What are gametes?
- a haploid reproductive cell, such as egg or sperm
- - gametes unite during sexual reproduction to produce diploid zygote
What is a locus?
– A gene’s specific location along the length of a chromosome
What is a life cycle?
– the generation-to-generation sequence of stages in the reproductive history of an organism, from conception to production of its own offspring
What is a karyotype and homologous chromosomes?
- karyotype: a display of the chromosome pairs of a cell arranged by size and shape resulting in homologous chromosomes
- homologous chromosomes: a pair of chromosomes of the same length, centromere position, and staining pattern that possess genes for the same characteristic at corresponding loci (both chromosomes of each pair carry genes controlling the same inherited characters. For example, if a gene for eye colour is situated at a particular locus on a certain chromosome, then the homologue of that chromosome will also have a version of the same gene specifying eye colour at the equivalent locus; one homologous chromosome is inherited from the organism’s father, the other from the mother (X and Y are not homologous in males)
What is fertilization and zygote?
- fertilization: the union of haploid gametes to produce a diploid zygote, culminating in the fusion of their nuclei (sperm fusing with egg)
- zygote: the resulting fertilized egg, a diploid
- mitosis of the zygote and its descendant cells generates all the somatic cells of the body
What are germ cells?
– gametes, which develop from specialized cells called germ cells in the gonads—ovaries in females and testes in males.
What is meiosis?
– a modified type of cell division in sexually reproducing organisms consisting of two rounds of cell division but only one round of DNA replication, resulting in cells with half the number of chromosome sets as the original cell
– Only diploid cells, however, can undergo meiosis because haploid cells have only a single set of chromosomes that cannot be further reduced
– Meiosis and mitosis are preceded by the duplication of chromosomes.
◦ meiosis is followed by two consecutive cell divisions and
◦ mitosis is followed by only one cell division.
– life cycle: Meiosis occurs in germ cells during the production of gametes, which undergo no further cell division prior to fertilization. After fertilization, the diploid zygote divides by mitosis, producing a multicellular organism that is diploid
What is meiosis I and meiosis II?
- Duplication of chromosomes followed by two consecutive cell divisions called meiosis I and meiosis II
- meiosis I separates homologous chromosomes and meiosis II separates sister chromatids
What happens during prophase 1 (crossing over)?
- Early in prophase I, the two members of a homologous pair associate loosely along their length. Each gene on one homologue is aligned precisely with the corresponding gene on the other homologue
- The DNA of two nonsister chromatids is broken by specific proteins at precisely corresponding points
- the formation of a zipperlike structure called the synaptonemal complex holds one homologue tightly to the other
- During this association, called synapsis, the DNA breaks are closed up so that each broken end is joined to the corresponding segment of the nonsister chromatid
- These points of crossing over become visible as chiasmata after the synaptonemal complex disassembles and the homologues move slightly apart from each other
- The homologues remain attached because sister chromatids are still held together by sister chromatid cohesion, even though some of the DNA may no longer be attached to its original chromosome. At least one crossover per chromosome must occur in order for the homologous pair to stay together as it moves to the metaphase I plate
What happens during mitosis I?
– At metaphase I of meiosis, chromosomes are positioned at the metaphase plate as pairs of homologues rather than individual chromosomes, as in metaphase of mitosis.
– There is an equal probability of the maternal or paternal chromosome facing a given pole.
– 23 chromosomes: 2^23 = 8.4 x 106
arrangements
What happens during anaphase I and II?
– At the onset of anaphase I, the release of cohesion along sister chromatid arms allows homologues to separate. At anaphase II, the release of sister chromatid cohesion at the centromeres allows the sister chromatids to separate.
What happens during prophase II?
– Chromosomes coil and become compact (if uncoiled after telophase I).
◦ Nuclear envelope, if re-formed, breaks up again.
What is the difference between mitosis and meiosis?
- DNA replication: occurs during interphase for both mitosis and meiosis
- Number of divisions: one in mitosis but two in meiosis
- Synapsis of homologous chromosomes: does not occur in mitosis but occurs in prophase I during meiosis
- # of daughter cells and genetic composition: two, each genetically identical with the same number of chromosomes in mitosis, but four, each haploid and genetically different from parents and each other in meiosis
