Reece et al., 9 Ed
Meiosis and Sexual Life Cycles
• The passage of genetic information is critical to the reproduction of cells
• The physical mechanism of inheritance depends on the behavior of chromosomes
• Genes are segments of DNA representing discrete units of hereditary information
• The physical location of a gene on the chromosome is called its locus (pl. - loci). Each
chromosome contains a sequence of several hundred to a few thousand genes
• The fidelity or the precision of DNA replication is important to heredity. Why?
II. Types of Reproduction
A. Asexual reproduction
• Asingle parent cell or organism gives rise to identical offspring
e.g., mitotic division of a mother cell gives rise to two daughter cells
multicellular hydra produces identical offspring by budding
Except for mutations, which are rare events, the offspring are identical to the parent. In other
words, the offspring of asexual reproduction are clones (identical copies) of their parents.
B. Sexual reproduction
• Results in greater variation in genetic material ▯ Offspring vary in genetic composition from
parents and siblings.
• Genetic material comes from two parents and genetic variability is created during meiosis
There are two types of cells in humans
i) Gametes - haploid cells that carry one set of chromosomes are referred to as “1n”
e.g., In humans, an egg (1n) and a sperm (1n) are the reproductive cells that fuse during
fertilization (syngamy) to produce a diploid (2n) zygote (a cell with 2 chromosome sets)
In the “1n” notation, the “n” represents the number of chromosomes in a set of chromosomes.
Human gametes (1n) have one set of 23 chromosomes, that is 1n = 23 Reece et al., 9 Ed
• The gametes are vehicles for carrying and passing on genetic information from parents to
ii) Somatic cells in humans are diploid cells and carry two sets of chromosomes (2n)
human ▯ 2n = 46 chromosomes
• The chromosomes in a cell vary in size, shape, centromere position and banding patterns
• In diploid cells, one can pair chromosomes in relation to size, shape and type (e.g., banding
pattern). This is called Karyotyping or the arrangement of chromosomes in relation to the
number, size, and type (Refer to figure 13.3)
• The similarity between paired chromosomes extends to loci on chromosomes.
• The chromosomes of a pair are called homologous chromosomes and will carrying genes
controlling the same inherited characters (i.e., each member of the pair will have a version of
the same gene specifying a particular inherited trait at equivalent loci)
There is one exception for humans
• one homologous pair - determines sex specific traits - sex chromosomes
e.g., females XX
The X and Y chromosomes are very different and only small parts are homologous
• The remaining 22 pairs of chromosomes are called autosomes and the members of each pair
What is the significance of the homologous pairs?
Their origin!!!!! ▯ one member of each homologous pair comes from each parent!
e.g., Human gametes ▯ haploid cells - single set of chromosomes (1n = 23)
Each parent contributes one set of chromosomes and therefore one member of each
Egg + sperm ▯ zygote
(1n) (1n) (2n)
Sexually reproducing species have a characteristic diploid and haploid number
Fruit fly gametes (1n) have one set of 4 chromosomes, that is 1n = 4
Fruit fly somatic cells (2n) have two sets of 4 chromosomes, that is 2n = 8 chromosomes
Mitosis generates daughter cells identical to the parental cell, but HOWARE GAMETES
Reece et al., 9 Ed
Reece et al., 9 Ed
• production of reproductive cells or gametes
• During meiosis chromosome number is halved by partitioning one member of each
homologous pair into different gametes
• Two rounds of cell division produce 4 daughter cells; each with only 1 set of chromosomes
(i.e., half the chromosome number)
Mitosis conserves chromosome number
Meiosis halves the chromosome number
Stages of Meiosis (Figure 13.8)
• replication of DNA
A. Meiosis I
• This meiotic division is referred to as a reduction division because it separates the members
of homologous pairs.
1. Prophase I
• chromosomes condense
• Synapsis = homologous chromosomes pair up along their length - precisely
aligned gene by gene - and form tetrads (i.e., four chromatids).
• The chromosome pairs are physically connected to each other along their length
by a zipper like protein complex known as the synaptonemal complex.
• Chiasmata (sing. - chiasma) - crossing over occurs between nonsister chromatids