cell division: meiosis

1. cell division: meiosis biology 1

2. Offspring acquire genes from parents by inheriting chromosomes • Two general strategies • Sexual reproduction • Asexual reproduction • Fertilization and meiosis alternate in a sexual lifecycle • Meiosis reduces chromosome number from diploid to haploid • Sexual reproduction produces genetic variation, a vital component of evolutionary adaptation

3. A glossary... • DNA is a nucleic acid composed of four different kinds of nucleotide in different sequences • Specific sequences of nucleotides that correspond to synthesis of a specific protein are called genes • Genes are joined together in strands called chromosomes • The lineal location of a gene on a chromosome is called a locus (plural: loci) • Different expressions of a gene at a particular locus are possible - these expressions are called alleles • Inheritance is possible because DNA is precisely replicated, with gene copies being passed onto offspring • Each species has a characteristic chromosome number - humans have 46

4. Asexual vs. sexual reproduction Genetic variation in sexual reproduction is a result of meiosis

5. The sexual life cycle • Human somatic cells contain 46 chromosomes (as determined by karyotyping) • Closer examination reveals that these 46 can be assigned into 23 pairs • 22 pairs are homologous pairs (ie, per pair, same set of loci). These are known as autosomes • 1 pair carries different loci - these are sex chromosomes • Each homologue from a pair is inherited from a specific parent • Thus, a human somatic cell consists of two sets of 23 chromosomes, each set inherited by a specific parent • A cell that possesses both sets is said to be diploid (2n) • A cell that has only one set is said to be haploid (n)

6. In a sexual life cycle, meiosis halves the chromosome number from diploid to haploid to create gametes • In fertilization, gametes fuse to become a single celled zygote which restores the diploid condition • Depending on species (and Kingdom), different periods of time spent in haploid and diploid phases • Occasionally, some organisms remain in either haploid or diploid state (although most organisms cycle)

7. Meiosis • Steps to meiosis in some ways mirror those in mitosis. However, meiosis consists of two divisions (Meiosis I and Meiosis II) • Produces 4 daughter cells • Each daughter cell is haploid • Meiosis plays a key role in generating variation • As in mitosis, replication of DNA occurs unseen while genetic material is uncoiled

8. Interphase I • Chromosomes replicate as in mitosis • Each duplicated chromosome consists of 2 sister chromatids attached at a centromere • BUT remember that in a diploid cell each chromosome (pair of chromatids) has a homologue • Therefore, following duplication, for any one gene, there will be two pairs of two alleles

9. Prophase I • Chromosomes condense and are visible • Homologues associate as a tetrad in the process of synapsis • During synapsis homologues may join at specific loci termed chiasma • At a chiasma, homologues may exchange a length of DNA (set of genes). This process is known as crossingover. The joint between the two homologues is known as a synaptonemal complex • In humans, 2-3 chiasmata per chromosone pair • Cell prepares for first division • Migration of centrosomes • Dispersion of nuclear membrane • Formation of meiotic spindle • Chromosomes begin to migrate • Prophase I accounts for 90% of the time spent in meiosis

10. Metaphase I • Tetrads align along metaphase plate • Centromeres of homologues point towards opposite poles of cell • Kinetochore microtubules connect to kinetochore sites in centromere of each homologue

11. Anaphase I • Homologues separate and move towards separate poles of the cell, pulled by depolymerization of kinetochore microtubules at kinetochore end • Sister chromatids remain intact and travel together to either pole • Homologue separation is not necessarily by parental assignment

12. Telophase I and Cytokinesis • Each pole now has a haploid set of chromosomes (homologues are at other pole), each homologue consisting of a chromatid pair • Cytokinesis separates cell into two daughter cells • In some cases nuclear membranes reform (interkinesis)

13. Prophase II • The goal of meiosis II is to separate sister chromatids • In prophase II, nuclear envelope disperses (if it reformed) • Spindle apparatus reforms and chromosomes start to move towards metaphase plate

14. Metaphase II • Chromosomes align on the metaphase plate, with each sister chromatid pointing towards a different pole of the cell • Each sister chromatid is joined to a kinetochore microtubule at the kinetochore

15. Anaphase II • Centromeres of sister chromatids separate • Sister chromatids are pulled apart and move towards separate poles of the cell Telophase II and cytokinesis • Nuclei form at opposite poles of the cell • Cytokinesis occurs producing 4 haploid daughter cells

16. Sexual life cycles produce variation • Genetic variation is essential for evolutionary adaptation • In meiosis, variation occurs by • Independent assortment • Homologues do not assign to different poles of the cell necessarily according to parental designation: 2n possible combinations • Crossing over • Alleles associated previously with other alleles on the same chromosome may now associate with different alleles from other homologue • Random fusion of gametes (2n x 2n) • Beyond meiosis, variation can also occur through mutation