Basics: Genes, Cells, and Signaling

WHY IT IS USEFUL TO KNOW ABOUT GENES
  • Many diseases and conditions are caused by gene mutations.
  • By studying the mechanisms of the way genes work scientists are developing new tests and treatments that can combat disease in completely new ways. Genetics is being called ‘the new frontier of medicine’.
  • This guide sets out to explain in detail how genes work. It also explains some of the terms used.
GENES ARE THE BODY’S BLUEPRINT
  • They provide the instructions for everything the body does.
  • They are contained in the cells in our body. (All living organisms from humans and animals to plants, bacteria and viruses have genes.)
  • Genes are passed down to us from our parents through eggs and sperm.
  • Genetic mutations can be inherited from one or both of our parents (germline mutations).
  • But it’s important to know that they also occur as the result of day to day living (somatic mutations).
  • Mutations can be damage to the gene itself or damage to the genetic control systems.
GENES ARE CONTAINED IN OUR CELLS
  • The human body has somewhere between 30 and 40 trillion cells.
  • There are hundreds of cell types each with specific jobs to do and they vary in size, shape and density.
TO UNDERSTAND HOW GENES WORK IT HELPS TO LOOK AT THE WAY CELLS WORK
  • The cell is a tiny sac containing a fluid called cytoplasm. The outer membrane has receptors on the external surface that respond to incoming chemical signals.
  • Floating around in the cytoplasm inside the cell are many different types of structures each with their own function.
  • At the heart is the nucleus – the command centre. This is where most of our genes are.
THE BODY IS CONSTANTLY RENEWING ITSELF
  • As they age, cells are damaged by wear and tear and eventually most self-destruct.
  • The body grows and renews itself by cells dividing and replicating to produce new cells. A cell’s life cycle varies from a few days for the gut lining to a person’s entire lifespan for some brain cells.
  • New cells are being created all the time. They develop into the appropriate kind of cell for job they have to do.
INSIDE THE NUCLEUS ARE CHROMOSOMES – THE PACKAGING FOR OUR DNA
  • Humans have 23 pairs of chromosomes. We inherit half from each of our parents. Eggs and sperm contain only one set of chromosomes each. In every generation each parent passes just one chromosome from each pair to their child and this ensures we’re all genetically different.
  • When a cell divides and replicates as part of its natural life cycle it must exactly copy the chromosomes and the DNA inside. This is when it is vulnerable to copying mistakes and genetic mutations.
  • Chromosomes are visible at the time cells divide. This is when they can be seen through the microscope. At other times the DNA is less structured.
TO UNDERSTAND HOW GENETIC MUTATIONS CAUSE ILLNESSES IT HELPS TO KNOW HOW GENES CONTROL THE BODY’S FU
  • DNA is packed up very tightly in a continuous double strand – the double helix. This is wrapped around spools called histones. This forms the chromosomes.
GENES ARE SECTIONS OF THE DNA STRAND

Genes are sections of the DNA strands

  • DNA is made up nucleotides. They are shown here as coloured ‘steps’ in the DNA ‘ladder’. They are linked along a ‘backbone’, the side rails of the ‘ladder’.
  • There are four nucleotides each with a different chemical base – adenine, thymine, guanine and cytosine – referred to as letters A, T, G and C. These pair up with each other linking the two strands, A with T and C with G (shown here in different colours).
  • Genes are sections of DNA strands – in humans most genes are between a thousand and a million nucleotides long.
  • To form the gene instructions code, the nucleotides are arranged in different combinations of three- letter groups (sequences). The code depends on the way the three chemicals are arranged.
  • When mutations occur, the code sequence is changed and the gene message is faulty.
GENES CONTROL WHAT THE BODY DOES BY MAKING PROTEINS
  • Proteins are the key molecules that form the structure of cells and control biological processes. They are involved in almost all cellular processes.
  • Genes create proteins by sending out a messenger from the nucleus into the cell to link together amino acids, the building blocks of proteins.
  • The specific sequence of DNA in a gene codes for the specific type of protein that is being made (gene expression).
  • Only some DNA codes for proteins. Some is used to control the timing and the amount of the expression.
  • The first step in making a protein is to make mRNA (messenger ribonucleic acid) from the DNA.
  • mRNA takes the coded instructions from the gene and carries them out of the nucleus into the cytoplasm where the proteins are made.
THE GENETIC CODE MUST BE COPIED EXACTLY
  • When mRNA is made from DNA an enzyme races along the DNA unzipping the double helix, reading the chemical code created by the nucleotides and producing a single strand of mRNA.
  • This process (transcription) happens at speed and the code must be copied exactly with all the chemicals in the right order. They are linked together at a rate of 20-50 per second.
MAKING THE PROTEIN
  • Once it moves outside the nucleus and into the cytoplasm, the mRNA can start linking the amino acids together to make a protein.
  • A structure called a ribosome (these float around in the cytoplasm or stick to special membranes in the cytoplasm) gathers around the mRNA chain and acts as the construction crew to build the protein. The code for each amino acid is read off, three letters at a time (translation). New amino acids are added at about five per second.
  • WHAT HAPPENS WHEN A GENETIC MUTATION OCCURS
  • Some diseases are caused by inherited mutations in the genes contained in mitochondria. Although most DNA is inside the cell nucleus, a very small amount is found in the mitochondria.
  • Mitochondria are hundreds of little bean shaped structures that float around in the cell’s cytoplasm. They have multiple jobs but their main one is to convert energy from food into a form the cell can use.
  • MITOCHONDRIAL DISEASES

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