The Nature of Heredity

1. The Nature of Heredity SBI3U0

2. Heredity • Genetics • The study of heredity and variation • Heredity refers to the fact that an individual can inherit traits from its parents • You will share characteristics with your parents • If your parents have black hair, you will likely have black hair • Bacterial cells will share characteristics with their parent cells • If a bacteria has cilia, its offspring will have cilia • If a bacteria causes the flu, its offspring will also cause the flu

3. DNA • Heredity is due to a certain class of molecules called nucleic acids • In humans (and plants, animals, fungi, etc) deoxyribonucleic acid (DNA) is the nucleic acid responsible for heredity • In bacteria it is a related molecule called RNA ribonucleic acid • DNA has an elegant structure • It has a sugar-phosphate back bone that supports the bases adenine (A), guanine (G), thymine (T), and cytosine (C)

4. DNA • DNA forms a double helix • With the bases in the middle • This means there are two strands side-by-side • The strands coil into a helical shape

5. Chromosomes • These helices of DNA coil themselves around proteins to create large, dense structures called chromosomes • Each chromosome contains the DNA sequences for several (often thousands) of traits • The specific DNA sequence (of bases) that codes for a certain trait is called a gene • The position of a gene on a chromosome is called a locus Loci

6. Cell Cycle • All cells have a specific life cycle that ensures that they will divide and reproduce new cells that have the appropriate genes to carry on living • Every cell must have a specific number of chromosomes to encode all of the proper genetic information for that cell • Dog cells contain 78 chromosomes • Tomato plant cells contain 24 chromosomes • Human cells contain 46 chromosomes • The cell cycle shows how cells live and then procreate

7. Cell Cycle • Why do cells need to procreate? • 1) Normal cell replacement • Cells die all of the time (~3 million a minute in your body) and they need to be replaced or your body would not function for very long • This occurs do to injury, not receiving enough food/oxygen, or by natural aging • All cells have different lifespans • Eg: Brain cells last 30-50 years, red blood cells last ~120 days, while stomach lining cells last ~2 days

8. Cell Cycle • 2) Regeneration • When you cut your skin or break a bone, new cells must be created to repair the damage • This ability is usually limited • Eg: if you cut your skin too deep you cannot grow back new skin cells, instead your body fills in the hole with scar tissue

9. Cell Cycle • Cells spend most of their time in “interphase” • During interphase there are two growth phases (G1 and G2) where the cell grows larger, increases its number of organelles and cytoplasm, and duplicates its chromosomes • After G2 the cell is ready for division

10. Cell Division • The cell division portion of the cycle is different depending on the type of cell • Most cells undergo mitosis, which is used exclusively for asexual reproduction • Meaning that there is one parent cell only • This occurs in most simple organisms, and in the somatic cells of multicellular organisms • Somatic cells refer to body cells that are not used to reproduce an entire organism (eg: skin cells, brain cells) • Some cells divide slightly differently, via meiosis. These cells are used for sexual reproduction and we will look at them later on

11. Mitosis • Mitosis describes the division of a single cell into an exact duplicate • We call the duplicate a daughter cell • Mitosis occurs in distinct stages • Prophase • Metaphase • Anaphase • Telophase • Cytokinesis

12. Interphase • Before mitosis begins the cell is in interphase • During this phase (G1, S, G2) the cell grows larger and duplicates all of its DNA • There are now 92 chromosomes in humans • There is a duplicate of each of the 46 chromosomes • The DNA replicates so that the daughter and parent cells each get a complete set of DNA

13. Prophase • In prophase, the chromosomes wrap themselves around special proteins and become short, thick, structures called chromatin • Each duplicate chromatin is referred to as a chromatid • The two sister chromatids (the two duplicates) are connected together at the centromere

14. Prophase • The nuclear membrane starts to dissolve • In animal cells, organelles called centrioles move to the poles of the cell • These centrioles attach themselves to, and organize, the series of microtubules that attach to the centromere of each chromatin

15. Prophase • Each centriole is connected to each chromatin • Thus each chromatin is connected to two microtubules • One going to the top of the cell and one going to the bottom

16. Metaphase • By the time metaphase occurs, the chromatin have arranged themselves along the equator of the cell • The chromatin are arranged so that one sister chromatid is facing the top of the cell while the other is facing the bottom

17. Anaphase • In anaphase, the spindle fibres pull the sister chromatids apart and start separating them to opposite sides of the cell • In this way, each side of the cell is getting a complete set of chromosomes • In humans, each cell gets 46 chromosomes

18. Telophase • Telophase is the beginning of the end of mitosis • The chromatids are separated to opposite poles of the cell • The nuclear membrane starts forming around the two groups of chromatin • The DNA starts to unspool from the proteins holding it together as chromatin • The cell membrane starts to pinch in forming a cleavage furrow

19. Cytokinesis • This is the separation of the two daughter cells from one another • The cleavage furrow grows and eventually pinches off the cells from each other, forming two distinct (and identical) cells • The organelles and cytoplasm is split between the two cells Animation

20. Cloning • Cloning is a term that refers to creating an exact copy of an organism from one single cell • This is exactly what happens in mitosis • The biotechnology technique is slightly different from normal mitosis however • Multicellular organisms have two classes of cells • Stem cells and Specialized cells • Specialized cells have specific functions and are quite different from one another (muscle vs. brain vs. skin etc.) • Stem cells can divide and create any type of specialized cell

21. Cloning • Since specialized cells do not normally divide to create different specialized cells • Muscle cells only divide into muscle cells, they do not create brain cells (for example) • Adults are mostly composed of specialized cells, with very few stem cells • Therefore creating a clone animal out of a specialized cell is difficult • Traditionally stem cells have been used for the purpose of cloning

22. Cloning • Recently there have been many breakthroughs in this area, and some specialized cells can now be turned into clone cells and used for cloning • This is still a very active area of research • Dolly the sheep • Many species of animals have been successfully cloned • All have had short life spans and exhibited many physiological problems, like arthritis, premature aging, and lung disease • Click and Clone

23. Applications of Cloning • We can genetically modify certain organisms to give them advantageous traits and then clone them to produce several identical organisms • Eg: Producing corn plants that can grow in cold weather, or cows that produce more milk than normal • Cloning allows us to produce several organisms with our specific modifications • We already do this for several of our fruits and vegetables • Homework pg 151 #1-9, 11, 12