1. Biology 2B Chromosomes and DNA

2. Chromosomes Chromosomes are only visible during cell division They consist of two strands (chromatids) of tightly coiled DNA joined at the centromere They contain genes which control the characteristics of organisms

3. DNA Nuclear DNA is arranged in structures called chromosomes, which are only visible when the cell is dividing. Normal cells have the diploid number of chromosomes. In humans, this is 46 chromosomes arranged as 23 pairs. Gametes have the haploid number of chromosomes. In humans, this is 23 chromosomes, one of each pair. Mitochondrial DNA is inherited only from the mother as it is the ovum that provides the cell and its organelles (mitochondria) for the new zygote. The sperm provides only nuclear DNA as the mitochondria in the flagella are rapidly destroyed. Mitochondrial DNA can be used to trace ancestry of people. DNA (deoxyribose nucleic acid) in found in the nucleus of all cells, and controls protein synthesis, and cell activity. DNA is also found in mitochondria

4. The structure of DNA DNA consists of two strands of alternating sugars and phosphates, with pairs of nuclear bases forming cross-links between the chains. This is called a double helix. A nucleotide is a nuclear base attached to a sugar and a phosphate The four bases are cytosine, guanine, adenine and thymine. Each base links up with its base pair - cytosine with guanine, adenine with thymine Each strand is coiled around the histones so they can fit into a small space. When not dividing, the tangled DNA is called chromatin.

5. How DNA works The base sequences on the DNA acts as a code that controls the action of the cell. Sections of DNA code which control one feature are called genes, so the DNA sequence is called the genetic code. In order to read the code, that section of DNA must untangle and uncoil from its histone. The code sequences are copied as mRNA, which is used to make proteins. Proteins are used as structural components (eg in cell membranes, skin and hair - keratin, messengers (eg hormones) and as enzymes. Protein synthesis occurs in the ribosomes.

6. DNA controls protein synthesis

7. Why are proteins important? Roles of proteins in the body include Structural proteins eg collagen, keratin Enzymes (organic catalysts) eg digestive enzymes Transport proteins eg haemoglobin Regulatory proteins eg hormones Protective proteins eg antibodies, clotting factors Therefore proteins determine what you will look like, and how your body functions

8. DNA replication When cells divide the DNA is copied or replicated, forming two strands joined at the centromere. DNA replication occurs during mitosis or meiosis in interphase, just before prophase starts. During DNA replication the strands separate New bases are added using the base pair rule adenine attaches to thymine thymine attaches to adenine cytosine attaches to guanine guanine attaches to cytosine The separation and attachment of new nucleotides are controlled by enzymes.

9. DNA replication 2