Download Introducing the Chromosome Yr 12 Biology

Introducing the Chromosome Yr 12 Biology
Linked to parts 3 – Chromosomal structure provides the key to inheritance

outline the roles of Sutton and Boveri in identifying the importance of chromosomes

describe the chemical nature of chromosomes and genes

explain the relationship between the structure and behaviour of chromosomes during meiosis and the
inheritance of genes
 explain the role of gamete formation and sexual reproduction in variability of offspring
What is a chromosome?
The word ‘chromosome’ comes from ‘chroma’ (meaning coloured) and ‘soma’ (meaning body). A
chromosome is a single molecule of _____________ coiled around a series of
__________________ called ______________. The DNA is compacted and wound up so much
that is can actually be seen under a light microscope when stained with certain dyes
What is a Karyotype?
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Mutations
A mutation caused by _____________, harmful ________________ or random ____________
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
A mutation can involve a rearrangement of the chromosomes already present. Chromosomes
can be rearranged by section being inverted, duplicted, deleted or translocated.
Chromosome Bands
Each chromosome has a unique banding ___________________ that is used to identify it. You
could think of it as a specific bar code. Naming the bands helps ________________ specific
sections of the chromosome. The dark bands represent areas of the chromosome that are more
________________ 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 ___________________ are from the same parent. The original sister
chromatid ________________________ itself just before cell division.
•This is a diagram of Chromosome 21. The
band numbers are written down the side
and next to that is a gene map. Genes
have numbers and letters that make up
their names.
•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 Contribution
In Germany at the turn of the 20th century, Theodor ____________________ studied sea urchins
and discovered that the nucleus rather than the cytoplasm of a cell is concerned with
__________________________. 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. In 1907 he fertilized sea urchin eggs with two sperm,
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which developed into 4N tetraploid cells instead of 2N diploid cells. Suggested that chromosomes
exchanged genes during meiosis – this was later known as crossing over.
Sutton’s Contribution
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 _________________. Sutton
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.
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. 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 in Chromosomes
Crossing over occurs when parts of _________________ chromosomes that are ______________
up next to each other during meiosis 1 break and then rejoin so that the parts of the two different
chromosomes are _________________ over. The point where the chromosomes break and cross
is known as the ____________________ . Does not occur every time chromosomes line up at
meiosis. Introduces extra ________________ variation into the offspring. The _________________
genes are apart on a chromosome, the _______________ crossing over can occur between them,
and the closer they are the less crossing over occurs.
Crossing over at Meiosis
List the 3 ways that genetic variation is introduced into the offspring during sexual reproduction:
1)
2)
3)
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