A. Genes, DNA, and Chromosomes : overview
1. Definitions
a. Heredity: the transmission of traits from one generation to the next, also heritance.
b. Genetics: the scientific study of heredity and hereditary variation.
c. Genes: Parents endow their offspring with coded information in the form of hereditary units
d. Locus: A gene’s specific location along the length of a chromosomes
2. Linkages between these terms
- From parents, (mother and father) their genes heredity to the daughter or son. From genetics, genes were specified along the length of a chromosomes and we called the location of gene is locus.
3. Relationships to sexual reproduction
- Asexual reproduction only requires a single individual as sole parents and copies of all its genes to its offspring. The genomes of the offspring are virtually exact copies of the parent’s genome.
- Sexual reproduction requires genes heredities from two parents and there are variation therefore genetics were important to study.
B. Sexual vs Asexual reproduction
1. Asexual reproduction
a. Definition: only organisms that reproduce asexually produce offspring that are exact copies of themselves.
b. Examples: hydra (relatively simple animal, reproduces by budding)
Redwoods (each tree grows from a single parent tree)
c. Advantages: only need 1 parent, offspring are identical to the parent, good genetic traits are conserved and reproduced.
d. Limitation: no variation (evolution), no protection or development for attacking by dieses or pest.
2. Sexual reproduction
a. Definition: two parents five rise to offspring that have unique combinations of genes inherited from the two parents.
b. Examples: human, pea plants, dinosaur etc.
c. Advantages: may be an improvement over both parents, evolution
d. Limitation: need two parents, good gene can be lost, may not be improvement over the parents.
C. The Variety of sexual life cycles
1. Human life cycle: A life cycle is the generation-to-generation sequence of stages in the reproductive history of an organism, from conception to production of its own offspring.
a. Chromosome number (if the number of chromosomes in a single set is represented by n)
1) Diploid: any cell with two chromosomes sets; has a diploid number of chromosomes, 2n.
2) Haploid: each has a haploid number of chromosomes (n).
3) Polyploids: any cell with more than two chromosomes sets.
b. Gametes: reproductive cells, the vehicles that transmit genes from one generation to the next.
1) Sperm: may contains an X or Y chromosomes (from father)
2) Egg: contains an X chromosomes (from mother)
c. Fertilization
- The union of gametes, culminating in fusion of their nuclei.
d. Zygote
- The resulting fertilized egg, is diploid because it contains twp haploid sets of chromosomes bearing genes representing the maternal and paternal family lines.
2. Life cycle comparisons
a. Animals
- The number of chromosome sets doubles at fertilization, but is halved during meiosis. For humans, the number of chromosomes in a haploid cells, consisting of one set; the number of chromosomes in the diploid zygote and all somatic cells arising from consisting of two sets.
b. Some fungi and algae
- After gametes fuse and form a diploid zygote, meiosis occurs without a multicellular diploid offspring developing. Meiosis produces not gametes but haploid cells that then divided by mitosis and give rise to either unicellular descendants or a haploid multicellular adult organism. The haploid organism carries out further mitoses, producing the cells that develop into gametes. The only diploid stage found in these species is the single-called zygote.
c. Plants (alternation of generation)
1) Sporophyte phase
- The muticellular diploid stage.
- Meiosis in the sporophyte produces haploid cells called spores.
2) Gametophyte phase
- Fusion of two haploid gametes at fertilization results in a diploid zygote. The sporophyte generation produces a gametophyte as its offspring, and the gametophyte generation produces the next sporophyte generation
D. Meiosis
1. Meiosis Ⅰ
- Has two cell divisions.
a. Prophase Ⅰ
1) General characteristics
- Basically it is the same as in prophase of mitosis.
- Longest phase of division
- Chiasmata observed
2) Synapsis – tetrad formation
- Synapsis: homologous chromosomes become connected, occurs as the chromosomes condense.
3) Chiasma
- A chiasma is the physical manifestation of crossing over
b. Metaphase Ⅰ
1) Characteristics
- Tetrads or bivalents align on the metaphase plate
- Centromeres of homologous pair point toward opposite pole.
2) Chromosomes alignment
- Chromosomes are positioned on the metaphase plate as pairs of homologs, rather than individual chromosomes, as in metaphase of mitosis.
c. Anaphase Ⅰ
1) Tetrad separation
- Homologous pairs separate
- Maternal and paternal chromosomes are now separated randomly
2) Fate of duplicate chromosomes
- Duplicate chromosomes are still attached at the centromeres.
d. Telophase Ⅰ
1) Characteristic
- Similar to telophas in mitosis
- Chromosomes may or may not unwind to chromatin
2) Cytokinesis
- Separates cytoplasm and 2 cells are formed.
2. Interkinesis
a. Characteristics
- No DNA synthesis occurs.
- May appear similar to interphase of mitosis
b. Duration
- May last for years, or the cell may go immediately into Meiosis Ⅱ
3. Meiosis Ⅱ
a. Prophase Ⅱ
b. Metaphase Ⅱ
c. Anaphase Ⅱ
d. Telophase Ⅱ
e. Cytokinesis
4. Results of meiosis
- 4 cells produced
- Chromosomes number halved
- Gametes or sex cells made
- Genetic variation increased
E. Comparison of meiosis and mitosis
1. DNA replication
Mitosis | Meiosis |
Occurs during interphase before mitosis begins | Occurs during interphase before meiosis Ⅰ begins |
2. Number of divisions
Mitosis | Meiosis |
One, including prophase, metaphase, anaphase, and telophase | Two, each including prophase, metaphase, anaphase, and telophase |
3. Synapsis
Mitosis | Meiosis |
Does not occur | Occurs during prophase Ⅰ along with crossing over between nonsister chromatids; resulting chiasmata hold paris together due to sister chromatid cohesion |
4. Number of daughter cells
5. Ploidy levels
Mitosis | Meiosis |
each diploid and genetically identical to the parent cell | each haploid, containing half as many chromosomes as the parent cell; genetically different from the parent cell and from each other |
6. Importances and uses
Mitosis | Meiosis |
Enable multicellular adult to arise from zygote;produces cells for growth, repair, and in some species, asexual reproduction | Produces gametes; reduces number of chromosomes by half and introduces genetic variability among the gametes |
F. Sexual sources of genetic variation
1. Independent assortment of chromosomes
a. Definition
- Because each homologous pair of chromosomes is positioned independently of the other pairs at metaphase Ⅰ, the first meiotic division results in each pair sorting its maternal and paternal homologs into daughter cells independently of every other pair
b. Mechanism
- The chance to inherit a single chromosomes of each pair is 1/2
c. Importance
d. Human example
- There are 23 pairs of chromosomes in humans
2. Crossing over
a. Definition
- The exchange of sister chromatid material during synapsis
b. Mechanism
- Very common during meiosis
- Frequency can be used to map the position of genes on chromosomes
c. Importance
- Breaks old linkage groups
- Creates new linkage groups increases genetic variation
3. Random fertilization
a. Definition
- The choice of which sperm fuses with which egg is random
b. Mechanism
- Therefore, which 8,388,608 kinds of sperms and 8,388,608 kinds of eggs, the number of possible combinations of offspring is over 64 million kinds
c. Importance
- We are really unique
meiotic nondisjunction : gamets with an abnormal chromosome number can arise by nondisjunction in either meiosis 1 and meiosis 2. In which the members of a pair of homologous chromosomes do not move apart properly during meiosis 1 or sister chromatids fail to separate during meiosis 2. In these case, one gamete receives two of the same type of chromosomes and another gamete receives no copy. 
this diagram shows me the process of what Mendel had done to Pea plants. First he cut out the stamens from the purple flower in order to prevent self-polinating. Second he purposely moved sperm- bearing pollen from stamens forom white to egg-bearing carpel of purple. and he planted the result seeds. The result was all purple flowers appeared and he kept on doing the same process with the F1 generagion again.