Cells and Genes

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Cell TheoryIn 1838, two German Scientists Theodor Schwann and Mattias Jakob Schleiden put forward a theory on the structure of living organisms.

This theory had a number of observations, including:

1. The cell is the unit of structure, physiology, and organization in living things.

2. The cell retains a dual existence as a distinct entity and a building block in the construction of organisms.

These observations have become parts of what is known as the Cell Theory

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Cells

• Cells are the smallest living subunits of an organism– Some organisms are unicellular, meaning

that they are comprised of only one cell• Ex bacteria, amoebas, etc

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Cells

• Other organisms are multicellular, meaning that they are comprised of many cells– Ex animals, birds, insects, human beings

• Human beings are said to contain about 200 different types of cells – each adapted to a specific task

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Cytology

• The study of the formation, structure, and function of cells – Focus on different aspects including:

• Chromosomes• DNA• Genetics

– Cytology = cyt/o (cell), -logy (study of)

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Cell Structure

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DNA• DNA stands for

deoxyribonucleic acid• Has two strands that twist

together to form a double helix

• DNA is read to create the proteins encoded in our more than 20,000 genes

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Chromosomes

• Chromosomes are pieces of DNA found within the nucleus of a cell– Normally cannot be seen individually– When a cell is preparing

to divide, the DNA condenses into dense rod-like structures that are copied before division occurs

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Chromosomes• Human body (somatic) cells normally have 46

chromosomes arranged into 23 pairs

• Sex cells or gametes contain 23 single chromosomes

• Chromosomes contain regions called genes, that can be “read” to give the cell instructions,or to produce a necessarysubstance

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Karyotype

• A karyotype image is a map of the number and appearance of chromosomes within a human cell– The chromosomes are treated with chemicals

so that characteristic bands of light and dark areas can be seen

– It may be requested if there is any suspicion of genetic disorder

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Genetics

• Genetics is the study of how genes are transferred from generation to generation– Genesis means producing or origin

• Geneticists study not only how genes function in normal conditions, but also how they function to create disease

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Genetic Inheritance

• Humans normally receive two copies (alleles) of every gene – one from their mother, and one from their father– Gametes have one copy of each chromosome– When a male gamete (sperm) fertilizes a

female gamete (ovum), the DNA mingles– The resulting fetus develops with this mix of

maternal and paternal DNA

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Inheritance– Dominant alleles will

cause their trait to be displayed even if only one copy of the gene is received

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Inheritance• Recessive alleles will

only cause their trait to be displayed if two copies of the gene are received

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Genetic Disorders

• Genetic disorders are usually due to a genetic mutation– Inherited– Acquired

• Mutations can be due to:– Deletions– Substitutions– Additions

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Trisomy 21 (Down Syndrome)

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Kleinfelter Syndrome

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Genetic Disorders

• Down Syndrome (trisomy 21)• Cystic Fibrosis (recessive)• Hemophilia (X-linked recessive)• Huntington Disease (dominant)• Phenylketonuria (recessive)• Tay-Sachs Disease (recessive)• Osteogenesis Imperfecta (dominant)

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Cell Division

• Mitosis– Normal Cell replication

• One cell with a diploid number of chromosomes (46) divides into two cells, each with the diploid number of chromosomes

• Produces identical cells• Used to replace cells in the body

– Mitosis Animation

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Cell Division

• Meiosis– Cell replication to form gametes

• Gametes are sex cells – the ovum and the spermatozoon

• Each gamete contains half of the usual number of chromosomes (23)

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Cell Replication• Meiosis has two parts:

– Meiosis I – begins when the chromosomes duplicate• The chromosomes group together so that all chromosomes

of one type are joined together (at this point the copies of the chromosomes can exchange information)

• The chromosomes then separate in a process similar to Mitosis

– Meiosis II – the process occurs again, without the chromosomes replicating to create the gametes

• Mitosis results in 2 daughter cells, Meiosis results in 4 daughter cells