Introducing:

1. Introducing: The Chromosome

2. http://www.rothamsted.bbsrc.ac.uk/notebook/courses/guide/chromo.htmhttp://www.rothamsted.bbsrc.ac.uk/notebook/courses/guide/chromo.htm What is a Chromosome? • The word ‘chromosome’ comes from ‘chroma’ (meaning coloured) and ‘soma’ (meaning body). • A chromosome is a single molecule of DNA coiled around a series of proteins called histones. • The DNA is compacted and wound up so much that is can actually be seen under a light microscope when stained with certain dyes. The supercoiled DNA can be seen under a light the microscope during the metaphase stage of mitosis Proteins

3. DNA, Chromosomes, Cells http://molekularbiologie.web.med.uni-muenchen.de/groups/imhof/start.html Photograph of a chromosome – they were first seen in 1840, but then no-one knew their function. All they knew was that they re-appeared every time a cell divided. http://www.mtsinai.on.ca/pdmg/Genetics/basic.htm

4. What is a Karyotype? Cytogeneticists (scientists that study chromosomes) cut out each chromosome and arrange them in homologous pairs. This is known as a karyotype and can be written as 46,XX An actual photograph taken down the microscope showing chromosomes in metaphase. There are 5 interphase cells also. Humans have 23 pairs of chromosomes – making 46 - known as the 2N or the diploid number. We have 22 pairs of autosomes and 1 pair of sex chromosomes Sex chromo-some pair

5. Anatomy of the Chromosome p arm centromere q arm 2 sister chromatids satellites homologous chromosome

6. Chromosome Bands Higher resolution of banding shown on this sister chromatid, this means more bands can be seen • Each chromosome has a unique banding pattern that is used to identify it. You could think of it as a specific bar code. Naming the bands helps identify specific sections of the chromosome. • The dark bands represent areas of the chromosome that are more condensed and the pale bands show less condensed sections. • The bands are used to identify parts of the chromosomes to know if there are any abnormalities within or between the chromosomes. • Both sister chromatids are from the same parent. The original sister chromatid duplicates itself just before cell division. Band p11 Band q25 A G-banded X chromosome

7. Mutations in Chromosomes A mutation caused by radiation, harmful chemicals or random chance can involve a change in the number of chromosomes, as shown here in a Down Syndrome karyotype with an extra chromosome 21. This karyotype is written as 47,XY,+21.

8. httpwww.dynagene.comeducationpopupdic1314.html Mutations in Chromosomes A mutation can involve a rearrangement of the chromosomes already present. Chromosomes can be rearranged by section being inverted, duplicted, deleted or translocated. In this karyotype the bottom of the q arm of chromosome number 5 has been translocated to to p arm of chromosome 8. Karyotype: 46,XY,t(5q;8p)

9. Mutations in Chromosomes Can you pick the mutation in this karyotype? How would you write this karyotype? Karyotype: 45,X A female with Turner’s Syndrome

10. http://hgp.gsc.riken.go.jp/chr21/ Genes on a Chromosome? • This is a diagram of Chromosome 21. The band numbers are written down the side and next to that is a gene map. It is called a map because it shows where the genes are located down the chromosome. Genes have numbers and letters that make up their names. • You can see how any rearrangement mutations in the chromosomes can alter the order and/or function of gene. • Numerical mutations will affect the number of genes by either removing one copy or providing extra copies. • During the Human Genome Project (which aimed to map the entire human genome) analysis of this chromosome revealed 127 known genes, 98 predicted genes, and 59 pseudogenes. Pseudogenes are sections of DNA that appear to be gene like, but do not function as genes.

11. http://www.biologie.uni-hamburg.de/b-online/e09/boveri.htm Boveri’s Contributions • In Germany at the turn of the 20th century, Theodor Boveri studied sea urchins and discovered that the nucleus rather than the cytoplasm of a cell is concerned with inheritance. • In 1896 he bred two different kinds of sea urchins and found that the characteristics mixed. Then he removed the nucleus from the female and noted that only the male traits were passed on into the offspring. • Suggested that chromosomes exchanged genes during meiosis – this was later known as crossing over. • He discovered that the number of chromosomes in each organism was fixed.

12. Sutton’s Contributions • In 1902 Walter Sutton (an American geneticist) proposed that chromosomes were actually involved in inheritance. • He suggested that the ‘factors’ (soon to be known as genes) that Mendel used to describe how character traits were inherited, were actually carried on chromosomes. • Although Sutton worked with grasshoppers, was not famous for any single experiment, but instead he brought together the research of many other scientists – including Boveri – to explain how the chromosomes were involved in heredity.

13. http://www.genomenewsnetwork.org/timeline/1902_Boveri_Sutton.shtmlhttp://www.genomenewsnetwork.org/timeline/1902_Boveri_Sutton.shtml Sutton’s Contributions The homologous chromosome pair separate, or ‘segregate’ at meiosis so that each gamete receives one chromosome from each pair. This is why I called it the ‘reduction division’. During meiosis each chromosome lines up independently next to its homologous pair – this supported Mendel’s theory of Independent assortment. After fertilization the zygote then has a full set on homologous chromosomes again.

14. The Sutton-Boveri Theory • The Sutton-Boveri theory, otherwise known as the ‘chromosome theory of inheritance’, stated that chromosomes carried the units of inheritance and occurred in distinct pairs. • The two scientists worked separately but came to the same conclusions. They never met. • As there were more inheritable traits than there were chromosomes they suggested that each chromosome must carry many factors representing a particular trait. • Their theory was initially discussed and debated, some embraced it while others strongly rejected it. By 1915 Thomas Hunt Morgan laid the controversy to rest with his genetic studies of the fruit fly Drosophila melanogaster.’

15. a A B b D d e E Crossing over between Chromosomes • Crossing over occurs when parts of homologous chromosomes that are lined up next to each other during meiosis 1 break and then rejoin so that the parts of the two different chromosomes are switched over. • The point where the chromosomes break and cross is the chiasma. • Does not occur every time chromosomes line up at meiosis. • Introduces extra genetic variation into the offspring. • The further genes are apart on a chromosome, the more crossing over can occur between them, and the closer they are the less crossing over occurs. More chance of crossing over occuring between genes A and E than… …between d and e (or b and d, etc)

16. Crossing over at Meiosis Meiosis 1 Meiosis comes from a Greek word for ‘making smaller’ These homologous pairs can line up either side of each other, this increases genetic variation between siblings when the chromosomes sort

17. Crossing over at Meiosis Meiosis 1

18. Crossing over at Meiosis Meiosis 11

19. Crossing over at Meiosis Meiosis 11

20. Crossing over at Meiosis Four daughter cells Only one of these haploid (1N) cells will be used in fertilization. This helps increase the genetic diversity.

21. References • Aubusson, P. and Kennedy, E. (2000) Biology in Context. The Spectrum of Life Oxford University Press, Melbourne, Australia. • Board of Studies (2002) STAGE 6 SYLLABUS Biology Board of Studies, NSW, Australia. • Castells-Brooke, Natalie (2003) Molecular Biology Notebook Retrieved from the site http://www.rothamsted.bbsrc.ac.uk/notebook/courses/guide/chromo.htm February 2004 • DYNAGENE: The Future of Diagnostic Genetics (2002) Retrieved from site http://www.dynagene.com/education/geninfo.html April 2004. • Human Genome Research Project (2003) Retrieved from the site http://hgp.gsc.riken.go.jp/top.html February 2004. • Humphreys, Kerri (2003) Biology. Blueprint of Life. Science Press, Australia. • Kinnear, J and Martin, M (2001) Biology 2 HSC Course: Jacaranda HSC Science John Wiley & Sons, Australia, Ltd. • Mount Sinai Hospital (2004) Basic Understanding of Genetics Retrieved from site http://www.mtsinai.on.ca/pdmg/Genetics/basic.htm February 2004. • Mudie, K. et.al. (2000) Heinemann Biology Malcom Parsons, National Library of Australia, Australia. • The Center for the Advancement of Genomics (2004) Theodore Boveri (1862 - 1915) and Walter Sutton (1877 – 1916) propose that chromosomes bear heredity factors in accordance with Mendelian Laws Retrieved from the site http://www.genomenewsnetwork.org/timeline/1902_Boveri_Sutton.shtml February 2004.