Genetics

1. Genetics Why do living things grow? • because their cells can reproduce and make new cells How do cells make new cells? • to make new cells, cells divide in two. The process by which cells reproduce is called cell division

2. The importance of cell division • healing and tissue repair • growth • reproduction of organisms Each of your body cells has two sets of 23 chromosomes. All cells are body cells except sex cells. Chromosomes are cell parts that determine what characteristics a living thing will have. Every time your body cells divide, each new cell receives both sets of chromosomes.

3. The nucleus controls cell division. Before the cell divides, each chromosome makes an exact copy of itself. After the chromosomes make duplicate copies, the nucleus divides. The division of the nucleus is called mitosis. The two new cells formed by cell division are called daughter cells. The two daughter cells are exactly alike- exact same traits. Cell division is a form of asexual reproduction. Only one parent cell divides to form daughter cells.

4. Cell Division Mitosis • Interphase • Cell prepares for cell division • Chromosomes are copied

7. Prophase • Centrioles appear and begin to move to the opposite ends of the cell • Spindle fibres begin to form between the centrioles.

8. 3. Metaphase • Spindle fibres attach to the chromosomes causing them to line up in the middle

9. 4. Anaphase • Spindle fibres contract and pull the pairs of chromosomes apart to the opposite ends of the cell

10. 5. Telophase • Cell membrane begins to indent • One complete set of chromosomes at each pole • Nuclear membrane reappears

11. 6. Cytokinesis • Cell membrane moves inward to create 2 new daughter cells • Each daughter cell has its own nucleus with identical chromosomes

14. Recap: Chromosomes • composition • In eukaryotes, chromosomesconsist of • a single molecule of DNA • structure • before a cellgetsready to divide by mitosis, eachchromosome is duplicated during whichphase of ofthecycle? • S phase

15. As mitosisbegins, chromosomesduplicate. The duplicatedchromosomes are called Dyads When first seen, theduplicates are held together at their Centromeres Whilethey are still attached, it is common to calltheduplicatedchromosomes Sisterchromatids

16. SUMMARY Mitosis Celldivisionproducesnewcells for cellgrowth and for thereplacement of wornoutcells in the body. Celldivisioninvolves a series of stepsthatproducetwogeneticallyidenticaldaughtercells. Two divisionsoccur during celldivision: nucleardivision (mitosis) and cytoplasmicdivision (cytokinesis). During interphase, genetic material is replicated. Cellsseemable to divideonly a finitenumber of times.

17. MEIOSIS • Meiosis is a process by which sex cellsor gametesare formed. • in humans it takesplace in the testes and ovum • it involvestwo stages of celldivisionthat have somesimilarities to thephases of mitosis • thesegametes, afterthey are formed, have nofutureunlesstheyunite to form a zygote.

18. In mitosis, the chromosome number of the daughter cells is the same as in the parent cell. In meiosis, the chromosome number of daughter cells is half the number of the parent cell. A human cell containing 46 chromosomes will undergo meiosis and produce gametes that have 23 chromosomes.

19. Each gamete will contain both the same number and the same kind of chromosomes. The number of chromosomes in a gamete is called the haploid chromosome number, or n; the number of chromosomesin all other cells is twice the haploid number and is called the diploid number, or 2n. In humans, the haploid chromosome number is 23 and the diploid chromosome number is 46.

20. Offspring carry genetic information from each of the parents. This explains why you might have your father’s eyes but your mother’s hair. Although you may look more like one parent than another, you receive genetic information from each parent. For example, your father gives you a chromosome with genes that code for eye colour, but so does your mother.

21. Each of the 23 chromosomes that you receive from your father is matched by 23 chromosomes from your mother. The paired chromosomes are called homologous chromosomes because they are similar in shape, size, and gene arrangement.

22. The genes in homologous chromosomes deal with the same traits. Each cell in your body, except the sex cells, contains 23 pairs of homologous chromosomes, or 46 chromosomes in total.Homologous chromosomes interact during meiosis. Your characteristics are determined by the manner in which the genes from homologous chromosomes interact.

24. Stages of Meiosis • involves two nuclear divisions that produce four haploid cells. • The first phase Meiosis I is often called reduction division because the diploid, or 2n, chromosome number is reduced to the haploid, or n, chromosome number • The second phase, meiosis II, is marked by a separation of the two chromatids.

26. Comparison of mitosis and meiosis in humans. Mitosis produces two diploid cells from one diploid cell. Meiosis produces four haploid cells from one diploid cell.

27. Meiosis I During prophase I, the nuclear membrane begins to dissolve, the centriole splits and its parts move to opposite poles within the cell, and spindle fibres are formed. The chromosomes come together in homologous pairs. The replicated chromosomes condense. Homologous chromosomes come together in synapsis and crossing-over occurs. Chromosomes attach to the spindle.

28. Each chromosome of the pair is a homologue and is composed of a pair of sister chromatids. The whole structure is then referred to as a tetrad because each pair is composed of four chromatids.

29. This process is referred to as synapsis. As the chromosomes synapse, the chromatids often intertwine. Sometimes the intertwined chromatids from different homologues break and exchange segments or undergo crossing over Crossing over permits the exchange of genetic material between homologous pairs of chromosomes.

31. Metaphase I follows prophase I. The homologous chromosomes attach themselves to the spindle fibres and line up along the equatorial plate.

32. Anaphase I, • homologous chromosomes move toward opposite poles. • the process is known as segregation. • at this point of meiosis, reduction division occurs. • one member of each homologous pair will be found in each of the new cells. • each chromosome consists of two sister chromatids Each chromosome separates from its homologue. They move to opposite poles of the cell

33. Telophase I • During telophase I, a membrane begins to form around each nucleus. • However, unlike in mitosis, the chromosomes in the two nuclei are not identical. • Each of the daughter nuclei contains one member of the chromosome pair.

34. Although homologous chromosomes are similar, they are not identical. • They do not carry exactly the same information. The cells are now ready to begin the second stage of meiosis. • The nucleus completes its division. • The chromosomes are still • composed of sister chromatids. • The cytoplasm divides after • telophase.

35. Summary of Meiosis I • tetrad: a pair of homologous chromosomes, each with two chromatids • synapsis: the pairing of homologous chromosomes • crossing over: the exchange of genetic material between two homologous chromosomes

36. Meiosis II Occurs at approximately the same time in each of the haploid daughter cells. During meiosis II, pairs of chromatids will separate and move to opposite poles. Note that, unlike with mitosis and meiosis I, there is no replication of chromosomes prior to meiosis II.

37. Prophase II signals the beginning of the second division. During this stage, • the nuclear membrane dissolves and the spindle fibres begin to form.

38. Metaphase II follows prophase II. It is signalled by the arrangement of the chromosomes, each with two chromatids, along the equatorial plate. • The chromatids remain pinned together by the centromere.

39. Anaphase II can be identified by the breaking of the attachment between the • two chromatids and by their movement to the opposite poles. This stage ends • when the nuclear membrane begins to form around the chromatids, now referred • to as chromosomes.

40. The cell then enters its final stage of meiosis: telophase II. During this stage, the • second nuclear division is completed and then the second division of cytoplasm • occurs. Four haploid daughter cells are produced from each meiotic division.

42. Reproduction and Cell Division

46. 1. Meiosis involves the formation of sex cells or gametes. 2. Cells undergoing meiosis pass through two divisions.