Cells, Chromosomes, DNA and RNA

DNA DNA is short for Deoxyribonucleic Acid It is the basic substance of heredity. DNA is organized into chromosomes which are found in the nucleus of cells. It carries the genetic instructions for growth, development, functioning and reproduction of living organisms. It also passes on the genetic information from one generation to the next.

Where is DNA located? Prokaryotic – Cells that lack a membrane bound nucleus Eukaryotic – Cells that contain a membrane bound nucleus The DNA of Prokaryotic cells is found in the nucleoid region or cytoplasm. The DNA of Eukaryotic cells is found in the nucleus.

DNA’s Structure DNA is a very long polymer with the basic shape of a twisted ladder or zipper called a double helix.

The Double Helix Structure The DNA double helix has two strands twisted together.

DNA’s Structure The DNA structure is contributed to the work of four scientists: James Watson Francis Crick Maurice Wilkins Rosalind Franklin

DNA Structure The sides of the ladder are sugar molecules and phosphate groups. The steps or rungs are nitrogen bases. Adenine – A Cytosine – C Guanine – G Thymine – T Arranged in base pairs. Chargraff’s Rule tells us… Adenine must always pair with Thymine Guanine must always pair with Cytosine

How does DNA copy itself? 16 How does DNA copy itself? Before a cell divides, the DNA strands unwind and separate. Each strand makes a new partner by adding the appropriate nucleotides. The result is that there are now two double-stranded DNA molecules in the nucleus So that when the cell divides, each nucleus contains identical DNA.

DNA Structure A gene is a section of DNA that codes for a protein. Each unique gene has a unique sequence of bases. This unique sequence of bases will code for the production of a unique protein. It is these proteins and combination of proteins that give us a unique phenotype.

Humans are made of cells. Within each cell’s nucleus are 46 chromosomes arranged in pairs for 23 chromosome pairs. Each chromosome is made up of genes. The genes are made of DNA

Proteins The main function of chromosomes is to control the production of proteins. Proteins help to determine the size, shape, color and many other traits of an organism. Proteins are made of amino acids. Each set of 3 DNA base codes code for a certain amino acid. This 3 letter code is called a codon. There are 64 codons; 61 that code for amino acids and 3 stop codons. Proteins are produced on ribosomes in the cytoplasm of cells.

So how does the DNA inside the nucleus get its genetic information to the ribosomes in the cytoplasm?

RNA RNA is short for Ribonucleic Acid RNA is made of Adenine, Guanine, Cytosine and Uracil RNA has only one strand. Messenger RNA (mRNA) copies the message from the DNA in the nucleus and carries it to the ribosomes in the cytoplasm. Transfer RNA (tRNA) reads the genetic code from the mRNA and makes protein chains.

DNA Facts and Information There are ≈ 3.4 billion base pairs in the human genome. There are 20,000 to 25,000 genes in the human genome. The diameter of a human DNA strand is ≈2nm. It would take 12,500 DNA strands to approximate the size of a human hair. The length of human DNA (all 46 chromosomes) stretched end-to-end is 6 feet. The length of all the DNA in the body would be over 113 billion miles (or the distance to the sun and back 610 times) Humans are 99.8% genetically identical – the remaining 0.2% accounts for the variation seen in the human population.