Blueprint of Life Topic 15: Chemical Nature of Chromosomes and Genes

1. Blueprint of LifeTopic 15: Chemical Nature of Chromosomes and Genes Biology in Focus, HSC Course Glenda Childrawi, Margaret Robson and Stephanie Hollis

2. DOT POINT(s) • describe the chemical nature of chromosomes and genes • identify that DNA is a double-stranded molecule twisted into a helix with each strand comprised of a sugar-phosphate backbone and attached bases – adenine (A), thymine (T), cytosine (C) and guanine (G) – connected to a complementary strand by pairing the bases, A-T and G-C

3. Introduction Although the work of Sutton and Boveri and Thomas Hunt Morgan showed that chromosomes are the physical basis of inheritance (that is, they carry the hereditary factors and these genes are arranged in a linear fashion, like beads on a string), the actual chemical nature and chemical structure of the hereditary material and genes remained unsolved until the 1940s. pinterest.com

4. Introduction At that stage the common expectation among scientists was that the secret of heredity would be found in proteins. In 1944, Oswald Avery published his findings, which contested this idea and suggested that DNA (not proteins) encodes the hereditary information. Many biologists were not convinced, as they thought that Avery’s DNA had been ‘contaminated’ by protein. www.britannica.com

5. Introduction The race was on in laboratories around the world to try to solve the puzzle—both protein and DNA structures were being investigated. In America, Linus Pauling discovered the structure of the alpha helix in the protein haemoglobin in 1948, ahead of several teams doing similar studies in Europe. en.wikipedia.org

6. Introduction There were two leading teams in England that were also working on the molecular structure of biological molecules at the time—one team at Cambridge University in the Cavendish laboratory under the leadership of Lawrence Bragg, and one team at King’s College in London under the leadership of John Randall. www.adelaide.edu.au

7. Introduction In 1951, both of these laboratories as well as that of Linus Pauling in America had teams doing similar research into the structure of DNA. In 1953, one of these teams, after building a detailed model, won the race—they discovered that DNA (deoxyribosenucleic acid) is the molecule that meets all the requirements of the hereditary material. www.dnai.org

8. Introduction According to the model, DNA: • Can carry, in coded form, all the instructions for the formation and functioning of cells, despite the fact that its ‘alphabet’ consists of only four nitrogenous bases. • Its structure allows self-replication. • It can be transferred (packaged in the form of chromosomes) by gametes from one generation to the next. www.accessexcellence.org

9. Introduction As a result of their innovation, their attention to sound scientific detail and their collaborative approach, Watson and Crick revealed that DNA is a double helix or ‘twisted ladder’. www.achievement.org

10. General Structure of DNA Chromosome is made up of two chemicals: 1. DNA, a long, thin thread-like macromolecule, which is the information-carrying part of the chromosome 2. Proteinsaround which the DNA is coiled, to keep it neatly ‘packaged’. www.turbosquid.com

11. General Structure of DNA A DNA molecule is made up of two chains or strands of small building blocks or monomers called nucleotides. www.brightondailyphoto.com

12. General Structure of DNA Each nucleotide consists of three parts—a phosphate, a sugar (deoxyribose sugar) and a nitrogenous base. www.brightondailyphoto.com

13. General Structure of DNA There are four types of bases and each nucleotide is named after the base that it carries—adenine, thymine, guanine or cytosine. These are often simply referred to by their first letters—A, T, G and C. The bases are arranged in a sequence along each strand of DNA— e.g. GGTCAGGCTTGAACGA—and so each DNA molecule is thousands of bases long. www.brightondailyphoto.com

14. General Structure of DNA The whole ‘ladder’ molecule, instead of being flat, spirals and is therefore known as the ‘double helix’. ■ X-ray crystallography suggested a helix measuring 3.4 nm for every turn and this fitted the model where exactly 10 base pairs would measure 3.4 nm in length and make up one twist of the helix. ■ The ratio of adenine to guanine and cytosine to thymine could be explained by their complementary base pairing. www.lazyboytech.com

15. General Structure of DNA The two complementary chains of DNA could ‘unzip’ or open up along the line of hydrogen bonds between the base pairs, allowing them to replicate. www.coriell.org

16. Chemical Structure of DNA The DNA molecule is a long chain molecule consisting of two complementary strands. Each strand is made up of a sequence of many nucleotides and the strands are held together by weak hydrogen bonds in the centre. The two strands in the double-helix model have an ‘antiparallel’ arrangement—that is, they run in opposite directions. Handout Copy of Diagram 3,2 whyfiles.org

17. Chemical Structure of DNA The vertical sides of the ladder are made up of alternating sugar and phosphate molecules (a ‘sugar–phosphate backbone’) and the ‘rungs’ of the ladder are pairs of nitrogenous bases (adenine, thymine, guanine and cytosine, or A, T, C and G respectively). www.britannica.com

18. Chemical Structure of DNA Each nucleotide is made up of a phosphate group, a sugar (deoxyribosesugar) and a nitrogenous base attached to the sugar. There are four types of nitrogenous bases: adenine, guanine, cytosine and thymine. www.brightondailyphoto.com

19. Chemical Structure of DNA Chemically, these bases have to pair in a particular manner: adenine with thymine and guanine with cytosine (that is, A-T and G-C), held together in the centre by hydrogen bonds, forming two complementary strands.

20. Genes and Chromosomes A gene is considered to be the smallest unit of heredity (that is, what Mendel called a ‘factor’). Chemically, each gene is a portion of DNA with a specific sequence of bases that encodes for a particular trait that can be passed from parent to offspring. www.bio.miami.edu

21. Genes and Chromosomes A locus is the position of a gene on a chromosome. The coded information within genes determines how living things look, behave and function—that is, it influences particular characteristics (phenotypes). A chromosome can therefore be described as a linear sequence of genes. www.virtualmedicalcentre.com

22. Genes and Chromosomes The total amount of genetic material that an organism has in each of its cells is called its genome. boydfuturist.wordpress.com

23. Genes and Chromosomes Specific staining techniques are used to show up banding patterns on chromosomes and these bands correspond on homologous pairs of chromosomes. The banding patterns can also be used to identify the positions of particular genes on chromosomes. With modern technology, particular genes can be marked with fluorescent tags that show up on the chromosome, assisting gene mapping. www.physics.uwo.ca

24. Genes and Chromosomes Task: List the following structures in order of size, from smallest to largest: chromosome, gene, DNA, nucleotide, base, genome. mbg.cornell.edu

25. Activity/Homework -Students to complete Structure of Nucleic Acids worksheet a