Unit 5 (Heredity)

Genetics

Study of heredity and hereditary variation

Heredity

The transmission of traits from one generation to the next

• Traits are passed from parent to offspring through genes

• Genes: segments of DNA that code for basic units of life

• Offspring aquires genes from parents by inheriting chromosomes

Asexual reproduction

• Single individual

• No fusion of gametes

• Clones: offspring are exact copies of parents

• Mutations are the only source of variation

• Can produce asexually through mitosis

Sexual Reproduction

• Two parents (male/female)

• Offspring are unique combinations of genes from parents

• Genetically varied from parents and siblings

Homologous Chromosomes

A pair of chromosomes that carry carry the same genetic information

• Same size, length, centromere position

• One from dad, one from mom

Karyotypes

A display of chromosome pairs ordered by size and length

Somatic (body) cells

• Diploid or two complete sets of each chromosome (2n)

• Humans: 2n - 46

Gametic (sex) cells

• Haploid or one set of each chromosome (n)

• Humans: n - 23

Eukaryotic DNA

Packaged in chromosomes

Autosomes

Chromosomes that do not determine sex (humans have 22 pairs)

Sex Chromosomes

X and Y

• Eggs: X (humans: 22 + x)

• Sperm: X or Y (humans: 22 + x or 22 + y)

Life cycle

Sequence of stages in the reproductive history of an organism from conception to its own reproduction

• Fertilization and meiosis alternate in sexual life cycles

• Fertilization: When a sperm cell (haploid) fuses with an egg (haploid) to form a zygote (diploid)

Meiosis

A process that creates haploid gamete cells in sexually reproducing diploid organisms

• Results in daughter cells with half the number of chromosomes as the parent cell

• Ex. humans

• Diploid: 2n = 46

• Meiosis produces sperm and eggs that are haploid (n=23)

• 2 rounds of division (Meiosis I and II)

Mitosis

• Occurs in somatic cells

• 1 division

• diploid daughter cells

• Daughter cells are genetically identical

Meiosis

• Forms gametes (sperm/egg)

• 2 divisions

• 4 haploid daughter cells

• Each daughter cell is genetically unique

Key Events in Meiosis

1. Prophase: synapsis and crossing over

2. Metaphase 1: tetrads (homologous pairs) line up at the metaphase plate

3. Anaphase 1: homologous pairs separate

Interphase

• G1

• S (DNA is copied)

• G2

Prophase I

• Synapsis: homologous chromosomes pair up and physically connect to each other forming a tetrad

• Crossing over (recombination): Occurs at the chiasmata and DNA is exchanged between the homologous pairs

• Every chromatid that is produced has a unique combination of DNA

Metaphase I

• Independent orientation: tetrads line up at the metaphase plate → independent assortment (random)

Anaphase I

Pair of homologous chromosomes separate

• Sister chromatids are still attached

Telophase I and Cytokinesis

• Nuclei and cytoplasm divide

• There is now a haploid set of chromosomes in each daughter cell

Prophase II

• No crossing over

• Spindle forms

Metaphase II

• Chromosomes line up at the metaphase plate

• Because of crossing over in meiosis I, the chromatids are unique

Anaphase II

• Sister chromatids separate and move towards opposite poles

Telophase II and Cytokinesis

• 4 haploid cells

• Nuclei reappear

• Each daughter cell is genetically unique

Meiosis conclusion

interacts with sub-cellular components and uses free energy that is required for the growth and reproduction of living systems