Introduction to Genetics PDF

# Chapter 17 - Introduction to genetics ## Section 4 - Protection and survival ### Chromosomes, genes and DNA * **Chromosomes:** Each cell contains an identical copy of the individual's entire genetic material in the nucleus, called chromatin. This material is diffuse and hard to see under a mic...

# Chapter 17 - Introduction to genetics ## Section 4 - Protection and survival ### Chromosomes, genes and DNA * **Chromosomes:** Each cell contains an identical copy of the individual's entire genetic material in the nucleus, called chromatin. This material is diffuse and hard to see under a microscope when the cell is resting but becomes highly visible as compact sausage-shaped structures called chromosomes when the cell prepares to divide. Chromosomes are arranged as 23 pairs, one from the mother and one from the father. A cell with 23 pairs of chromosomes is termed diploid. Gametes (spermatozoa and ova) with only 23 chromosomes are called haploid. * **Homologous chromosomes:** Chromosomes of the same pair are called **homologous chromosomes**. The largest pair is number 1. The first 22 pairs are **autosomes** and contain the same amount of genetic material. The pair 23 are called **sex chromosomes** and determine the individual's gender. * **Karyotype:** The complete set of chromosomes from a cell is the **karyotype**. * **Genes:** About 99% of DNA does not code for protein. These filler regions contain signals to start and stop protein synthesis. The sequences of DNA that code for proteins are called **genes**. Genes normally exist in pairs. * **DNA:** DNA is a double-stranded molecule, made of two chains of nucleotides. Nucleotides consist of a sugar, a phosphate group, and a base. Each base binds to another base on the other sugar/phosphate chain, forming the rungs of a ladder. The two chains are twisted around one another, giving a double helix. The DNA-histone material is called **chromatin** which is packaged into chromosomes shortly before the cell divides. * **The genetic code:** DNA carries information that determines all of the organism's biological activities and is transmitted from one generation to the next. * **Mitochondrial DNA:** Present in each cell, it is passed from one generation to another via the ovum. ### Mutation * A hereditable alteration in the normal genetic make-up of a cell. Most mutations occur spontaneously because of DNA replications and cell divisions. * **Lethal mutation:** Disrupts some essential cellular function resulting in cell death. * **Non-lethal mutation:** Does not kill the cell but alters its function, e.g. the development of a tumor. ### Protein synthesis * **mRNA (Messenger ribonucleic acid):** A single-stranded chain of nucleotides synthesized in the nucleus from the gene, when the cell needs to make protein for which that gene codes. It carries the genetic instructions from the nucleus to the cytoplasm. * **Transcription:** The process of DNA to mRNA synthesis. The enzyme uses only one of the two strands of DNA, adding the complementary base: cytosine to guanine, thymine to adenine, and adenine to uracil. * **Translation:** The synthesis of the final protein using the information carried on mRNA. It takes place on ribosomes in the cytoplasm. The ribosome reads the base sequence of the mRNA in triplets, called codons, and adds the appropriate amino acids to the growing protein molecule as it goes. ### Gene expression * Although all nucleated cells have an identical set of genes, each cell type uses only those genes related directly to its own particular function. This selective gene expression is controlled by various regulatory substances, and the genes not needed by the cell are kept switched off. ### Cell division * **Mitosis:** The process of cell division, where two identical diploid daughter cells are produced. This is important in body growth and repair. * **Meiosis:** The process of cell division, where the daughter cells have only half the normal chromosome number and are haploid. It is important in the production of gametes. * **DNA replication:** The only biological molecule capable of self-replication. Mistakes in copying of DNA can lead to the development of a tumor. The enzyme responsible for DNA replication moves along the base sequence on each strand, reading the genetic code and adding the complementary base to the newly forming chain. ### The genetic basis of inheritance * **Autosomal inheritance:** Each pair of homologous chromosomes contains genes for the same traits. Corresponding alleles contain genes concerned with the same trait. * **Homozygous:** Individual with two identical forms of a gene. * **Heterozygous:** Individual with one dominant and one recessive gene. * **Punnett squares:** A diagram that shows all the possible combinations of a gene. * **Co-dominance:** More than two alleles may code for a trait and more than one allele may be dominant. * **Sex-linked inheritance:** The Y chromosome is much smaller than the X chromosome. ### Ageing and genetics * **Cumulative exposure to potential mutagens:** Leads to a diminishing ability to repair DNA, which results in mutations. * **Cell senescence:** Progressive impairment of cell function. The number of times a cell can divide is somewhere between 50 and 60 divisions. ### Genetic basis of disease * **Cancer:** Caused by mutation of cellular DNA. * **Inherited disease:** Many diseases are passed directly from parent to child via a faulty gene. * **Chromosomal abnormalities:** Usually caused by a gamete carrying abnormal chromosomes and are often lethal. * **Down's syndrome:** Individuals have three copies of chromosome 21. * **Cri-du-chat syndrome:** Part of chromosome 5 is missing. * **Abnormalities of the sex chromosomes:** If the sex chromosomes fail to separate normally during meiosis, the daughter cells will have an incorrect number. ### Review and revise * **Phenylketonuria:** An inborn error of metabolism caused by a faulty gene responsible for producing an enzyme called phenylalanine hydroxylase. This enzyme converts phenylalanine to tyrosine in the liver. If the enzyme is absent, phenylalanine accumulates in the liver and overflows into the blood, resulting in brain damage and mental retardation. * **Mitochondrial abnormalities:** Defects in mitochondrial DNA can cause inherited disorders. Spontaneous mutations in this DNA can also occur in maturity, leading to onset of disease in adults.