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Introduction to Plant Development BIO 274-01 Plant Morphology Fall 2007 Discussion 2 Plant Morphology – study of the morphology (physical form and external structures) of plants Plant morphology examines the pattern of development, the process by which structures originate and mature as a plant grows Plant Anatomy – study of the internal structure of plants, examines plant at cellular level Plant Physiology – study of the function of physiology (mechanical, physical and biochemical functions of living organisms ) of plants Plant Development – studies the formation of the embryo from a zygote, seed germination, development of mature vegetative plant from the embryo, the formation of flowers, fruits and seeds and environmental factors that affect these growth processes. Molecular genetics and the use of molecular biology tools has greatly improved our understanding of plant development. Plant morphologist, anatomist, and physiologist all use molecular genetics to investigate various aspects of plant development. ¾ Why is genetics so important in understanding plant development? Genetics helps researchers identify genes that control plant development. Many of the scientist that study plant development use a “toolbox” of mutants that affect a developmental process These mutations are recognized by their phenotype; abnormalities in development Phenotype – physical appearance of an organism; description of its traits Genotype – genetic make-up of an organism Characterization of genetic mutants is often used to connect these aberrant phenotypes to the genes responsible for that developmental process Many of these genes will be discussed this semester so it is important that you understand the basic principles of gene expression Genetic Screens Plants carrying various mutations will often have an abnormal phenotype when compared to wild-type plants – Wild-type – the typical form of an organism, strain, gene or characteristic as it occurs in nature – Mutant – an individual, organism, or genetic character arising from an instance of mutation – Mutation – changes to the base pair sequence of genetic material One plant the is commonly used in genetic and molecular studies is Arabidopsis thaliana Arabidopsis thaliana is a small weed that is used as a model organism in plant biology – Model Organism – a species that is extensively studied to understand particular biological phenomena Studies of Arabidopsis have helped to identify the structure and function of many genes involved in plant development The advantages of using this organism are – – – – – – it has a small genome; ~ 26,000 genes the complete DNA sequence is known reproduces quickly can be grown easily can reproduce as a hermaphrodite functions have been assigned to ~ 18,000 genes Fig. 21-107 Understanding Gene Expression Topic Primer 1 Understanding gene expression is critical to understanding the role that molecular genetics plays in plant development. Gene expression is defined as as the process by which a genes DNA sequence is converted into the structures and functions of a cell. The Flow of Genetic Information – The Central Dogma -- In order for genes to be expressed they must first be transcribed into messenger RNA and then translated into protein DNA Genes are composed of DNA DNA composed of nucleotides = nitrogenous base, phosphate group and sugar (ribose or deoxyribose) DNA is a double helix with sugarphosphate backbones to outside and bases to inside The bases of DNA are adenine (A), guanine (G), thymine (T) and cytosine (C) Adenine must always base pair with thymine and guanine must always base pair with cytosine Thus one strand of DNA is the complement of the other Two strands are antiparallel meaning that if one has a 5’ – 3’ polarity then the other has a 3’ to 5’ polarity mRNA DNA maintains the genetic code, this code is transcribed into mRNA mRNA holds the information for making a polypeptide (protein) so it is said to “code for” or “encode” a polypeptide mRNA has the same sequence as the nontemplate strand except uracil (U) substitutes for thymine (T) The strand that is complementary to the mRNA is called the template, anticoding or antisense strand The strand that has the same sequence as the mRNA is called the nontemplate, coding or sense strand Proteins The information contained in mRNA must be translated so that proteins can be synthesized ¾ Why are proteins important? – Provide structure that helps give cells integrity and shape – Serve as hormones to carry signals from one cell to another – Bind and carry substances – Control the activity of genes – Serve as enzymes that catalyze essential chemical reactions Proteins ¾ How are proteins made? The information carried by the mRNA determines the sequence of amino acids that will make up a polypeptide chain (protein) Involves converting the nucleic acid language”, of the genetic code, to protein “language” and is therefore called translation The general rule is that each gene in the DNA gives rise to a single protein, there are however, exceptions During translation, the bases of mRNA are read off in groups of three, which are known as codons Each codon represents a particular amino acid There are 64 different codons in the genetic code Some amino acids are encoded by more than one codon Codon Table mRNA Code – Base sequence: GAU – GCC – GUA – AUC – GAC – UGU Protein – 3 letter code: Asp Ala Val Ile Asp Cys Gene Mutations Gene mutations are defined as changes in the genetic material (DNA) Mutations can be caused by – Copying errors that occur during cell division – Exposure to radiation – Exposure to chemicals (mutagens) Gene mutations have varying effects on organisms depending on whether the mutation alters the function of an essential or non-essential gene product Gene mutations can be classified according to their structural modifications; some examples are – – – – – Missense mutations Nonsense mutations Insertions Deletions Frameshift mutations The result of these mutations is a change in the protein product; the altered protein may not function properly Note the following examples taken from the genetics home reference website, visit the website to review other examples of gene mutations – http://ghr.nlm.nih.gov/handbook/mutationsanddisorders/possiblemutati ons – Link available on Blackboard Missense mutation – change in one DNA base pair that results in the substitution of one amino acid for another in the protein made by the gene Insertion – changes the number of DNA bases in a gene by adding a piece of DNA Key Concepts – Discussion 2 Upon completion of this topic students should be able to: – Explain the role molecular genetics plays in understanding plant development – Define the terms genotype, phenotype, gene expression, wild-type, mutant and model organism – Describe the characteristics that make Arabidopsis thaliana a good model organism for plant development – Explain the central dogma – Transcribe a gene sequence – Translate a mRNA sequence – List the classifications of gene mutations and explain how these mutations affect translation (protein product)
