1. Meiosis & Sexual Life Cycles
Chapter 13

2. Offspring acquire genes from parents
Genes are segments of DNA that program specific traits.
Genetic info is transmitted as specific sequences of the four deoxyribonucleotides in DNA
Most genes program cells to synthesize specific enzymes and other proteins which combine to form the organisms traits.
Almost all of the DNA in a eukaryotic cell is subdivided into chromosomes in the nucleus.
Living species have a characteristic number of chromosomes.
Chromosomes consist of DNA and various proteins
Each chromosome has hundreds or thousands of genes, each at a specific location or locus

3. Asexual Reproduction
Single individual is the sole parent to donate genes to its offspring
Single-celled eukaryotes (like ameba) can reproduce asexually by mitotic cell division
Some multicellular eukaryotes, like Hydra, can reproduce by budding, producing a mass of cells by mitosis
An individual that reproduces asexually gives rise to a clone
genetic differences among clones may arise through mutations

4. Sexual Reproduction
Two parents produce offspring that have unique combinations of genes inherited from the two parents.
Offspring vary genetically from their siblings and parents.
Life cycle= generation-to-generation sequence of stages in the reproductive history of an organism
It begins at conception of an organism and continues until the organism produces its own offspring.

5. Human chromosomes
Somatic cells = body cells (all cells other than egg or sperm) have 46 chromosomes
Chromosomes can be distinguished by size, position of the centromere, pattern of staining with certain dyes.
Homologous chromosomes = pairs of chromosomes that carry genes that control the same inherited characters.
Homologous chrom. Have the same length, centromere postion and staining pattern.
A karyotype is an image of the 46 chromosomes arranged in pairs in order of size.
Sex chromosomes: X and Y. These are not homologous – different genes are present on X and Y
Human females have XX
Human males have XY
Autosomes: non-sex chromosomes
22 pairs

7. Sexual reproduction results in homologous pairs of chromosomes
Humans inherit one chromosome of each homologous pair from each parent
Number of chromosomes in a single set is represented by n
Gametes (sperm and ovum) have one set of chromosomes → 22 autosomes and an X or a Y
Gametes have the haploid number of chromosomes
Any cell with two sets of chromosomes is called diploid and has a diploid number of chromosomes represented by 2n.
Human haploid number = 23
Human diploid number = 46

9. The Human Life Cycle
Begins when haploid sperm fuses with haploid ovum (syngamy = fusion)
the fusion results in fertilization which produces a diploid zygote
Mitosis of the zygote produces the somatic cells of the body
Gametes are produced by meiosis which halves the chromosome number in these cells.

11. Organisms display a variety of sexual life cycles
Fertilization and meiosis alternate in all sexual life cycles
Timing of meiosis and fertilization varies among species
Three main types of life cycles:
Most animals including humans – gametes are the only haploid cells
Plants and some algae have alternation of generations – includes a multicellular haploid (gametophyte) and a multicellular diploid (sporophyte) phase
Fungi and some protists: zygote is the only diploid phase.

13. Meiosis reduces the chromosome number
Meiosis is preceded by replication of chromosomes.
Two divisions
Meiosis I = separates homologous chromosomes
Meiosis II = separates sister chromatids
Results in 4 haploid daughter cells.

14. Details of Meiosis I
Prophase I – occupies more than 90% of time required for meiosis
Chromosomes condense
Homologous chromosomes pair up = synapsis = forms a tetrad
Crossing over may occur
Metaphase I
Anaphase I – homologous pairs separate
Telophase I & cytokinesis

16. Details of Meiosis II
No chromosome replication occurs between meiosis I and meiosis II
Prophase II
Metaphase II – sister chromatids are arranged at the metaphase plate
Anaphase II – sister chromatids separate
Telophase II & cytokinesis

18. Differences between mitosis & meiosis
Chromosome number is reduced from diploid to haploid in meiosis but is conserved in mitosis
Mitosis produces daughter cells tha are genetically identical to the parent and to each other
Meiosis produces cells that are genetically distinct from the parent cell and from each other

19. Events unique to meiosis
Synapsis occurs during prophase I
Homologous pairs line up along the metaphase plate during metaphase I
Homologous chromosomes separate during anaphase I

21. Genetic variation that results from sexual reproduction contributes to evolution
Mutations are the original source of genetic diversity
Shuffling of genes during meiosis and fertilization produce offspring with their own unique set of traits.

22. Three mechanisms contribute to genetic variation
Independent assortment of chromosomes
Random orientation of homologous pairs of chromosomes at the metaphase plate during meiosis I.
Each homologous pair segregates independently of the other homologous pairs
The number of combinations possible when chromosomes assort independently is 2n
For humans where n=23 there are 223 (more than 8 million) possible combinations of chromosomes

24. Three mechanisms contribute to genetic variation (cont.)
2. Crossing over produces recombinant chromosomes, which combine genes inherited from each parent.
Crossing over occurs during synapsis in
prophase I
Homologous portions of two nonsister chromatids trade places.
In humans this occurs an average of one to three times per chromosome pair

26. Three mechanisms contribute to genetic variation (cont.)
3. Random fertilization adds to genetic variation
Any sperm can fuse with any egg
The ovum is one of more than 8 million possible chromosome combinations
The sperm is one of more than 8 million possible chromosome combinations
The resulting zygote could contain any one of more than 70 trillion possible combinations of chromosomes.
Crossing over adds even more variation to this.

27. Genetic variation and evolutionary adaptation
A population evolves through the differential reproductive success of its variant members
The individuals best suited to the local environment leave the most offspring, transmitting their genes in the process.
If the environment changes or a population moves to a new environment, new genetic combinations that work best in the new conditions will produce more offspring.