1. Sexual Reproduction and Meiosis
Chapter 8

2. Sex
Asexual reproduction - individual inherits all its chromosomes from a single parent
parthenogenesis - development of an adult from an unfertilized egg

3. Sexual reproduction - Fusion of two gametes to produce a single zygote.
Introduces greater genetic variation, allows genetic recombination.
With exception of self-fertilizing organisms (e.g. some plants), zygote has gametes from two different parents.

5. Reduction Division
In sexual reproduction, gametes fuse (fertilization) to produce a zygote.
Gamete formation involves a mechanism (meiosis) that reduces the number of chromosomes to half that found in other cells.
Adult body cells are diploid (2N)
Gamete cells are haploid (1N)

6. A diploid cell
Has two sets of each of its chromosomes
In a human has 46 chromosomes (2n = 46)
In a cell in which DNA synthesis has occurred
All the chromosomes are duplicated and thus each consists of two identical sister chromatids

7. Unlike somatic cells
Gametes, sperm and egg cells are haploid cells, containing only one set of chromosomes = 1n = 23

8. Sexual Life Cycle

9. At sexual maturity
The ovaries and testes produce haploid gametes by meiosis
During fertilization
- These gametes, sperm and ovum, fuse, forming a diploid zygote
The zygote develops into an adult organism
Most cells in the body produced by mitosis.
Only gametes are produced by meiosis.

10. MEIOSIS
Meiosis reduces the number of chromosome sets from diploid to haploid
Meiosis
Takes place in two sets of divisions, meiosis I and meiosis II
Cell division results in 4 haploid (1N) cells from a single parent cell
N = 23 in humans

11. The Stages of Meiosis Meiosis I
An overview of meiosis
Meiosis I
Reduces the number of chromosomes from diploid to haploid
Meiosis II
Produces four haploid daughter cells
Interphase
Homologous pair
of chromosomes
in diploid parent cell
Chromosomes
replicate
Homologous pair of replicated chromosomes
Sister
chromatids
Diploid cell with
replicated
chromosomes 1 2
Homologous
separate
Haploid cells with
replicated chromosomes
Sister chromatids
Haploid cells with unreplicated chromosomes
Meiosis I
Meiosis II

12. Interphase and meiosis I
Centrosomes
(with centriole pairs)
Sister
chromatids
Chiasmata
Spindle
Tetrad
Nuclear
envelope
Chromatin
Centromere
(with kinetochore)
Microtubule
attached to
kinetochore
Tertads line up
Metaphase
plate
Homologous
chromosomes
separate
Sister chromatids
remain attached
Pairs of homologous
chromosomes split up
Chromosomes duplicate
Homologous chromosomes
(red and blue) pair and exchange
segments; 2n = 6 in this example
INTERPHASE
MEIOSIS I: Separates homologous chromosomes
PROPHASE I
METAPHASE I
ANAPHASE I

13. Chiasmata indicate point where the two chromatids are intertwined.
Prophase I
Homologous chromosomes become closely associated in synapsis, exchange segments via crossing over, and then separate.
Chiasmata indicate point where the two chromatids are intertwined.
chiasmata

14. Metaphase I
Spindle microtubules attach to kinetochore proteins on the outside of each centromere.
Joined pairs of homologues lines up on metaphase plate.
orientation of each pair is random

15. Random orientation of chromosomes in Metaphase
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Random orientation of chromosomes in Metaphase

17. function as one. Microtubules can attach to only one side of
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Homologues do not pair; kinetochores of sister chromatids remain separate; microtubules attach to both
kinetochores on
opposite sides of the
centromere.
Chiasmata hold homologues together. The kinetochores of sister chromatids fuse and
function as one. Microtubules
can attach to only one side of
each centromere.
Meiosis I
Mitosis
Metaphase I
Metaphase
Chiasmata
Microtubules pull the
homologous chromosomes
apart, but sister
chromatids are
held together.
Microtubules pull sister
chromatids apart.
Anaphase I
Anaphase

18. Anaphase I
Spindle fibers begin to shorten and pull whole centromeres toward poles.
Each pole receives a member of each homologous pair.
complete set of haploid chromosomes
random orientation results in independent assortment

20. Telophase I
Chromosomes are segregated into two clusters; one at each pole.
Nuclear membrane re-forms around each daughter cell.
Sister chromatids are no longer identical due to crossing over.

22. The two newly divided cells
DO NOT undergo REPLICATION before MEIOSIS II, they
Simply continue to the second meiotic division.
NO INTERPHASE II

23. Second Meiotic Division
Meiosis II resembles normal mitotic division.
prophase II - nuclear envelope breaks down and second meiotic division begins
metaphase II - spindle fibers bind to both sides of centromere and line up in the middle
anaphase II - spindle fibers contract and sister chromatids move to opposite poles
telophase II - nuclear envelope re-forms and chromosomes uncoil
Final result - four haploid cells

24. PROPHASE II
METAPHASE II

25. ANAPHASE II
TELOPHASE II

27. A Comparison of Mitosis and Meiosis
Meiosis and mitosis can be distinguished from mitosis
By three events in Meiosis l

28. 1. Synapsis and crossing over
Homologous chromosomes physically connect and exchange genetic information. Chromatids are not identical.

29. 2. Tetrads on the metaphase plate
At metaphase I of meiosis, paired homologous chromosomes (tetrads) are positioned on the metaphase plates
TETRADS

30. 3. Separation of homologues
At anaphase I of meiosis, homologous pairs move toward opposite poles of the cell
In anaphase II of meiosis, the sister chromatids separate.

31. (before chromosome replication)
MITOSIS
MEIOSIS
Prophase
Duplicated chromosome
(two sister chromatids)
Chromosome
replication
Parent cell
(before chromosome replication)
Chiasma (site of
crossing over)
MEIOSIS I
Prophase I
Tetrad formed by
synapsis of homologous
chromosomes
Metaphase
Chromosomes
positioned at the
metaphase plate
Tetrads
Metaphase I
Anaphase I
Telophase I
Haploid
n = 3
MEIOSIS II
Daughter
cells of
meiosis I
Homologues
separate
during
anaphase I;
sister
chromatids
remain together
Daughter cells of meiosis II n
Sister chromatids separate during anaphase II
Anaphase
Telophase
Sister chromatids
separate during
anaphase 2n
Daughter cells
of mitosis
2n = 6

32. SIGNIFICANT RESULTS OF MEIOSIS:
Four haploid cells are produced because two rounds of division follow one round of chromosome replication = sex cells or gametes
Daughter cells are NOT identical to their parent(s) like in Mitosis
Causes genetic variation (recombination) in offspring

33. Ways in which Meiosis creates Genetic Variation
a. crossing over - recombination
b. independent assortment of chromosomes into gametes
c. Random union of gametes to form a zygote

34. Independent Assortment

35. Random fertilization
At least 8 million combinations from Mom, and another 8 million from Dad …
>64 trillion combinations for a diploid zygote!!!

36. Formation of Gametes
In humans meiosis occurs in the testes and ovaries.
In the testes, meiosis results in the production of four sperm = SPERMATOGENESIS
In the ovaries, meiosis results in the production of one egg and three polar bodies that disintegrate. Thus, all the cytoplasm goes to the one egg cell and it is larger than sperm = OOGENESIS

38. Mutations
A mutation is a permanent change in the DNA sequence of a gene. Sometimes mutations in DNA can cause changes in the way a cell behaves. This is because genes contain the instructions necessary for a cell to work. If some of the instructions to the cell are wrong, then the cell may not know what it is supposed to do!

39. There are two ways in which DNA can become mutated:
1) Mutations can be inherited. This means that if a parent has a mutation in his or her DNA, then the mutation is passed on to his or her children – GERM CELL MUTATION
2) Mutations can be acquired. This happens when environmental agents damage DNA, or when mistakes occur when a cell copies its DNA prior to cell division – SOMATIC CELL MUTATION

40. Mutations may occur on the whole chromosome or within a single gene.
CHROMOSOME MUTATIONS
Changes in the structure of a chromosome
Types:
1) deletion – loss of a piece of chromosome due to chromosome breakage.
2) inversion – chromosome breaks off and then reattaches to the same chromosome
3) translocation – chromosome breaks off and reattaches to a different chromosome
4) nondisjunction – failure of chromosome to separate from its homologue during meiosis

41. CHROMOSOME MUTATIONS Changes in the structure of a chromosome Types:
1) deletion – loss of a piece of chromosome due to chromosome breakage.
2) inversion – chromosome breaks off and then reattaches to the same chromosome
3) translocation – chromosome breaks off and reattaches to a different chromosome
4) nondisjunction – failure of chromosome to separate from its homologue during meiosis

42. Lead to abnormal conditions like Down’s syndrome – extra Chromosome #21

43. Gene Mutations
Involve large segments of DNA or a single nucleotide within a codon.
Types
1. Point mutation – substition, addition, or removal of a single nucleotide
2. Frame shift – addition or deletion of a single nucleotide
Much more harmful because affects entire protein