1. Meiosis

2. Meiosis: Halves the Chromosome Number
Special type of cell division
Used only for sexual reproduction
Halves the chromosome number prior to fertilization
Parents diploid
Meiosis produces haploid gametes
Gametes fuse in fertilization to form diploid zygote
Becomes the next diploid generation

3. One process, three aspects
Meiosis
Sexual reproduction
Heredity

4. Heredity DNA Genes = specific sequences of nucleotides
Inheritance possible because
a. DNA replication is an exact process
b. Sperm and ova are combined in nucleus of fertilized egg
Transmission of genes depends on behavior of chromosomes

5. Chromosome
Single long DNA molecule; highly folded and coiled with proteins
Contains genetic information arranged in linear fashion
Contains 100s or 1000s of genes, each in a specific region of DNA; locus
Each species has a specific number of chromosomes

6. Homologous Pairs of Chromosomes
In diploid body cells chromosomes occur in pairs
Humans have 23 different types of chromosomes
Diploid cells have two of each type
Chromosomes of the same type are said to be homologous
They have the same length
Their centromeres are positioned in the same place
One came from the father (the paternal homolog) the other from the mother (the maternal homolog)
When stained, they show similar banding patterns
Because they have genes controlling the same traits at the same positions

7. Homologous Chromosomes

8. Homologous Pairs of Chromosomes
Homologous chromosomes have genes controlling the same trait at the same position
Each gene occurs in duplicate
A maternal copy from the mother
A paternal copy from the father
Many genes exist in several variant forms in a large population
Homologous copies of a gene may encode identical or differing genetic information
The variants that exist for a gene are called alleles
An individual may have:
Identical alleles for a specific gene on both homologs (homozygous for the trait), or
A maternal allele that differs from the corresponding paternal allele (heterozygous for the trait)

9. Asexual vs. sexual reproduction
Asexual Reproduction
Sexual Reproduction
Single individual is sole parent.
Two parents give rise to offspring.
Single parent passes on all its
genes to its offspring.
Each parent passes on half its
genes to its offspring.
Offspring are genetically
identical to the parent.
Offspring have a unique
combination of genes inherited
from both parents
Results in a clone. Rarely
differences occur and as a
result of mutation.
Results in greater genetic
variation; offspring vary
genetically from their siblings
and parents.

10. Sexual Life Cycles Alternation of generations The only haploid cells
spores
The only diploid stage

11. Terms to review Life cycle Somatic cell
Homologous chromosome pairs (homologues)
Except for X/Y they carry same genetic loci (locus)
Karyotype
Use human lymphocytes
Can find chromosomal abnormalities
Autosomes (22 pairs)
Sex chromosomes (1 pair)
Diploid (2n)
Haploid (n); gametes
Sperm cells/ova (23 chromosomes)
Only cells in body not produced by mitosis
Fertilization
Zygote

12. Overview of Meiosis

13. Phases of Meiosis I: Prophase I & Metaphase I
Meiosis I (reductional division):
Prophase I
Each chromosome internally duplicated (consists of two identical sister chromatids)
Homologous chromosomes pair up – synapsis
Physically align themselves against each other end to end
End view would show four chromatids – Tetrad
Metaphase I
Homologous pairs arranged onto the metaphase plate

14. Phases of Meiosis I: Anaphase I & Telophase I
Meiosis I (cont.):
Anaphase I
Synapsis breaks up
Homologous chromosomes separate from one another
Homologues move towards opposite poles
Each is still an internally duplicate chromosome with two chromatids
Telophase I
Daughter cells have one internally duplicate chromosome from each homologous pair
One (internally duplicate) chromosome of each type (1n, haploid)

15. Phases of Meiosis I: Cytokinesis I & Interkinesis
Meiosis I (cont.):
Cytokinesis I
Two daughter cells
Both with one internally duplicate chromosome of each type
Haploid
Meiosis I is reductional (halves chromosome number)
Interkinesis
Similar to mitotic interphase
Usually shorter
No replication of DNA

16. Genetic variations Sexual life cycles --> variations
Independent assortment of chromosomes
Crossing over during prophase I
Random fusion of gametes during fertilization

17. Genetic Variation: Crossing Over
Meiosis brings about genetic variation in two key ways:
Crossing-over between homologous chromosomes, and
Independent assortment of homologous chromosomes
1. Crossing Over:
Exchange of genetic material between nonsister chromatids during meiosis I
At synapsis, a nucleoprotein lattice (called the synaptonemal complex) appears between homologues
Holds homologues together
Aligns DNA of nonsister chromatids
Allows crossing-over to occur
Then homologues separate and are distributed to different daughter cells

18. Crossing Over

19. Crossing over Protein strands hold homologues in register with
each other
DNA loops
Protein cross
bridges

20. Crossing over (2) Harlequin chromosomes demonstrate reciprocal
exchange between
sister chromatids

21. Genetic Variation: Independent Assortment
When homologues align at the metaphase plate:
They separate in a random manner
The maternal or paternal homologue may be oriented toward either pole of mother cell
Causes random mixing of blocks of alleles into gametes

22. Independent Assortment

23. Independent assortment
Homologous pairs
behave independently
of each other
Consider the possible combinations of 23 pairs !
Fertilization = (223)2 = 64 x 1012 variations
2n = 223 = 8 x 106 (1 parent)

24. Recombination

25. Genetic Variation: Fertilization
When gametes fuse at fertilization:
Chromosomes donated by the parents are combined
In humans, (223)2 = 70,368,744,000,000 chromosomally different zygotes are possible
If crossing-over occurs only once
(423)2, or 4,951,760,200,000,000,000,000,000,000 genetically different zygotes are possible

26. Genetic Variation: Significance
Asexual reproduction produces genetically identical clones
Sexual reproduction cause novel genetic recombinations
Asexual reproduction is advantageous when environment is stable
However, if environment changes, genetic variability introduced by sexual reproduction may be advantageous

27. Phases of Meiosis II: Similar to Mitosis
Metaphase II
Overview
Unremarkable
Virtually indistinguishable from mitosis of two haploid cells
Prophase II – Chromosomes condense
Metaphase II – chromosomes align at metaphase plate

28. Sister chromatids separate and become daughter chromosomes
Anaphase II
Centromere dissolves
Sister chromatids separate and become daughter chromosomes
Telophase II and cytokinesis II
Four haploid cells
All genetically unique

29. Meiosis I & II in Plant Cells

30. Meiosis versus Mitosis
Requires two nuclear divisions
Chromosomes synapse and cross over
Centromeres survive Anaphase I
Halves chromosome number
Produces four daughter nuclei
Produces daughter cells genetically different from parent and each other
Used only for sexual reproduction
Mitosis
Requires one nuclear division
Chromosomes do not synapse nor cross over
Centromeres dissolve in mitotic anaphase
Preserves chromosome number
Produces two daughter nuclei
Produces daughter cells genetically identical to parent and to each other
Used for asexual reproduction and growth

31. Meiosis Compared to Mitosis

33. Meiosis I Compared to Mitosis

34. Meiosis II Compared to Mitosis

35. Life Cycle Basics:Plants
Haploid multicellular “individuals” alternate with diploid multicellular “individuals”
The haploid individual:
Known as the gametophyte
May be larger or smaller than the diploid individual
The diploid individual:
Known as the sporophyte
May be larger or smaller than the haploid individual

36. Life Cycle Basics:Plants
Mosses are haploid most of their life cycle
Ferns & higher plants have mostly diploid life cycles
In fungi and most algae, only the zygote is diploid
In plants, algae, & fungi, gametes produced by haploid individuals

37. Life Cycle Basics: Animals
In familiar animals:
“Individuals” are diploid; produce haploid gametes
Only haploid part of life cycle is the gametes
The products of meiosis are always gametes

38. Life Cycle Basics: Animals
Meiosis occurs only during gametogenesis
Production of sperm
Spermatogenesis
All four cells become sperm
Production of eggs
Oogenesis
Only one of four nuclei get cytoplasm
Becomes the egg or ovum
Others wither away as polar bodies

39. The Human Life Cycle Sperm and egg are produced by meiosis
A sperm and egg fuse at fertilization
Results in a zygote
The one-celled stage of an individual of the next generation
Undergoes mitosis
Results in multicellular embryo that gradually takes on features determined when zygote was formed

40. All growth occurs as mitotic division
The Human Life Cycle
All growth occurs as mitotic division
As a result of mitosis, each somatic cell in body
Has same number of chromosomes as zygote
Has genetic makeup determined when zygote was formed

41. The Human Life Cycle

42. Gametogenesis in Mammals

43. Oogenesis
Long interkinesis

44. Spermatogenesis

45. Meiosis

46. Mitosis