Introducing:The

Chromosome

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

http://www.rothamsted.bbsrc.ac.uk/notebook/courses/guide/chromo.htm

DNA, Chromosomes, Cells

                                                      

             

http://molekularbiologie.web.med.uni-muenchen.de/groups/imhof/start.html

http://www.mtsinai.on.ca/pdmg/Genetics/basic.htm

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.

What is a Karyotype?

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

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

Sex chromo-some pair

Anatomy of the Chromosomep arm

centromere

q arm

2 sister chromatids

satellites

homologous chromosome

Chromosome BandsHigher resolution of banding shown on this sister chromatid, this means more bands can be seen

Band q25

•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

A G-banded X chromosome

Mutations in ChromosomesA 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.

Mutations in ChromosomesA 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)

httpwww.dynagene.comeducationpopupdic1314.html

Mutations in ChromosomesCan you pick the mutation in this karyotype? How would you

write this karyotype?

Karyotype: 45,X

A female with

Turner’s Syndrome

Genes on a Chromosome?

http://hgp.gsc.riken.go.jp/chr21/

•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.

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.

http://www.biologie.uni-hamburg.de/b-online/e09/boveri.htm

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.

Sutton’s Contributions

http://www.genomenewsnetwork.org/timeline/1902_Boveri_Sutton.shtml

During meiosis each chromosome lines up independently

next to its homologous pair –

this supported Mendel’s theory of

Independent assortment.

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’.

After fertilization the zygote then has a full set on

homologous chromosomes

again.

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.’

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.

A a

B b

D d

E e

More chance of crossing over occuring between genes A and E than…

…between d and e (or b and d, etc)

Crossing over at MeiosisMeiosis 1

These homologous pairs can line up either side of each other, this increases genetic variation between siblings when the chromosomes sort

Meiosis comes from a Greek word for ‘making smaller’

Crossing over at MeiosisMeiosis 1

Crossing over at MeiosisMeiosis 11

Crossing over at MeiosisMeiosis 11

Crossing over at MeiosisFour daughter cellsOnly one of these haploid (1N) cells

will be used in fertilization. This helps increase the genetic diversity.

• 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.

References