1. Meiosis and Sexual Life Cycles
Chapter 13

2. Heredity and Inheritance
Extreme variation among living things
Due to heredity – transmission of traits from one generation to the next
Transferred in the form of genes – specific sequences of DNA nucleotides

3. Heredity and Inheritance
Genes are passed via gametes (sex cells)
Uniting of gametes causes fusing of the nuclei
DNA is packaged into chromosomes
A specific genes location on a chromosome is its locus
Somatic cells have a full set of chromosomes
Somatic cells are body cells
Gametes sperm and egg
Chromosomes can contain a few hundred to a few thousand genes
Locus = location

4. Sexual versus Asexual Reproduction
Two types of reproduction exist
Sexual
Typically two parents
Genetic variability
Asexual
One parent
Creates clone
Asexual reproduction is when parent passes all their traits on to the next generation

5. Chromosomes Human cells have 23 pairs of chromosomes
Distinguished from one another by banding patterns – homologous chromosomes
Same gene, same position BUT possible different versions
One difference – X and Y – sex crhomosome
Arranged on a karyotype
Karyotype is an ordered arrangement of paired homologs
Homologous chromosomes will have the same length, centromere position and staining pattern
Many of the genes on X do not have counterparts on the Y

6. Chromosomes One chromosome inherited from each parent
Number of chromosomes in a single set is represented by n
Diploid cell has two sets (normal for humans) of chromosomes (2n)
Haploid cell has a single set (n)
Sperm and egg are haploid
Somatic cells are diploid

7. Chromosomes and Fertilization
Fertilization results in a zygote
Zygote then begins mitosis to generate somatic cells
Only cells not produced by mitosis are eggs and sperm – why?
Zygote is a fertilized egg and becomes diploid

8. Meiosis I
Prophase I
A tetrad is made of each homologous set due to replication
Homologs connected along length – synaptonemal complex (synapsis)
Crossing over occurs resulting in chiasmata
Rest remains the same as mitosis – metaphase, anaphase, telophase
No duplication of chromosomes a the end of meisosis I
Followed by meiosis II

9. Variety of Sexual Life Cycles
3 types of life cycles
1st - Normal Meiosis
2nd - Alternation of generations
Both diploid and haploid multicellular organisms
3rd – Gametes fuse to make diploid zygote
Only haploid cells produced
Alternation of generations
Multicellular diploid stage is called a sporophyte
Meiosis of sporophyte produces haploid cells called spores
Spore divides mitotically generating a haploid gametophyte (multicellular)
Gametophyte generates gametes by mitosis
Fusion of two gametes at fertilization results in a diploid sporophyte
Occurs in plants and some algae
3rd
Meiosis does not produce games but rather mitosis produces haploid unicellular organisms or haploid multicellular organisms
Haploid organisms produce gametes by mitosis
Only diploid stage is the zygote
Occurs in most fungi and some protists

10. Meiosis vs. Mitosis Meiosis Mitosis Sexual reproduction
Reduction from diploid to haploid
Homologous chromosomes
Synapsis and crossing over
4 cells produced
Mitosis
Asexual reproduction
Conserves the diploid number
Sister chromatids
No synapsis/crossing over
2 cells produced

11. Genetic Variation by Sexual Reproduction
50% chance of a gamete of getting either the maternal or paternal chromosome
Independent Assortment
The chromosomes will assort independently of all other chromosomes
Crossing over
Produces recombinant chromosomes
Random Fertilization
8.4 million possible chromosome combination
Fertilization produces 70 trillion possible combinations
Recombinant chromosomes are chromosomes from 2 different parents

12. Evolution and Genetic Variation
Individuals best suited for an environment leave more offspring
Changing environment-sexual reproduction
Stable environment-asexual reproduction?
Bdelloid rotifer
Asexual ensures passing on of favorable traits, less energy expensive
Bdelloid does parthonegenesis for reproduction
Live in a wide variety of environments-does this mean parthonegenesis is better?
In certain environments, the area may dry and the rotifers go into a suspended state-during this time their cell membranes can crack allowing other rotifer or even other organisms DNA to enter