discussing a gene and we want to describe how big is a gene, we might say that the gene is a thousand base pairs long.

If it's a really big gene, it may be 10, base pairs, or essentially 10 kilobases long. Lawrence C. Brody, Ph. It is impossible for researchers to see double-stranded DNA with the naked eye — unless, that is, they have a large amount of it. Modern laboratory techniques allow scientists to extract DNA from tissue samples, thereby pooling together miniscule amounts of DNA from thousands of individual cells.

When this DNA is collected and purified, the result is a whitish, sticky substance that is somewhat translucent. To actually visualize the double-helical structure of DNA, researchers require special imaging technology, such as the X-ray diffraction used by Rosalind Franklin. However, it is possible to see chromosomes with a standard light microscope, as long as the chromosomes are in their most condensed form.

To see chromosomes in this way, scientists must first use a chemical process that attaches the chromosomes to a glass slide and stains or "paints" them. Staining makes the chromosomes easier to see under the microscope. In addition, the banding patterns that appear on individual chromosomes as a result of the staining process are unique to each pair of chromosomes, so they allow researchers to distinguish different chromosomes from one another.

Then, after a scientist has visualized all of the chromosomes within a cell and captured images of them, he or she can arrange these images to make a composite picture called a karyotype Figure This page appears in the following eBook.

Aa Aa Aa. What components make up DNA? Figure 1: A single nucleotide contains a nitrogenous base red , a deoxyribose sugar molecule gray , and a phosphate group attached to the 5' side of the sugar indicated by light gray. Opposite to the 5' side of the sugar molecule is the 3' side dark gray , which has a free hydroxyl group attached not shown. Figure 2: The four nitrogenous bases that compose DNA nucleotides are shown in bright colors: adenine A, green , thymine T, red , cytosine C, orange , and guanine G, blue.

Although nucleotides derive their names from the nitrogenous bases they contain, they owe much of their structure and bonding capabilities to their deoxyribose molecule. The central portion of this molecule contains five carbon atoms arranged in the shape of a ring, and each carbon in the ring is referred to by a number followed by the prime symbol '. Of these carbons, the 5' carbon atom is particularly notable, because it is the site at which the phosphate group is attached to the nucleotide.

Appropriately, the area surrounding this carbon atom is known as the 5' end of the nucleotide. Opposite the 5' carbon, on the other side of the deoxyribose ring, is the 3' carbon, which is not attached to a phosphate group. This portion of the nucleotide is typically referred to as the 3' end Figure 1.

When nucleotides join together in a series, they form a structure known as a polynucleotide. At each point of juncture within a polynucleotide, the 5' end of one nucleotide attaches to the 3' end of the adjacent nucleotide through a connection called a phosphodiester bond Figure 3.

It is this alternating sugar-phosphate arrangement that forms the "backbone" of a DNA molecule. Figure 3: All polynucleotides contain an alternating sugar-phosphate backbone.

This backbone is formed when the 3' end dark gray of one nucleotide attaches to the 5' phosphate end light gray of an adjacent nucleotide by way of a phosphodiester bond. How is the DNA strand organized? Figure 4: Double-stranded DNA consists of two polynucleotide chains whose nitrogenous bases are connected by hydrogen bonds. Within this arrangement, each strand mirrors the other as a result of the anti-parallel orientation of the sugar-phosphate backbones, as well as the complementary nature of the A-T and C-G base pairing.

Figure Detail. Figure 6: The double helix looks like a twisted ladder. How is DNA packaged inside cells? Figure 7: To better fit within the cell, long pieces of double-stranded DNA are tightly packed into structures called chromosomes. What does real chromatin look like? The bases have different shapes and pair up together in specific combinations: A pairs with T, and C pairs with G to make base pairs. Put three billion of these base pairs together in the right order, and you have a complete set of human DNA—the human genome.

This amounts to a DNA molecule about a metre long. You have two copies of the genome, one from your father and one from your mother. The metre-long sequence is cut up into 23 bits, which are then tightly packaged as chromosomes in each of our microscopic cells.

No two people have exactly the same DNA sequence. Identical twins are a bit of an exception to this rule, because they came from a single egg that split into two, forming two copies of the same DNA. Three billion is a lot of base pairs. Rather, within this long string, there are distinct sections of DNA that affect a particular characteristic or condition.

These stretches of DNA are known as genes. Their base pair sequence provides the instructions for bringing together the right amino acids in the right order to build a protein. Some genes are small, only around base pairs; others contain over one million.