1. Meiosis and Sexual Reproduction
Chapter 12
Meiosis and Sexual Reproduction

2. The diagram can be used to illustrate a process directly
involved in?
A) tissue repair
B) Meiosis
C) Recombination
D) sexual reproduction

3. A sperm cell of an alligator has 16 chromosomes
A sperm cell of an alligator has 16 chromosomes. What is the total number of chromosomes normally present in a stomach cell of this alligator?
A) 8 B) C) D) 48

4. A) meiotic cell division B) mitotic cell division C) fertilization
The diagram represents a single-celled organism, such as an ameba, undergoing the changes shown. This could best be described as?
A) meiotic cell division
B) mitotic cell division
C) fertilization
D) recombination

5. 12.1 Why Sex?
In asexual reproduction, a single individual gives rise to offspring that are identical to itself and others
In sexual reproduction, two individuals mix their genetic material

6. Introducing Alleles
Somatic (body) cells of sexually- reproducing eukaryotes contain pairs of homologous chromosomes:
One from the mother and one from the father
Homologous chromosomes:
Carry genes (one from the mother and one from the father) of the same characteristics
Different forms of the same gene are called alleles

7. Introducing Alleles
Paired genes on homologous chromosomes may vary in DNA sequences as alleles
Arise by mutation
Are the basis of differences in shared traits
Offspring of sexual reproducers inherit new combinations of parental alleles
Results in new combinations of traits

8. Introducing Alleles Genes occur in pairs on homologous chromosomes.
The members of each pair of genes may be identical, or they may differ slightly, as alleles.
Figure 12.2 Genes on chromosomes. Different forms of a gene are called alleles. A B

9. On the Advantages of Sex
Sex mixes up the genes of two parents, so offspring have unique combinations of traits
Diversity offers a better chance of surviving environmental change than clones

10. 12.2 Why Is Meiosis Necessary for Sexual Reproduction?
Asexual reproduction produces clones
Sexual reproduction mixes up alleles from two parents
Involves the fusion of gametes, mature reproductive cells
Meiosis: a nuclear division mechanism that occurs in reproductive cells of eukaryotes

11. Meiosis Halves the Chromosome Number
Gametes are specialized cells that are the basis of sexual reproduction
Derive from germ cells: immature reproductive cells
All gametes are haploid (n), but they differ in other details
Gamete formation differs among plants and animals

12. testis ovary anther ovary
Figure 12.4 {Animated} Examples of reproductive organs in A animals and B plants.
ovary B

13. Meiosis Halves the Chromosome Number
The two consecutive nuclear divisions are called meiosis I and meiosis II.

14. Meiosis Halves the Chromosome Number
In males, the diploid germ cell develops into sperm
In females, a diploid germ cell becomes an egg

15. Meiosis Halves the Chromosome Number
Occurs in the gametes – mature reproductive cells
Gametes are haploid (n)
Their chromosome number is half of the diploid (2n) number
Meiosis is a nuclear division

16. Meiosis Halves the Chromosome Number
Nuclear division that halves the chromosome number in the reproductive cells
Ensures that offspring have the same number of chromosomes as the parents

17. Meiosis Halves the Chromosome Number
DNA replication occurs prior to meiosis:
The nucleus is diploid (2n) with two sets of chromosomes, one from each parent
During meiosis, chromosomes of a diploid nucleus become distributed into four haploid nuclei
Meiosis halves diploid (2n) chromosome to the haploid (n) number for forthcoming gametes

18. Meiosis Halves the Chromosome Number1 2
Figure 12.5 How meiosis halves the chromosome number.
1 Chromosomes are duplicated before meiosis begins. During meiosis I, each chromosome in the nucleus pairs with its homologous partner. The nucleus contains two of each chromosome, so it is diploid (2n).
2 Homologous partners separate and are packaged into two new nuclei. Each new nucleus contains one of each chromosome, so it is haploid (n). The chromosomes are still duplicated.
3 Sister chromatids separate in meiosis II and are packaged into four new nuclei. Each new nucleus contains one of each chromosome, so it is haploid (n). The chromosomes are now unduplicated. 3

19. Fertilization Restores the Chromosome Number
Haploid gametes form by meiosis
The diploid chromosome number is restored at fertilization
Two haploid gametes fuse a zygote is formed
A zygote is a cell formed by the fusion of two gametes
The first cell of a new individual

20. Haploid gametes (n  23) Egg cell n n Sperm cell MEIOSIS FERTILIZATION
Multicellular
diploid adults
(2n  46)
Diploid
zygote
(2n  46)
Figure 8.12 The human life cycle 2n
MITOSIS
and development
Key
Haploid (n)
Diploid (2n)
Figure 8.12

21. Fertilization Restores the Chromosome Number
If meiosis did not precede fertilization, the chromosome number would double with every generation
Chromosome number changes can have drastic consequences in animals
An individual’s set of chromosomes is like a fine-tuned blueprint that must be followed exactly to have normal functions

22. Figure 8.13-3 Homologous chromosomes separate. Chromosomes duplicate.
Sister
chromatids
separate.
Pair of homologous
chromosomes in
diploid parent cell
Duplicated pair of
homologous
chromosomes
Sister
chromatids
Figure 8.13 How meiosis halves chromosome number. (Step 3)
INTERPHASE BEFORE MEIOSIS
MEIOSIS I
MEIOSIS II
Figure

23. (before chromosome duplication)
MITOSIS
MEIOSIS
Prophase
Prophase I
MEIOSIS I
Chromosome
duplication
Chromosome
duplication
Duplicated chromosome
(two sister chromatids)
Parent cell
(before chromosome duplication)
2n  4
Homologous
chromosomes come
together in pairs.
Site of crossing over
between homologous
(nonsister) chromatids
Metaphase
Metaphase I
Chromosomes
align at the
middle of the
cell.
Homologous pairs
align at the middle
of the cell.
Anaphase
Telophase
Anaphase I
Telophase I
Chromosome with two
sister chromatids
Sister
chromatids
separate
during
anaphase.
Figure 8.15 Comparing mitosis and meiosis
Homologous
chromosomes
separate during
anaphase I;
sister chromatids
remain together.
Haploid
n  2 2n 2n
Daughter
cells of meiosis I
Daughter cells
of mitosis
MEIOSIS II
Sister chromatids
separate during
anaphase II. n n n n
Daughter cells of meiosis II
Figure 8.15

24. 12.4 How Meiosis Introduces Variations in Traits
Two events in meiosis introduce novel combinations of alleles into gametes:
Crossing over in prophase I
Segregation of chromosomes into gametes
Along with fertilization, these events contribute to the variation in combinations of traits among the offspring of sexually reproducing species

25. Crossing Over in Prophase I
Chromatids of homologous chromosomes condense and align along their length and exchange segments
Introduces novel combinations of traits among offspring

26. Figure 8.18-5 Prophase I Duplicated pair of of meiosis homologous
chromosomes
Homologous chromatids
exchange corresponding
segments.
Chiasma, site of
crossing over
Metaphase I
Spindle
microtubule
Sister chromatids
remain joined at their
centromeres.
Metaphase II
Figure 8.18 The results of crossing over during meiosis for a single pair of homologous chromosomes. (Step 5)
Gametes
Recombinant
chromosomes
combine genetic
information from
different parents.
Recombinant chromosomes
Figure

27. Crossing Over During meiosis, chromosomes cross over each other
This allows for genetic shuffling and increases genetic variability
Offspring have a unique combination of genes inherited from both parents

28. Chromosome Segregation
When homologous chromosomes separate in meiosis I, one of each chromosome pair goes to each of the two new nuclei
For each chromosome pair, the maternal or paternal version is equally likely to end up in either nucleus

29. Independent Assortment of Chromosomes or Chromosome Segregation
Metaphase of
meiosis I
Metaphase
of meiosis II
Figure 8.16 Results of alternative arrangements of chromosomes at metaphase of meiosis I. (Step 3)
Gametes
Combination a
Combination b
Combination c
Combination d

30. Chromosome Segregation
Human gametes have 23 pairs of homologous chromosomes
Each time a human germ cell undergoes meiosis the four gametes that form end up with one of 8,388,608 (or 223) possible combinations of homologous chromosomes
Crossing over increases these combinations
If each gamete represents one of 8,388,608 different chromosome combinations, at fertilization, humans would have 8,388,608 × 8,388,608, or more than 70 trillion, different possible chromosome combinations.

31. Chromosome Segregation
The chance that the maternal or paternal version of any chromosome will end up in a particular nucleus is 50 percent
Due to the way the spindle segregates the homologous chromosome during meiosis I
In prophase I, chromosomes are attached to spindle poles
Each homologous partner becomes attached to opposite spindle poles

32. 12.6 Application: How To Survive 80 Million Years Without Sex
In nature, there are a few all-female species of fishes, reptiles, and birds but not mammals
Females have been reproducing themselves for 80 million years through cloning themselves
Asexual reproduction is a poor long-term strategy because it lacks crossing over that leads to genetic diversity

33. How To Survive 80 Million Years Without Sex
Figure A bdelloid rotifer. All of these tiny animals are female.

34. NONDISJUNCTION IN MEIOSIS I NONDISJUNCTION IN MEIOSIS II
Pair of homologous
chromosomes fails
to separate.
Meiosis II
Nondisjunction:
Pair of sister
chromatids
fails to separate.
Gametes
Figure 8.20 Two types of nondisjunction. (Step 3)
Number of
chromosomes
n  1
n  1
n – 1
n – 1
n  1
n – 1 n n
Abnormal gametes
Abnormal gametes
Normal gametes

35. Abnormal egg cell with extra chromosome n  1 Normal sperm cell
Figure 8.21 Fertilization after nondisjunction in the mother.
Normal
sperm cell
Abnormal zygote
with extra
chromosome 2n  1
n (normal)

36. LM
Figure 8.22 Trisomy 21 and Down syndrome.
Chromosome 21

37. Infants with Down syndrome90 80 70 60
Infants with Down syndrome
(per 1,000 births) 50 40 30 20
Figure 8.23 Maternal age and Down syndrome. 10 20 25 30 35 40 45 50
Age of mother

38. Why crossing over is important…
A male bird with large wings and weak muscles mated with a female bird with small wings and strong muscles.
Describe all the possible characteristics that could occur in their offspring.
Would any combination(s) be advantageous?
Would any combination(s) be disadvantageous?
How could these variations affect the survival of the organisms?

39. 10/8 Protein Synthesis 9 10/10 Genes 10 10/15 Mitosis 11 10/17 Meiosis12
10/22
Genotype &
Phenotype 13
10/24
Biotechnology 14
10/29
Exam #2
10/31
Begin Evolution