Biology I for Non-Majors

1. Biology I for Non-Majors Cell Division

2. DNA and Genomes • DNA (deoxyribonucleic acid) is the genetic material of living organisms • In humans, DNA is found in almost all the cells of the body • It provides the instructions cells need to grow, function, and respond to their environment • In eukaryotes such as plants and animals, the great majority of DNA is found in the nucleus and is called nuclear DNA • In bacteria and other prokaryotes, most of the DNA is found in a central region of the cell called the nucleoid • A cell’s set of DNA is called its genome

3. Chromosomes • Each species has its own characteristic number of chromosomes • Humans have 46 in a typical body cell • Like many species of animals and plants, humans are diploid (2n) • Most of their chromosomes come in matched sets known as homologous pairs • The 46 chromosomes of a human cell are organized into 23 pairs • The two members of each pair are said to be homologues of one another  • Human sperm and eggs, which have only one homologous chromosome from each pair, are said to be haploid (1n) • The X and Y chromosomes are known as sex chromosomes • The other 44 human chromosomes are called autosomes

4. Chromosomal Structure and Compaction • DNA • Chromatin • Nucleosome • Chromatin Fibers • Sister chromatids

5. Interphase • The first stage of interphase is called the G1 phase (first gap)  • The cell is quite active at the biochemical level • The cell is accumulating the building blocks of chromosomal DNA and the associated proteins as well as accumulating sufficient energy reserves to complete the task of replicating each chromosome in the nucleus • Next is the S phase • DNA replication results in the formation of identical pairs of DNA molecules • Last is the G2 phase • The cell replenishes its energy stores and synthesizes proteins necessary for chromosome manipulation • Some cell organelles are duplicated • The cytoskeleton is dismantled to provide resources for the mitotic phase

6. Mitosis • Prophase • The nuclear envelope starts to dissociate into small vesicles, and the membranous organelles fragment and disperse toward the periphery of the cell • The nucleolus disappears • The centrosomes begin to move to opposite poles of the cell • Microtubules that will form the mitotic spindle extend between the centrosomes, pushing them farther apart as the microtubule fibers lengthen • The sister chromatids begin to coil more tightly and become visible under a light microscope • Each sister chromatid develops a protein structure called a kinetochore in the centromeric region

7. Mitosis • Prometaphase • The nuclear envelope is fully broken down and chromosomes are attached to microtubules from both poles of the mitotic spindle • Metaphase • All the chromosomes are aligned in a plane called the metaphase plate, or the equatorial plane, midway between the two poles of the cell • The chromosomes are maximally condensed

8. Mitosis • Anaphase • The sister chromatids separate at the centromere • The cell becomes visibly elongated (oval shaped) as the polar microtubules slide against each other at the metaphase plate where they overlap • Telophase • The chromosomes reach the opposite poles and begin to decondense (unravel), relaxing into a chromatin configuration • Nuclear envelopes form around the chromosomes • Nucleosomes appear within the nuclear area

10. Cytokinesis • Cytokinesis is the second main stage of the mitotic phase during which cell division is completed via the physical separation of the cytoplasmic components into two daughter cells • In cells such as animal cells that lack cell walls, cytokinesis follows the onset of anaphase • In plant cells, a new cell wall must form between the daughter cells

11. Cell Cycle Checkpoints • External Regulation • Death of a nearby cell • Release of growth-promoting hormones • Crowding of cells • Cell size • Internal Regulation • Near the end of G1 • At the G2/M transition • During metaphase 

12. Cell Division and Cancer • Cancer comprises many different diseases caused by a common mechanism: uncontrolled cell growth • The genes that code for the positive cell cycle regulators are called proto-oncogenes • Normal genes that, when mutated in certain ways, become oncogenes, genes that cause a cell to become cancerous • Tumor suppressor genes are segments of DNA that code for negative regulator proteins • When activated can prevent the cell from undergoing uncontrolled division

13. Sexual Life Cycles • Sexual reproduction was an early evolutionary innovation after the appearance of eukaryotic cells • Fertilization and meiosis alternate in sexual life cycles • Three main categories of life cycles in multicellular organisms • Diploid-dominant, in which the multicellular diploid stage is the most obvious life stage, such as with most animals including humans • Haploid-dominant, in which the multicellular haploid stage is the most obvious life stage, such as with all fungi and some algae • Alternation of generations, in which the two stages are apparent to different degrees depending on the group, as with plants and some algae

14. Meiosis • Sexual reproduction, specifically meiosis and fertilization, introduces variation into offspring that may account for the evolutionary success of sexual reproduction • Meiosis employs many of the same mechanisms as mitosis • The starting nucleus is always diploid and the nuclei that result at the end of a meiotic cell division are haploid • To achieve this reduction in chromosome number, meiosis consists of one round of chromosome duplication and two rounds of nuclear division

15. Meiosis I • Prophase I • The nuclear envelope begins to break down, the proteins associated with homologous chromosomes bring the pair close to each other • The tight pairing of the homologous chromosomes is called synapsis • The synaptonemal complex supports the exchange of chromosomal segments between non-sister homologous chromatids, a process called crossing over

16. Meiosis I • Metaphase I • The homologous chromosomes are arranged in the center of the cell with the kinetochores facing opposite poles • The homologous pairs orient themselves randomly at the equator • This randomness is the physical basis for the creation of the second form of genetic variation in offspring

17. Meiosis I • Anaphase I • The microtubules pull the linked chromosomes apart • The sister chromatids remain tightly bound together at the centromere • Telophase I and Cytokinesis • The separated chromosomes arrive at opposite poles • The remainder of the typical telophase events may or may not occur, depending on the species • In some organisms cytokinesis occurs with reformation of the nuclear envelope; in other cases, the envelope does not reform • Two haploid cells are the end result of the first meiotic division

18. Meiosis II • In some species, cells enter a brief interphase, or interkinesis, before entering meiosis II • Interkinesis lacks an S phase, so chromosomes are not duplicated • Prophase II • If the chromosomes decondensed in telophase I, they condense again • If nuclear envelopes were formed, they fragment into vesicles • The centrosomes that were duplicated during interkinesis move away from each other toward opposite poles, and new spindles are formed • The nuclear envelopes are completely broken down, and the spindle is fully formed

19. Meiosis II • Metaphase II • The sister chromatids are maximally condensed and aligned at the equator of the cell • Anaphase II • The sister chromatids are pulled apart by the kinetochore microtubules and move toward opposite poles • Non-kinetochore microtubules elongate the cell • Telophase II and Cytokinesis • The chromosomes arrive at opposite poles and begin to decondense • Nuclear envelopes form around the chromosomes • Cytokinesis separates the two cells into four unique haploid cells

21. Meiosis and Genetic Diversity • Meiosis generates genetic diversity in 2 ways • Crossing over: the points where homologues cross over and exchange genetic material are chosen more or less at random, and they will be different in each cell that goes through meiosis • Random orientation of homologue pairs: the random orientation of homologue pairs during metaphase of meiosis I is another important source of gamete diversity • Genetic uniqueness is further enhanced during random fertilization

22. Karyotypes • A karyotype is the number and appearance of chromosomes, and includes their length, banding pattern, and centromere position • A karyogram (or ideogram) is a chart showing the karyotype • The simplest use of a karyotype (or its karyogram image) is to identify abnormal chromosomal numbers

23. Chromosomal Abnormalities • Disorders of chromosome number include the duplication or loss of entire chromosomes, as well as changes in the number of complete sets of chromosomes • Caused by nondisjunction, which occurs when pairs of homologous chromosomes or sister chromatids fail to separate during meiosis • Nondisjunction can occur during either meiosis I or II • An individual with the appropriate number of chromosomes for their species is called euploid • An individual with an error in chromosome number is described as aneuploid • An individual with more than the correct number of chromosome sets (two for diploid species) is called polyploid

24. Quick Review • What are the two types of chromosomes in humans? • List the major layers of chromosome structure • What are the sub-phases of interphase? • Draw the stage of mitosis • How does cytokinesis differ between animals and plants? • What are the major cell cycle checkpoints? • How do errors in cell division relate to cancer? • Define the major sexual life cycles. • What are the stages meiosis I and II? • How and when is genetic unique generated? • What are common chromosomal errors and how are they diagnosed?