Meiosis and Sexual Life Cycles
Offspring acquire genes from their parents by inheriting chromosomes
Fertilization and meiosis alternate in sexual life cycles
Meiosis reduces the number of chromosome sets from diploid to haploid
Parents endow their offspring with coded information in the form of genes
Humans have 46 chromosomes in their somatic cells – cells other than the gametes and their precursors
In plants and animals, reproductive cells called gametes transmit genes from one generation to the next
Each chromosome has hundreds or thousands of genes, each at a specific location, or locus, along the length of the chromosome
In asexual reproduction, a single individual is the sole parent to donate genes to its offspring
An individual that reproduces asexually gives rise to a clone, a group of genetically identical individuals
In sexual reproduction, two parents produce offspring that have unique combinations of genes inherited from the two parents
A life cycle is the generation-to-generation sequence of stages in the reproductive history of an organism
Images of the 46 human chromosomes can be arranged in pairs in order of size to produce a karyotype display
These homologous chromosome pairs carry genes that control the same inherited characters
Two distinct sex chromosomes, the X and the Y, are an exception to the general pattern of homologous chromosomes in human somatic cells
The other 22 pairs of chromosomes are called autosomes
Any cell with two sets of chromosomes is called a diploid cell and has a diploid number of chromosomes, abbreviated as 2n
A cell with a single chromosome set is a haploid cell, abbreviated as n
The union of these gametes, culminating in the fusion of their nuclei, is fertilization
Gametes undergo the process of meiosis, in which the chromosome number is halved
Plants and some algae have a second type of life cycle called alternation of generations
In meiosis, there are two consecutive cell divisions, meiosis I and meiosis II, resulting in four daughter cells
Meiosis I is preceded by interphase, in which the chromosomes are duplicated to form sister chromatids
During prophase I, duplicated homologs pair up and the formation of the synaptonemal complex between them holds them in synapsis
During metaphase I of meiosis, pairs of homologs, rather than individual chromosomes, as in mitosis, line up on the metaphase plate
During anaphase I of meiosis, the duplicated chromosomes of each homologous pair move toward opposite poles, while the sister chromatids of each duplicated chromosome remain attached
Genetic variation produced in sexual life cycles contributes to evolution
Independent assortment of chromosomes contributes to genetic variability due to the random orientation of homologous pairs of chromosomes at the metaphase plate during meiosis I
Crossing over produces recombinant chromosomes, which combine genes inherited from each parent
Evolutionary adaptation depends on a population’s genetic variation
As the environment changes, the population may survive if some members can cope effectively with the new conditions
New and different combinations of alleles may work better than those that previously prevailed