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Transcript of At the end of this lesson you should be able to 1. Identify two causes of variation 2. Identify the...
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Variations and Mutations
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At the end of this lesson you should be able to
1. Identify two causes of variation 2. Identify the characteristics of mutant alleles3. Identify two types of mutations4.Give an example of each type5.List two agents responsible for increased
mutation rates
Lesson Objectives
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Chromosomes and Genetics
Chromosomes are long pieces of DNA, with supporting proteins
Genes are short regions of this DNA that hold the information needed to build and maintain the body
Genes have fixed locations: each gene is in a particular place on a particular chromosome
Diploids have 2 copies of each chromosome, one from each parent. This means 2 copies of each gene.
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Human Chromosomes
We have 46 chromosomes, or 23 pairs.
44 of them are called autosomes and are numbered 1 through 22. Chromosome 1 is the longest, 22 is the shortest.
The other 2 chromosomes are the sex chromosomes: the X chromosome and the Y chromosome.
Males have and X and a Y; females have 2 X’s: XY vs. XX.
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Male Karyotype
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Female Karyotype
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The basic rule:
If the Y chromosome is present, the person is male.
If absent, the person is female.
Sex Determination
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1. What is the difference between a chromosome and a gene?
2. What is the difference between the terms haploid and diploid?
3. How many pairs of chromosomes do we have?
4. What are pair number 23 called?5. What is the difference between the male and
the female karyotype?
Learning Check
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the X and Y chromosomes separate and go
into different sperm cells:
½ the sperm carry the X and the other half
carry the Y.
All eggs have one of the mother’s X
chromosomes
The Y chromosome has the main sex-
determining gene on it, called SRY
Meiosis
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About 4 weeks after fertilization, an embryo
that contains the SRY gene develops testes,
the primary male sex organ.
The testes secrete the hormone
testosterone.
Testosterone signals the other cells of the
embryo to develop in the male pattern.
Sex Determination
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Causes of Inherited Variations
1. Sexual reproduction
2. The independent assortment of
chromosomes when gametes are
being formed
3. Crossing Over during Meiosis – this
allows genes to move between
chromosomes
4. Mutations
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Fact
When your parents cells undergo meiosis to produce sperm/egg cells
8 million different combinations of their chromosomes are possible
Every zygote is a 1 in 64,000,000,000,000 combination of chromosomes!
This leads to great variation in species
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Offspring are similar to their parents but are not identical
Cause – crossing over
Differences between parents and offspring
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Crossing Over
http://www.sciencelearn.org.nz/var/sciencelearn/storage/images/contexts/uniquely-me/sci-media/images/chromosomes-crossing-over/464438-1-eng-NZ/Chromosomes-crossing-over.jpg
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Fusion of gametes
Sperm and egg cell fuse to form a zygote
The combination of chromosomes leads to great variation in species
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1. What happens during meiosis?
2. What does meiosis lead to?
3. What are the causes of variation?
4. Why are offspring not identical to their
parents?
Learning Check
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Chromosome Number Variations
Except for the X and Y, humans don’t survive with only 1 copy of any chromosome. Also, 3 copies is lethal in most cases.
Down Syndrome is the most common
human aneuploidy. It is also called trisomy-21, meaning 3 copies of chromosome number 21.
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Aneuploidy
http://www.ncbi.nlm.nih.gov
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Aneuploidy
http://www.ncbi.nlm.nih.gov
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Downs syndrome
http://www.genome.gov/Pages/Hyperion/DIR/VIP/Glossary/Illustration/Pdf/trisomy.pdf
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Chromosome Structure Variations Chromosomes can be broken by X-rays
and by certain chemicals.
The broken ends spontaneously rejoin, but if there are multiple breaks, the ends join at random.
This leads to alterations in chromosome structure.
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Structure Variation Examples
There are lots of ways chromosomes can change structure, so the syndromes are not as well defined as with number variations.
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23
Point mutations
These are changes in just one base pair of a gene.
If a point mutation occurs in a gamete or in a cell that gives rise to gametes, it may be transmitted to offspring and to a succession of future generations e.g. sickle cell anaemia.
© Biology Support Service 2007
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Sickle Cell Anaemia is caused by a point mutation-
A mutation of a single base pair in the gene that codes for one of the polypeptides of haemoglobin.
http://evolution.berkeley.edu/evolibrary/article/mutations_06
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Protein folding
25
Just one amino acid could affect how a protein folds
Campbell and Reece (2002) Biology
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As a result, damage is caused to the red blood cell membrane, resulting in a cellular lifespan of only a few days.
Due to the sickle cell shape, small capillaries may become blocked.
www.cnn.com/sickle/cells.lage.jpg
ww.sciencemuseum.org.uk/genes/223.asp
www.pueblo.gsa.gov/sicklecell/496-sick.html
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Cri-du-chat syndrome comes from a deletion of one end of chromosome 5, so the person only has 1 copy of all the genes on this end of the chromosome.
Cri Du Chat Syndrome
Missing part of Chromosome 5
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Radiation High energy radiation from a radioactive material or from X-rays
These X rays attack the DNA molecule and alters it in many ways.
Radiation can also cause double strand breaks in the DNA molecule, which the cell's repair mechanisms cannot put right
Remember Chernobyl
Mutations – Other Causes
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Sunlight contains UV radiation (the component that causes a suntan)
This can cause an abnormal cross link in DNA to form between certain adjacent bases.
In most normal cases the cells can repair this damage, but sometimes a mutation can occur
Unprotected exposure to UV radiation by the human skin can lead to skin cancer and extensive skin tumors.
Dying for a Suntan
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A chemical in cigarette smoke causes mutations in a crucial gene associated with cancer.
Cigarette smoke caused mutations in a gene called P53.
Smoking
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http://www.cdc.gov/tobacco/data_statistics/sgr/2010/consumer_booklet/chemicals_smoke/index.htm
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http://freehosting.tomaweb.com/QuitSmoking/Neck_Tumor.htm
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http://freehosting.tomaweb.com/QuitSmoking/Oral_Cancer_Sucks.htm
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1. What is meant by aneuploidy?
2. Give an example?
3. What are the causes of structural variation in
chromosomes?
4. Give an example of a structural variation?
5. What are the causes of mutatoins?
Learning Check
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Can you ……………… 1. Define variation and mutation2. List the causes of variations and mutations3. List the types of mutations4. Discuss the causes for and effects of
increased mutation rates
What have you learned?
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End