Download Introducing:

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.unimuenchen.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 reappeared every time a cell divided.
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
chromosome
pair
Anatomy of the Chromosome
p arm
centromere
q arm
2 sister
chromatids
satellites
homologous
chromosome
Chromosome Bands
•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.
Higher
resolution of
banding shown
on this sister
chromatid, this
means more
bands can be
seen
Band p11
Band q25
A G-banded X
chromosome
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.
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)
httpwww.dynagene.comeducationpopupdic1314.html
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
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.
http://hgp.gsc.riken.go.jp/chr21/
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
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.
http://www.genomenewsnetwork.org/timeline/1902_Boveri_Sutton.shtml
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.
A
a
B
b
D
d
E
e
•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)
Crossing over at Meiosis
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
Meiosis 1
Crossing over at Meiosis
Meiosis 1
Crossing over at Meiosis
Meiosis 11
Crossing over at Meiosis
Meiosis 11
Crossing over at Meiosis
Only one of these haploid (1N) cells
will be used in fertilization. This
helps increase the genetic diversity.
Four daughter cells
References
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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.