1. Meiosis and Sexual Life Cycles
Chapter 8

2. Genetics
Heredity- is the transmission of traits from one generation to the next.
Variation- shows that offspring differ somewhat in appearance from parents and siblings.

3. Genetics Study of heredity and hereditary variation
Offspring acquire genes from parents by inheriting chromosome
Genes:
Are the units of heredity
Are segment of DNA
We inherit one set of chromosomes from our mother and one from our father

4. Overview of meiosis
Meiosis is the type of nuclear division that reduces the chromosome number from the diploid number (2n) to the haploid number (n)
Diploid (2n)- refers to the total number of chromosomes, which insist in 2 sets
Haploid (n)- is half the diploid number, or a single set of chromosomes

6. Chromosomes
In humans
Each somatic cell has 46 chromosomes, made up of two sets
2n=46
During meiosis haploid (n) cells are created (sperm and egg)
Gametes
Gamete + Gamete = zygote

8. Chromosomes In diploid body cells, chromosomes occur in pairs
Humans have 23 different types of chromosomes
homologous chromosomes
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)
Genes for the traits located in same position
Different sequences=alleles

9. Homologous chromosomes
Karyotype- a map showing the pairs of chromosomes
22 pairs of autosomes
1 pair of sex chromosomes

10. Meiosis Reduces the number of chromosome sets from diploid to haploid
Takes place in two set of divisions, meiosis I and meiosis II
Meiosis I
Reduces the number of chromosomes from diploid to haploid
Meiosis II
Produces four haploid daughter cells

11. Overview Meiosis I Meiosis II Overview of Meiosis
Chromosomes are replicated prior to meiosis I
Each chromosome consists of two identical sister chromatids
Homologous chromosomes pair up – synapsis
Homologous pairs align themselves against each other side by side at the metaphase plate
The two members of a homologous pair separate
Each daughter cell receives one duplicated chromosome from each pair
Meiosis II
DNA is not replicated between meiosis I and meiosis II
Sister chromatids separate and move to opposite poles
The four daughter cells contain one daughter chromosome from each pair
Each daughter chromosome consists of a single chromatid
The daughter cells are haploid

14. Meiosis Compared to Mitosis
Biology, 9th ed,Sylvia Mader
Chapter 10
Meiosis Compared to Mitosis
Meiosis
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
Meiosis & Sexual Reproduction

15. Genetic Variation Significance of genetic variation:
Asexual reproduction produces genetically identical clones
Sexual reproduction causes genetic recombinations among members of a population
Asexual reproduction is advantageous when the environment is stable
However, if the environment changes, genetic variability introduced by sexual reproduction may be advantageous
Some offspring may have a better chance of survival

16. Crossing Over
Exchange of genetic material between non-sister chromatids during meiosis I
At synapsis, a nucleoprotein lattice appears between homologues
Then homologues separate and are distributed to different daughter cells

17. Crossing Over Occurs During Meiosis I
Biology, 9th ed,Sylvia Mader
Chapter 10
Crossing Over Occurs During Meiosis I
nucleoprotein lattice
sister chromatids
of a chromosome
sister chromatids
of its homologue A A a a A a B B b b B b
chiasmata of
nonsister
chromatids
1 and 3 c C C c C c D D d d D d 1 2 3 4 1 2 3 4 1 2 3 4
Bivalent
forms
Crossing-over
has occurred
Daughter
chromosomes a. b. c. d.
Meiosis & Sexual Reproduction

18. Independent assortment
When homologous chromosome pairs align at the metaphase plate:
They separate in a random manner
Causes random mixing of blocks of alleles into gametes

19. Problems
Nondisjunction- failure to separate chromosomes into daughter cells
Results in some new cells with 2n + 1 (3) or 2n – 1 (1)

20. Nondisjunction Most common is Down Syndrome
Recognized by these characteristics:
short stature
eyelid fold
flat face
stubby fingers
wide gap between first and second toes

21. Nondisjunction in sex chromosomes
Results from inheriting too many or too few X or Y chromosomes
Nondisjunction during oogenesis or spermatogenesis
Turner syndrome (XO)
Female with a single X chromosome
Short, with broad chest and widely spaced nipples
Can be of normal intelligence and function with hormone therapy
Klinefelter syndrome (XXY)
Male with underdeveloped testes and prostate; some breast overdevelopment
Long arms and legs; large hands
Near normal intelligence unless XXXY, XXXXY, etc.
No matter how many X chromosomes are present, the presence of a chromosome Y renders the individual male

23. Other changes Deletion One or both ends of a chromosome breaks off
Two simultaneous breaks lead to loss of an internal segment
Duplication
Presence of a chromosomal segment more than once in the same chromosome
Translocation
A segment from one chromosome moves to a non-homologous chromosome
Follows breakage of two non-homologous chromosomes and improper re-assembly
Inversion
Occurs as a result of two breaks in a chromosome
The internal segment is reversed before re-insertion
Genes occur in reverse order in the inverted segment

24. Chromosome mutations

25. Differences in the life cycle
In plants, haploid multicellular “individuals” alternate with diploid multicellular “individuals” (alternation of generations)
The haploid individual:
Is called the gametophyte
The diploid individual:
Is called the sporophyte
Mosses are haploid for most of their life cycle
In fungi and most algae, only the zygote is diploid
Ferns & higher plants are diploid for most of their life cycles
In plants, algae, and fungi gametes are produced by haploid individuals

26. Plant Life Cycle