DNA Structure Reading

DNA – The Blueprint for Life

DNA, or deoxyribonucleic acid, is the major method that all living organisms store the information to build cells and bodies as well as manage the various functions of the organisms. Heredity is the passing of characteristics from parents to offspring. DNA is responsible for heredity because it is passed from generation to generation. Identical copies of DNA are present in (almost) every cell of an organism, providing each cell with a complete set of genetic instructions. Even more, the DNA of each individual is unique, providing the instructions that make each of us unique and therefore the variation in all life. Encoded in the DNA is instructions for making the proteins that do the majority of the work in the organism.

The Fourth Biomolecule

Like all biomolecules, DNA is an organic polymer built from certain monomers. Monomers of DNA (and other nucleic acids) are nucleotides. Nucleotides contain the atoms carbon, hydrogen, oxygen, nitrogen and phosphorus. Each nucleotide is made of three parts: a sugar, a phosphate group, and a nitrogen-containing base. The bonding of nucleotide monomers to each other forms a DNA polymer. The two strands on the side of the polymer are formed from repeating units of a phosphate group attached to another sugar, and so on. This is known as the sugar-phosphate backbone. Attached to each sugar is one of four different nitrogen-containing bases. These bases are adenine (A), cytosine (C), guanine (G) and thymine (T). Just as the meaning of a word is determined by which letters are strung together, the information in a molecule of DNA is determined by the sequence of bases. One molecule may have the sequence guanine, guanine, cytosine, thymine, adenine, or GGCTA. Another molecule may have the sequence CCTGA. Because the sequence of nucleotides differs, the genetic information is different, causing the variation between organisms and individuals.

The Double Helix

In the early 1950s a group of scientist all contributed the discovery of the structure of DNA. Rosalind Franklin took a picture of the molecule DNA. At the same time, James Watson and Francis Crick were attempting to use models to determine the structure. Without Franklin’s permission, Watson viewed Franklin’s picture, and completed the work solving the structure of DNA. With the clues from Franklin’s picture, Watson and Crick determined that DNA is made of two interlocking strands of DNA that wind around each other forming a double helix. The structure of DNA is like a ladder. The two sugar-phosphate backbones are like the long vertical sides of the ladder that give it height. A nitrogen-containing base sticking out from the backbone represents a rung on the ladder – or a half-rung. The bases extending from each of the two strands (each sugar-phosphate backbone) meet in the center and bind to each other. Because of the structure of the nitrogen-containing bases, they have specific binding patterns. Adenine (A) always pairs with thymine (T), while cytosine (C) always pairs with guanine (G). Each pair that is bound across the double helix is said to be complementary. Accordingly, A is complementary to T, and C is complementary to G. So while the sequence of nucleotides along the length of any one DNA strand can vary, the bases of the second strand in the double helix are determined by the sequence of the bases on the first strand. Each base must pair with its complementary base into what is called a base pair.

Chromosomes are Polymers of DNA

The full set of DNA in an individual organism is called its genome. In eukaryotes, the DNA is broken into several long, but distinct molecules chromosomes. Humans, for example, have three billion base pairs of DNA, divided into 23 unique, individual chromosomes. However, humans have two copies of each chromosomes (one from each parent), for a total of 46 chromosomes and 23 chromosomes pairs. This would be a total of about six billion total base pairs. Different organisms can have different numbers of chromosomes. Corn contains 10 chromosome pairs (20 total), goldfish have 50 chromosome pairs (100 total), flatworms have 8 chromosome pairs (16 total), and dogs have 39 chromosome pairs (78 total).

DNA is Self-Replicating

One of the most amazing parts of DNA is that it contains the instructions to make a copy of itself. DNA replication is the process by which an exact copy of each chromosome is made. During DNA replication, the two strands of a DNA polymer (the chromosome) separate because the bonding between complementary base pairs is disrupted. Each strand then acts as a blueprint, or template, to build a new, complementary strand of DNA. Nucleotides that are complementary to the bases in the strand are lined up and linked together to form a new DNA strand based on the original (template) strand. When the process is complete, the chromosomes form have one strand from the original chromosome, and one strand that is completely new.

Different Cells Use Different Instructions

All of the cells in an organism, except for sex cells like egg and sperm, have the exact same DNA in their nuclei – the DNA is identical. However, cells have different roles. Some cells carry oxygen, some absorb nutrients, some build structure, and some cells send chemical signals throughout the body. All of these cells have the exact same DNA, but only certain parts of that DNA are used by that cell. In other words, different cells use instructions from the DNA. The other regions are silent.

Consider the DNA as a cookbook for the body. Each recipe has information for a certain cell type. The cell only reads the recipe (parts of the DNA) it needs to use.

Reading Questions – Answer in your journal.

  1. Describe the monomers and polymer of DNA. How are they related to one another?
  2. How many polymers of DNA are in the human body?
  3. Explain where your DNA comes from?
  4. Explain the steps involved in DNA replication.
  5. What is the template for the information transfer of DNA?
  6. What is the final product of DNA replication?
  7. How is it possible to have different types of cells in the human body? Ex. Fat cell, Muscle Cell, Red Blood Cell
  8. Compare the DNA of two cells in an organism. How are they different? How are they similar?