Variations and Mutations

Lesson Objectives At the end of this lesson you should be able to   Define variation and mutation List the causes of variations and mutations List the types of mutations Discuss the causes for and effects of increased mutation rates

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.

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.

Male Karyotype

Female Karyotype

Sex Determination The basic rule: If the Y chromosome is present, the person is male. If absent, the person is female.

Meiosis 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

Sex Determination 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.

Learning Check What is the difference between a chromosome and a gene? What is the difference between the terms haploid and diploid? How many pairs of chromosomes do we have? What are pair number 23 called? What is the difference between the male and the female karyotype?

Causes of Inherited Variations Sexual reproduction (fusion of gametes) Crossing Over during Meiosis – this allows genes to move between chromosomes Mutations The independent assortment of chromosomes when gametes are being formed

Fusion of gametes Sperm and egg cell fuse to form a zygote The combination of chromosomes leads to great variation in species

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

But why do some children look a lot like their parents? On the next slide try to match up the parent with their child. In some cases its quite easy. Why?

Whose Child?

Whose Child?

Differences between parents and offspring Offspring are similar to their parents but are not identical Cause – crossing over Before one of the pairs of chromosomes are passed on into the gamete(sex cell like sperm or egg) they undergo crossing over. See next slide

Either one of these ends up in the sex cell Crossing Over Either one of these ends up in the sex cell

Learning Check What happens during meiosis? What does meiosis lead to? What are the causes of variation? Why are offspring not identical to their parents?

Mutations The last few slides explain how mutations can cause variation

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.

Aneuploidy

Downs syndrome

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.

Structure Variation Examples There are lots of ways chromosomes can change structure, so the syndromes are not as well defined as with number variations.

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

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.

Just one amino acid could affect how a protein folds Protein folding Campbell and Reece (2002) Biology Just one amino acid could affect how a protein folds © Biology Support Service 2007

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

Missing part of Chromosome 5 Cri Du Chat Syndrome 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. Missing part of Chromosome 5

Mutations – Other Causes 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

Dying for a Suntan 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.

Smoking A chemical in cigarette smoke causes mutations in a crucial gene associated with cancer. Cigarette smoke caused mutations in a gene called P53.

Learning Check What is meant by aneuploidy? Give an example? What are the causes of structural variation in chromosomes? Give an example of a structural variation? What are the causes of mutatoins?

What have you learned? Can you ……………… Define variation and mutation   Define variation and mutation List the causes of variations and mutations List the types of mutations Discuss the causes for and effects of increased mutation rates

The Theory of Natural Selection and the Survival of the Fittest Evolution The Theory of Natural Selection and the Survival of the Fittest

Lesson Objectives Define evolution At the end of this lesson you should be able to Define evolution Outline the Darwin and Wallace Theory of natural selection Discuss the evidence of evolution from either fossil, comparative anatomy or the study of embryos. (Fossil evidence was chosen for this unit)

Evolution Evolution is the changing of one species into another that takes place through natural selection It takes place over a long period of time

Darwin And Wallace Darwin presented the theory of evolution by natural selection in 1858 Much of Darwin’s work was done on the Galapagos islands Darwin proposed a mechanism for evolution by natural selection Alfred Wallace working in Borneo also proposed the same theory

Observations made by Darwin Over breeding Population numbers remain constant Inherited variations occur in populations

Conclusions drawn by Darwin There is a struggle for existence - competition will occur Natural Selection - only the fittest will survive

The Mechanism of Evolution Too many offspring are produced This leads to competition All populations have genetic variations Individuals that are best adapted to the environment survive As the environment changes new adaptations emerge Over a long period of time a new species forms

Learning Check Define Evolution Who were the two people who proposed the theory of evolution Where did Darwin carry out his work? What were the observations and conclusions made by Darwin? Outline the mechanism of evolution

Evidence for Evolution The fossil Record Palaeontology is the study of fossils. A fossil is the remains of an organism Fossils can be dated using radioactive carbon

Fossils Fossils show changes compared to modern organisms Some fossil organisms are extinct Modern fossils show increased complexity Fossil evidence can be linked to environmental change

Dinosaurs 65 million years ago dinosaurs became extinct Iridium an element found in meteorites was laid down This would suggest that a large meteorite hit earth creating large amounts of rock dust. This dust led to the earth becoming colder and darker causing plants and animals to become extinct.

Evolution of the horse The ancestor of the modern horse evolved 60 million years ago Horse Fossils were 0.4m tall 30 million years ago Horse fossils were 0.6m tall 15 million years ago Horse fossils were 1m tall 1 million years ago Equus – the modern horse evolved Horse fossils were 1.6m tall

Miohippus Hyracotherium Neohipparion Equus

Learning Check Outline the evidence for evolution How do fossils provide evidence for evolution Can you outline the evolution of the modern horse?

What have you learned? Define evolution Can you………………………… Define evolution Outline the Darwin and Wallace Theory of natural selection Discuss the evidence of evolution from the study of fossils

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