Meiosis
• The ability to pass on traits is called heredity.
– This ability is one of the unifying themes of
biology as individual units of heredity (genes) are
passed from one generation to the next in
reproductive cells called gametes.
– Organisms can reproduce in 2 basic ways; sexually
and asexually.
Sexual Reproduction
• gametes from 2 different parents, created during
meiosis, blend genes to create variation within the
species
– gametes are produced via meiosis and each have 1/2 the
DNA of the somatic cells from which they were derived
Asexual Reproduction
• offspring reproduced via mitosis
– each offspring is a clone of the parent
– examples range from budding (hydra) and binary fission
(bacteria) where offspring separate and grow to plants, algae,
and fungi that have offspring with 1/2 DNA of the parent
Reproductive Cycle Humans
• In the human reproductive cycle the somatic cell carries 46
chromosomes: 22 pairs of autosomes & 2 sex chromosomes.
– The somatic cell is said to be diploid (2n = 46) where each cell retains 2
copies of each piece of DNA
• gametes are said to be haploid (n = 23) having only one copy of each.
– Each pair (excluding sex cells) is said to be homologous
• share a similar centromere region, gene locus, and staining pattern when
condensed.
• The display of the homologous chromosomes when condensed is the
organisms karyotype.
– haploid cells are produced in the testis and ovaries (n = 3) via meiosis
– cells are joined via fertilization to form a zygote and are now diploid
(2n = 46)
– mitosis and cell differentiation form an individual with similar traits to
the parents
Alternate forms of reproduction
• Plants & some algae
– alterations of generation - both diploid and haploid
are multicellular
• diploid is the sporophyte stage
– meiosis produces spores
• spores grow into an organism called a gametophyte through
mitotic division of the haploid cell
– gametophyte make gametes via mitosis which fuse to form a
diploid zygote
• diploid organism grows into a sporophyte
Alternate forms of reproduction
• Most fungi and some protists
– haploid multicellular organism
– fertilization forms a diploid zygote that immediately
undergoes meiosis to produce haploid adults