Deoxyribonucleic Acid, DNA, is a molecular polymer containing the genetic information of life. The primary function of DNA is to store and transmit genetic information that tells cells which proteins to make and when to make them.

This molecule has a particular shape, called a double helix. James Watson and Francis Crick proposed the "twisted ladder" shape in 1953.

Polymers are complex molecules composed of repeating subunits. These subunits in DNA and RNA are called nucleotides.

Each nucleotide has three parts:

  1. A sugar molecule

• Deoxyribose - the 5-carbon sugar in DNA
   
• Ribose - the 5-carbon sugar in RNA
   
  • The "deoxy" prefix indicates the sugar in DNA has less oxygen than ribose sugar.

  3. One of these four nitrogen bases:

• Purines - have a double ring of carbon and nitrogen atoms.
   
  • Adenine (A)
  • Guanine (G)

• Pyrimidines - have a single ring of carbon and nitrogen atoms.
   
  • Thymine (T) - replaced by uracil in RNA
  • Cytosine (C)

Base-pairing rules: these rules describe the behavior of the bases.

1. Adenine always bonds with thymine by forming two hydrogen bonds. (AT)
    
2. Guanine always bonds with cytosine by forming three hydrogen bonds. (GC)

 
Ribonucleic Acid, RNA, is responsible for the movement of genetic information from DNA in the cell nucleus to the sites of protein synthesis in the cytoplasm (ribosomes).

RNA differs from DNA in three important structural ways:

• RNA is a single-strand molecule, unlike DNA's double-strand.
   
• RNA contains ribose sugar instead of DNA's deoxyribos sugar.
   
• RNA contains the nitrogen base uracil instead of DNA's thymine. (U instead of T)

Three Types of RNA:

• Messenger RNA (mRNA) - a single uncoiled strand that transmits information from DNA to the ribosomes during protein synthesis.
• Transfer RNA (tRNA) - a single folded strand that bonds with a specific amino acid.
• Ribosomal RNA (rRNA) - a globular form that is the major constituent of the ribosomes.

The Central Dogma of Molecular Biology:

Chromosomes:

DNA is normally found in long strands in the cell nucleus called chromatin. The coiled structures known as chromosome are found only during cell division.

One half of a chromosome (chromatid) is the original DNA and the other half is the duplicated DNA.

The giemsa band pattern seen on this single chromatid does not occur on natural chromosomes. It is obtained by applying giemsa stain.

The light and dark bands, referred to as R-bands and G-bands, do not show actual gene locations. They do make chromosome alterations more visible.

Every species has a distinct chromosome number. Human body cells have 46 chromosomes, arranged in 23 pairs.

 
Other terms related to chromosomes:

• Gene - a segment of DNA on a chromosome that controls a trait.
   
• Allele - any of the several alternative forms of a gene.
   
• Homologous chromosomes - the two members of each pair of chromosomes.
   
• Diploid (2N) - a cell containing both chromosomes of a homologous pair.
   
• Haploid (N) - a cell containing only one chromosome of a pair.

The Cell Cycle

Parts of the Cell Cycle:

• G1 Phase: The gaps in the cycle are times used to replenish energy.
   
  • This phase is sometimes called Interphase.
     
  • Specialized cells in higher organisms perform their normal functions in this longest cycle phase.
     
  • Cells grow during the G1 phase.
   
• S Phase:
   
  • Synthesis means "to make".
     
  • After a period of growth in G1, the cell's original DNA is replicated in this phase of the cycle.
   
• G2 Phase:
   
  • DNA replication is an energy-consuming process. The time needed to build up its energy reserves produces this second "gap".
   
• M Phase:
   
  • Mitosis is the process by which DNA is divided into two identical sets before the cell divides.
   
  Cytokinesis is the division of the cytoplasm to produce two "daughter" cells. Cytokinesis is generally considered the end of the Mitosis phase.

Meiosis is the division of sex cells (gametes).

The process duplicates DNA once, and divides the cell twice, reducing the number of chromosomes by half. The two divisions of genetic material provide more opportunity for genetic variation.

• Meiosis I (1^st cell division)
   
  • DNA is replicated.
     
  • The parent cell divides, producing two daughter cells.
   
• Meiosis II (2^nd cell division)
   
  • There is no DNA replication.
     
  • Each daughter cell divides.
     
  • At the end of Meiosis, there are four daughter cells from each parent cell. Each daughter cell has half the number of chromosomes of the parent.