Genes and Chromosomes

DNA: The Molecule of Heredity Scientists have found that the substance Deoxyribosenucleic Acid (DNA), contained in chromosomes, is responsible for the control of heredity.

Structure and Function of DNA The Nucleotide base is an organic ring structure that contains one or more atoms of nitrogen. In DNA, there are four possible nucleotide bases-Adenine, Guanine, Cytosine, and Thymine.

Structure and Function of DNA The DNA molecule is composed of two strands twisted in the shape of a double helix.

Replication of DNA Replication occurs every time a cell reproduces by mitosis or a gamete is formed by meiosis, DNA is copied. DNA molecule is composed of two strands, each containing a sequence of nucleotides.

Replication of DNA During replication, each strand serves as a template to make a new DNA molecule. DNA replication begins as an enzyme breaks the hydrogen bases that hold the two strands together, unzipping the DNA molecule.

From DNA to Protein Living organisms have their own code, the genetic code, in which the sequence of nucleotides in DNA represents information.

Genes and Proteins Proteins, such as enzymes, control chemical reactions that perform key life functions such as building ATP or digesting food.

Genes and Proteins Other proteins are produced to build and repair cell structures, such as microtubules or transport proteins in membranes. Proteins determine the structure and function of organisms.

The DNA Code When scientists worked out the structure of DNA, it became clear to them that the sequence of nitrogen bases along one of the two strands was a code for the synthesis of proteins.

The DNA Code This code is known as the genetic code. A sequence of three nucleotide bases forms the code for and amino acid, the DNA code is often called the triplet code.

The DNA Code Each set of three nucleotide bases for an amino acid that match the triplet code of DNA on the mRNA are known as a codon. The genetic code was figured out by studying the DNA of the bacterium Escherichia coli.

Transcription-From DNA to RNA Enzymes make an RNA copy of a DNA strand in a process called transcription. The RNA copy that carries information from DNA out into the cytoplasm of the cell is called Messenger RNA (mRNA).

Transcription-From DNA to RNA Some portions of DNA code for the RNA that makes up the ribosomes. This is called ribosomal RNA (rRNA)

Translation-From RNA to Protein Translation is the process of converting the information in a sequence of nitrogen bases in mRNA into a sequence of amino acids that make up protein.

Translation-From RNA to Protein If proteins are to be built, the 20 different amino acids dissolved in the cytoplasm must be brought to the ribosome's it is called Transfer RNA (tRNA).

Translation-From RNA to Protein The end result of translation is the formation of the large variety of proteins that make up the structure of organisms and help them to function.

Genetic Changes Mistakes in the DNA code can produce similar results. Sometimes there is no effect on an organism, but often mistakes in DNA can cause serious consequences for individual organisms.

Mutation-A Change in DNA Accuracy is important to ensure the genetic continuity of both new cells and offspring. Sometimes mistakes can occur in the genetic material. Any mistake or change in the DNA sequence is called a mutation.

Mutation-A Change in DNA Point mutation is a change in a single base pair in DNA. A simple analogy can illustrate point mutations. Read the sentences on the next slide to see what happens when a single letter in a sentence is changed.

Mutation-A Change in DNA –THE DOG BIT THE CAT. –THE DOG BIT THE CAR. As you can see, changing a single letter changes the meaning of this sentence.

Mutation-A Change in DNA A change in a single nitrogen base can change the entire structure of a protein. A mutation in which a single base is added or deleted from DNA is called a frameshift mutation.

Chromosomal Mutations Chromosomes often break and then rejoin backwards or even to the wrong chromosomes. Changes in chromosomes are called chromosomal mutations.

Chromosomal Mutations Chromosomal mutations occur in all living organisms, but they are especially common in plants. Few chromosome mutations are passed on to the next generation because the zygote usually dies.

Errors in Disjunction The failure of homologous chromosomes to separate properly during meiosis is called nondisjunction. In one case of nondisjunction, two kinds of gametes are formed as a result of nondisjunction.

Errors in Disjunction The effects of non-disjunction are often seen when gametes fuse in fertilisation. When a gamete with an extra chromosome is fertilized by a normal gamete, the zygote will have an extra chromosome. This condition is called trisomy.

Errors in Disjunction Another case of non- disjunction involves a total lack of separation of homologous chromosomes.

Errors in Disjunction While organisms with extra chromosomes often survive, organisms lacking one or more chromosomes usually do not.

Errors in Disjunction When a gamete with a missing chromosome is fertilized by a normal gamete, the resulting zygote will lack a chromosome. The condition is called monosomy.

Errors in Disjunction An example of monosomy is Turner’s Syndrome (XO). These are females that have one X chromosome only.

Causes of Mutations Mutations are generally random events. Mutations that occur at random are called spontaneous mutations. It is known that many environmental agents also cause mutation.

Causes of Mutations Exposure to X rays, ultraviolet light, radioactive substances, or certain chemicals can cause changes in DNA. Mutations often result in sterility or the lack of normal development in an organism.

Causes of Mutations If these mutations occur in human gametes, they can cause birth defects. If they occur in body cells, the mutations can lead to cancer.