1. Section 11-4
Interest Grabber
How Many Chromosomes?
Normal human body cells each contain 46 chromosomes. The cell division process that body cells undergo is called mitosis and produces daughter cells that are virtually identical to the parent cell. Working with a partner, discuss and answer the questions that follow.
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2. Interest Grabber continued
Section 11-4
1. How many chromosomes would a sperm or an egg contain if either one resulted from the process of mitosis?
2. If a sperm containing 46 chromosomes fused with an egg containing 46 chromosomes, how many chromosomes would the resulting fertilized egg contain? Do you think this would create any problems in the developing embryo? (Shrimp)
3. In order to produce a fertilized egg with the appropriate number of chromosomes (46), how many chromosomes should each sperm and egg have?
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3. 11–4 Meiosis Section Outline A. Chromosome Number B. Phases of Meiosis
1. Meiosis I
2. Meiosis II
C. Gamete Formation
D. Comparing Mitosis and Meiosis
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4. I.Chromosomes
Chromosomes- condensed DNA before division = two copies (chromatids) of the same genetic code

5. 1.Different species have different numbers of chromosomes.
A.Chromosome Number
1.Different species have different numbers of chromosomes.
Fruit flies =8, dogs = 78, chimps = 48
2. Chromosomes paired because one set from each parent.
3. Human cells (except sex cells) have 23 pairs = 46 total chromosomes: 23 from mom and 23 from dad

6. B. HOMOLOGOUS Chromosomes
Chromosome pairs (1 from each parent)
that code for the same type of genes but not necessarily the same expression.
EX: a pair of homologous chromosomes may have genes for hair color, but the one from mom codes for brown hair and the one from dad codes for red.
We all have two copies of our genetic code.

7. Homologous Pairs
In all offspring

8. HOMOLOGOUS Chromosomes

9. Checkpoint Homologous Shoes
Observe the shoes on the next slide. What do you notice? How are pairs of shoes similar to pairs of chromosomes?
Now compare within the pairs. What do you notice?
ANY QUESTIONS?

11. Where are genes located in the cell?
On the chromosomes.
The genes segregate during gamete formation so that each will have just one full set of instructions.
What is diploid? (2N)
Literally two sets, it is a cell that contains two sets of chromosomes.

12. KEY CONCEPT During meiosis, diploid cells undergo two cell divisions that result in haploid cells.

13. Human diploid (2n) cells = 46
C. DIPLOID v HAPLOID
Diploid (2n) = “two sets” - a cell with paired homologous chromosomes
most cells have a set from each parent
Human diploid (2n) cells = 46
23 from each parent
n = #chromosomes from each parent
2n = total # of chromosomes

14. 2. Haploid (n) - a cell that contains only one set of the chromosomes
C. DIPLOID v HAPLOID
2. Haploid (n) - a cell that contains only one set of the chromosomes
Human haploid cells (n) = 23
Sex cells or GAMETES

15. What was the purpose of mitosis?
D. Mitosis Chromosomes
What was the purpose of mitosis?
To divide into two cells that are Identical to original cell so same chromosome number and exact genes
Diploid cells undergo mitosis to produce diploid daughter cells
If one cell in a dog with 2n=78 undergoes mitosis, how many chromosomes will new cell have?

16. II. Sex and Chromosomes
If a new human is created from the union of two cells (one from the father and mother)…
How then, do you not double your chromosome number each time? 46
sperm
(father) 92 46
zygote
(cell)
egg
(mother)

17. Why is this a problem?
If sex cells (gametes) were diploid, each generation would double its chromosome number.
Two gametes with 46 would produce a zygote with 92 chromosomes.
In the next generation, = 184 chromosomes…and in the next generation we would have 368, and….. 46 92 46
zygote
(cell)

18. Obviously, this can’t happen Cell would have too many instructions
Why is this a problem?
Obviously, this can’t happen
Cell would have too many instructions
Nucleus would burst with too many DNA molecules
A single DNA strand is about 5 cm (2 in) long
Our 46 DNA molecules paled end to end would be about 2 meters long

19. GAMETES - haploid sex cells
A.Sex cells are special.
The cells used to pass on genes from parent to offspring are different than every other cell in your body.
GAMETES - haploid sex cells
Sperm(male) and egg (female)
Have half the normal number of chromosomes (hapliod)
produced by meiosis, not mitosis.

20. Checkpoint
Meiosis challenge
Are cells from meiosis the same or different than the parent cell?
How are they different?
Why is this important?
What type of cells undergo meiosis?
If one cell in a dog with 2n=78 undergoes meiosis, how many chromosomes will new cell have?
ANY QUESTIONS?

21. What is Meiosis?
The process where diploid cells form haploid cells. Reduction division separates the chromosome pairs in a diploid cell.

22. Cells go through two rounds of division in meiosis.
Meiosis reduces chromosome number and creates genetic diversity.

23. Pairs of homologous chromosomes separate in meiosis I.
Meiosis I and meiosis II each have four phases, similar to those in mitosis.
Pairs of homologous chromosomes separate in meiosis I.
Homologous chromosomes are similar but not identical.
Sister chromatids divide in meiosis II.
Sister chromatids are copies of the same chromosome.
homologous chromosomes
sister
chromatids
sister
chromatids

24. Meiosis I occurs after DNA has been replicated.
Meiosis I divides homologous chromosomes in four phases

25. Meiosis
A. 2 STAGES:
Meiosis I – separates homologous chromosomes to form two new haploid cells

26. Meiosis
A. 2 STAGES:
Meiosis I – separates homologous chromosomes to form two new haploid cells
Meiosis II – separates chromatids in the two cells from first stage to form four haploid cells

27. B.Phases of Meiosis I Interphase I Prophase I Metaphase I Anaphase I
During meiosis, the number of chromosomes per cell is cut in half through the separation of the homologous chromosomes. The result of meiosis is 4 haploid cells that are genetically different from one another and from the original cell.
Prophase I
Metaphase I
Anaphase I
Telophase I and Cytokinesis

28. Phases of Meiosis I: Interphase I
DNA replicates – makes sister chromatids.
Interphase I - Cells undergo a round of DNA replication, forming duplicate chromosomes.

29. Phases of Meiosis I: Prophase I
Spindles form
Nucleus disappears
Chromosomes condense
Homologous chromosomes pair up to make a TETRAD
Crossing Over occurs.
MEIOSIS I Prophase I - Each chromosome pairs with its corresponding homologous chromosome to form a tetrad.
Tetrad - paired homologous chromosomes = 4 chromatids

30. CROSSING OVER
Homologous chromosomes (tetrads) exchange portions of their chromatids during prophase I = new combinations of GENES.
Crossing-over occurs during meiosis. (1) Homologous chromosomes form a tetrad. (2) Chromatids cross over one another. (3) The crossed sections of the chromatids are exchanged.

31. Phases of Meiosis I: Metaphase I
Tetrads line up with homologues on each side of the line (NOT IDENTICAL)
Tetrads line up in random order = genetic variations
MEIOSIS I Metaphase I - Spindle fibers attach to the chromosomes.
Crossed over sections

32. Meiosis I = New Combinations
Two different ways to line up with 2 pairs = 22 or 4 combinations.
With our 23 pairs = 223 or 8,388,608 combinations!

33. Phases of Meiosis I: Anaphase I
Spindles pull the homologous chromosomes apart (separates tetrads)
MEIOSIS I Anaphase I - The fibers pull the homologous chromosomes toward opposite ends of the cell.

34. Phases of Meiosis I: Telophase I and Cytokinesis
Nuclear membranes reform.
Cytokinesis = makes 2 cells
New cells are haploid and genetically different than parent cell or each other.
MEIOSIS I Telophase I and Cytokinesis - Nuclear membranes form. The cell separates into two cells.

35. Meiosis II divides sister chromatids in four phases.
DNA is not replicated between meiosis I and meiosis II.

36. Telophase II and Cytokinesis
Phases of Meiosis II
Meiosis II
Meiosis I
Prophase II
Metaphase II
Anaphase II
Telophase II and Cytokinesis
During meiosis, the number of chromosomes per cell is cut in half through the separation of the homologous chromosomes. The result of meiosis is 4 haploid cells that are genetically different from one another and from the original cell.
Second stage of meiosis is exactly like mitosis, but with haploid cells.
Chromosomes line up, chromatids travel to opposite sides of the cell, and the cell splits.

37. Telophase II and Cytokinesis
Phases of Meiosis II
Meiosis II
Meiosis I
Prophase II
Metaphase II
Anaphase II
Telophase II and Cytokinesis
During meiosis, the number of chromosomes per cell is cut in half through the separation of the homologous chromosomes. The result of meiosis is 4 haploid cells that are genetically different from one another and from the original cell.
Both cells from Meiosis I go through Meiosis II
= Makes 4 haploid cells with different combinations of genes on the chromosomes.

38. Meiosis differs from mitosis in significant ways.
Meiosis has two cell divisions while mitosis has one.
In mitosis, homologous chromosomes never pair up.
Meiosis results in haploid cells; mitosis results in diploid cells.

39. Haploid cells develop into mature gametes.
Gametogenesis is the production of gametes.
Gametogenesis differs between females and males.
Sperm become streamlined and motile.
Sperm primarily contribute DNA to an embryo.
Eggs contribute DNA, cytoplasm, and organelles to an embryo.
During meiosis, the egg gets most of the contents; the other cells form polar bodies.

40. How can siblings look alike but not exactly the same if they come from the same parents with the same DNA?

41. Because not all of the cells produced in meiosis are identical.
Meiosis produces new combinations of your parents genes!
The gene combinations that a person gets from his or her parents will be different, to varying degrees, than the combination a sibling may get.

42. ANY QUESTIONS? What is crossing over and when does it occur?
Checkpoint
What is crossing over and when does it occur?
How do you get new genetic combinations in metaphase I?
Are cells haploid or diploid at the end of meiosis I?
What are male gametes called and how many are made during meiosis?
Why is only one egg made during female meiosis?
ANY QUESTIONS?

43. Some genes appear to be inherited together, or “linked.” If two genes
Interest Grabber
Section 11-5
Forever Linked?
Some genes appear to be inherited together, or “linked.” If two genes
are found on the same chromosome, does it mean they are linked forever?
Study the diagram, which shows four genes labeled A–E and a–e, and then answer the questions on the next slide.
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44. Section 11-5
Interest Grabber continued
1. In how many places can crossing over result in genes A and b being on the same chromosome?
2. In how many places can crossing over result in genes A and c being on the same chromosome? Genes A and e?
3. How does the distance between two genes on a chromosome affect the chances that crossing over will recombine those genes?
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45. Section 11-4
Crossing-Over
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46. Crossing-Over
Section 11-4
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47. Section 11-4
Crossing-Over
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48. 11–5 Linkage and Gene Maps Section Outline A. Gene Linkage
B. Gene Maps
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49. KEY CONCEPT Independent assortment and crossing over during meiosis result in genetic diversity.

50. VI. Sexual Reproduction
Fertilization - union of two haploid gametes to form a unique diploid zygote
Two parents = offspring with genetic diversity
More variation because random which sperm will fertilize which egg : 223 x 223 = 70 trillion combos
Zygote = fertilized egg
Humans - sperm(23 chromosomes) + egg (23 chromosomes) = zygote with 46 chromosomes.

51. N=23
N=23
2N=46

52. VII. Determining your gender
Autosomes - 22 pairs of our chromosomes have the same type of information
Sex chromosomes – differ from male to female
X X= female
XY = male
Human Karyotype

53. IX. MEIOSIS – The PURPOSE
Meiosis creates haploid gametes with new gene combinations for sexual reproduction = increased genetic diversity of offspring.
How?
DNA gets mixed up during crossing over of prophase I
DNA gets mixed up during line up in metaphase I
Produces haploid cells so don’t constantly double DNA each generation
DNA gets combined from two different parents during fertilization (sexual reproduction).

54. Sexual reproduction creates unique combinations of genes.
Sexual reproduction creates unique combination of genes.
independent assortment of chromosomes in meiosis
random fertilization of gametes
Unique phenotypes may give a reproductive advantage to some organisms.

55. Crossing over during meiosis increases genetic diversity.
Crossing over is the exchange of chromosome segments between homologous chromosomes.
occurs during prophase I of meiosis I
results in new combinations of genes

56. Chromosomes contain many genes.
The farther apart two genes are located on a chromosome, the more likely they are to be separated by crossing over.
Genes located close together on a chromosome tend to be inherited together, which is called genetic linkage.
Genetic linkage allows the distance between two genes to be calculated.

57. What is gene linkage?
A group of genes that are inherited together.
Each chromosome is a group of linked genes.
These groupings of genes, or chromosomes will assort independently

58. Linked genes can be transferred to other chromosomes during cross over.
The rate at which crossing over occurs can be used to map the gene location on the chromosome.

59. Mapping of Earth’s Features Mapping of Cells, Chromosomes, and Genes
Comparative Scale of a Gene Map
Section 11-5
Mapping of Earth’s Features
Mapping of Cells, Chromosomes, and Genes
Cell
Earth
Country
Chromosome
Chromosome fragment
State
Gene
City
People
Nucleotide base pairs
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60. Exact location on chromosomes Chromosome 2
Section 11-5
Figure Gene Map
of the Fruit Fly
Exact location on chromosomes
Chromosome 2
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61. Why is diversity important?
Helps a population survive environmental change
Ex: I am legend
slight gene variations are necessary to produce changes in anatomy that enable organisms to adapt and survive. More diversity = more variations from which to choose
Helps a population survive unfavorable mutation
If no variation, then every offspring would have the mutation.
If lethal, all genes from that line would be wiped out.
Evolution depends on genetic diversity.
No change in species without variation.
Ex: wisdom teeth,

62. Meiosis + Fertilization + Mitosis = YOU
Meiosis = sperm and egg from your parents
Fertilization (sex!)– joining of egg and sperm formed single cell zygote
Mitosis = zygote cell divides into trillions of cells or baby you.
Mitosis continues in you for growth, development and repair.
Meiosis occurs in your ovaries or testes so you can make gametes for sex too!

63. Checkpoint
Is it the parent cell that makes gametes haploid or diploid? Would it undergo mitosis or meiosis?
In what ways does meiosis increase genetic diversity in a population?
What is a zygote? (How is it formed?)
To grow into an adult, would cells undergo mitosis or meiosis?
ANY QUESTIONS?