AP Biology

1. AP Biology Exam Critical Concepts Cell Communication and the cell cycle Chapters 11-13

2. Chapter 11 Cell Communication • Signal Transduction Pathway: a signal on a cell’s surface is converted into a specific cellular response • Local signaling: Cells in a multicellular organism communicate by chemical messengers • Animal cells may communicate by direct contact, or cell-cell recognition • Cells receiving signals go through three processes: 1. Reception 2. Transduction 3. Response

3. Chapter 11 Cell Communication cont. • Reception: The binding between a signal molecule (ligand) and receptor is highly specific • A shape change in a receptor is often the initial transduction of the signal • Most signal receptors are plasma membrane proteins • G protein-coupled receptors • Receptor tyrosine kinases • Ion channel receptors • Intracellular receptors: receptor proteins inside the cell, found in the cytosol or nucleus of target cells

4. Chapter 11 Cell Communication cont. • Transduction: multiple steps that can amplify a signal • Multistep pathways provide more opportunities for coordination and regulation of the cellular response • Like falling dominoes, the receptor activates another protein, which activates another, and so on, until the protein producing the response is activated • Second messengers: small, nonprotein, water-soluble molecules or ions that spread throughout a cell by diffusion • Cyclic AMP (cAMP): one of the most widely used second messengers

5. Chapter 12 Cell Cycle • Unicellular organisms: division of one cell reproduces the entire organism • Multicellular organisms: cell division needed for: • Development from a fertilized cell • Growth • Repair • Daughter cells: duplicated cells with identical DNA • Genome: all the DNA in a cell • Single DNA molecule (common in prokaryotic cells) • A number of DNA molecules (common in eukaryotic cells)

6. Chapter 12 Cell Cycle cont. • Chromosomes: packaged DNA molecules in a cell are into • Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus • Somatic cells: non-reproductive cells with two sets of chromosomes • Gametes: reproductive cells (sperm and eggs) have half as many chromosomes as adult • Chromatin: complex of DNA and protein that condenses during cell division

7. Chapter 12 Cell Cycle cont. • Response: sometimes called the “output response” • leads to regulation of one or more cellular activities • synthesis of enzymes or other proteins • may function as a transcription factor • Apoptosis: programmed or controlled cell suicide • cell is chopped and packaged into vesicles that are digested by scavenger cells • prevents enzymes from leaking out of a dying cell and damaging neighboring cells • triggered by: • An extracellular death-signaling ligand • DNA damage in the nucleus • Protein misfolding in the endoplasmic reticulum

8. Chapter 12 Cell Cycle cont. • In preparation for cell division DNA is replicated and the chromosomes condense • Each duplicated chromosome has two sister chromatids, which separate during cell division • Centromere: narrow “waist” of the duplicated chromosome, where the two chromatids are most closely attached

9. Chapter 12 Cell Cycle cont. • Eukaryotic cell division consists of: • Mitosis, the division of the nucleus • Cytokinesis, the division of the cytoplasm • Cell cycle: includes two major phases • Mitotic (M): mitosis and cytokinesis • Interphase: cell growth & copying of chromosomes (about 90% of the cell cycle) • G1 phase (“first gap”) resting & checkpoint • S phase (“synthesis”) • G2 phase (“second gap”) resting & checkpoint • The cell grows during all three phases, but chromosomes are duplicated only during the S phase

10. Chapter 12 Cell Cycle cont. INTERPHASE G1 S (DNA synthesis) Cytokinesis G2 Mitosis MITOTIC (M) PHASE

11. Chapter 12 Cell Cycle cont. • Mitosis divided into the following steps • Prophase • Prometaphase • Metaphase • Anaphase • Telophase • Cytokinesis: development of two identical cells • underway by late telophase

12. Chapter 12 Cell Cycle cont. • Prophase: • Assembly of spindle microtubules begins in the centrosome • Mitotic spindle is an apparatus of microtubules that controls chromosome movement during mitosis • The centrosome replicates • forming two centrosomes that migrate to opposite ends of the cell, as spindle microtubules grow out from them • Aster: a radial array of short microtubules • extends from each centrosome

13. Chapter 12 Cell Cycle cont. • Prometaphase and Metaphase: • Some spindle microtubules attach to the kinetochores of chromosomes and begin to move the chromosomes • At metaphase the chromosomes are all lined up at the • metaphase plate, the midway point between the spindle’s two poles

14. Chapter 12 Cell Cycle cont. • Anaphase: • Sister chromatids separate and move along the kinetochore microtubules toward opposite ends of the cell • The microtubules shorten by depolymerizing at their kinetochore ends • Telophase: • Nonkinetochore microtubules from opposite poles overlap and push against each other, elongating the cell • Genetically identical daughter nuclei form at opposite ends of the cell

15. Chapter 12 Cell Cycle cont. • Cytokinesis: • In animal cells • Cleavage: develops into cell membrane • forming a cleavage furrow • In plant cells • Cell plate forms which develops into the cell wall • Binary fission: splitting of cell prokaryotic cell • One cell is clone of the other • Since prokaryotes evolved before eukaryotes, mitosis probably evolved from binary fission

16. Chapter 12 Cell Cycle cont. Cell cycle & Mitosis review G1 S Cytokinesis G2 Mitosis MITOTIC (M) PHASE Prophase Telophase and Cytokinesis Prometaphase Anaphase Metaphase

17. Chapter 12 Cell Cycle cont. • Regulation of the cell cycle: • Frequency of cell division varies with the type of cell • Cell cycle differences result from regulation at the molecular level • The cell cycle appears to be driven by specific chemical signals present in the cytoplasm • Cell cycle control: sequential events of the cell cycle are directed by a distinct system, which is similar to a clock • regulated by both internal and external controls • checkpoints where the cell cycle stops until a go-ahead signal is received

18. G1 checkpoint Chapter 12 Cell Cycle cont. Control system S G1 G2 M M checkpoint G2 checkpoint

19. Chapter 12 Cell Cycle cont. • G1 checkpoint most important one • go-ahead signal at the G1 checkpoint the cell usually complete the S, G2, and M phases • G0 phase: non-dividing state if no go-ahead signal • Two regulatory proteins involved in cell cycle control: • cyclins • cyclin-dependent kinases (Cdks) • The activity of cyclins and Cdks fluctuates during the cell cycle • MPF (maturation-promoting factor) is a cyclin-Cdk complex that triggers a cell’s passage past the G2 checkpoint into the M phase

20. Chapter 12 Cell Cycle cont. • Internal cell cycle signal: • kinetochores not attached to spindle microtubules send a molecular signal that delays anaphase • External signals: • growth factors, proteins released by certain cells that stimulate other cells to divide • density-dependent inhibition, in which crowded cells stop dividing • Most animal cells also exhibit anchorage dependence, in which they must be attached to a substratum in order to divide

21. Chapter 12 Cell Cycle cont. • Cancer cells do not respond normally to the body’s control mechanisms • may not need growth factors to grow and divide: • may make their own growth factor • may convey a growth factor’s signal without the presence of the growth factor • may have an abnormal cell cycle control system • Transformation: A normal cell is converted to a cancerous cell • Cancer cells form tumors, masses of abnormal cells within otherwise normal tissue

22. Chapter 12 Cell Cycle cont. • If abnormal cells remain at the original site, the lump is called a benign tumor • Malignant tumors invade surrounding tissues and can metastasize, exporting cancer cells to other parts of the body, where they may form secondary tumors

23. Chapter 12 Cell Cycle cont. G0 Cell cycle G1 - the first gap phase, during which the cell grows to a threshold size. The cell actively takes up materials from its environment to form new organelles and cytoplasm. S - during which the cell replicates its DNA and MTOCs (microtubule organising centres: i.e. centrioles, centrosomes, etc.) G2 - the second gap phase, during which the cell condenses its genome and MTOCs begin to form a spindle which will later separate the chromosomes. M - mitosis (or meiosis), during which the genome is passive, whilst the cytoskeleton actively moves the chromosomes about.

24. Chapter 12 Cell Cycle cont.

25. Chapter 13 Meiosis • Genes: units of heredity • made up of segments of DNA • Locus: a specific location on a certain chromosome • packaged into chromosomes • One set of chromosomes is inherited from each parent • Gametes: reproductive cells passed to the next generation through sperm and eggs • Somatic cells: Human non sex cells • have 23 pairs of chromosomes • Karyotype: an ordered display of the pairs of chromosomes from a cell

26. Chapter 13 Meiosis cont • Asexual reproduction: one parent produces genetically identical offspring by mitosis • Clone: group of genetically identical individuals from the same parent • Sexual reproduction: two parents give rise to offspring that have unique combinations of genes inherited from the two parents • Homologous chromosomes: chromosomes from each parent • in each pair are called or homologs • homologous pair are the same length • carry genes from the same inherited characters

27. Chapter 13 Meiosis cont • Sex chromosomes: human reproductive cells • called X and Y • females have a homologous X X • males have one X and one Y • 2n has two sets of chromosomes • For humans, the diploid number is 46 (2n = 46) • In a cell in which DNA synthesis has occurred, each chromosome is replicated • Sister chromatids: each replicated chromosome consists of two identical

28. Chapter 13 Meiosis cont • Autosomes: the 22 pairs of chromosomes that do not determine sex • one chromosome from each parent • Gamete: sperm or egg • haploid (n): contains a single set of chromosomes • Humans: the haploid number (n = 23) • Each set of 23 consists of 22 autosomes and a single sex chromosome • Ovum: unfertilized egg, the sex chromosome is X • Sperm: the sex chromosome may be either X or Y

29. Chapter 13 Meiosis cont • Fertilization: the union of gametes (the sperm and the egg) • Zygote: fertilized egg • one set of chromosomes from each parent • produces somatic cells by mitosis • develops into an adult • At sexual maturity, the ovaries and testes produce haploid gametes • Gametes are the only types of human cells produced by meiosis, rather than mitosis • Meiosis results in one set of chromosomes in each gamete

30. Chapter 13 Meiosis cont • Meiosis : two sets of cell divisions • meiosis I • meiosis II • The two cell divisions result in four daughter cells, rather than the two daughter cells in mitosis • Each daughter cell has only half as many chromosomes as the parent cell

31. Chapter 13 Meiosis cont • Meiosis I: the first cell division • homologous chromosomes separate • results in two haploid daughter cells with replicated chromosomes • called the reductional division • Meiosis II: the second cell division • sister chromatids separate • results in four haploid daughter cells with replicated chromosome • called the equational division

32. Chapter 13 Meiosis cont • Meiosis I and II Occurs in four phases: – Prophase I – Metaphase I – Anaphase I – Telophase I and cytokinesis

33. Chapter 13 Meiosis cont • Prophase I: Occupies more than 90% of meiosis time • Chromosomes begin to condense • Synapsis: homologous chromosomes pair up, gene by gene • Crossing over: nonsister chromatids exchange DNA segments • Pairs form a tetrad, a group of four chromatids • Metaphase I • Tetrads line up at the metaphase plate, with one chromosome facing each pole • Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad

34. Chapter 13 Meiosis cont • Anaphase I • Pairs of homologous chromosomes separate • One chromosome moves toward each pole, guided by the spindle apparatus • Sister chromatids remain attached at the centromere and move as one unit toward the pole • Telophase I: • In the beginning each half of the cell has a haploid set of chromosomes; each chromosome consists of two sister chromatids

35. Chapter 13 Meiosis cont • Cytokinesis: usually occurs simultaneously • two haploid daughter cells • In animal cells, a cleavage furrow forms • in plant cells, a cell plate forms • No chromosome replication occurs during meiosis • Meiosis II: is very similar to mitosis • also occurs in four phases: – Prophase II – Metaphase II – Anaphase II – Telophase II and cytokinesis

36. Chapter 13 Meiosis cont • Meiosis II: is very similar to mitosis • also occurs in four phases: – Prophase II – Metaphase II – Anaphase II –Telophase II and cytokinesis • Prophase II: A spindle apparatus forms • In late prophase II, chromosomes (each still composed of two chromatids) move toward the metaphase plate

37. Chapter 13 Meiosis cont • Metaphase II: Sister chromatids are arranged at the metaphase plate • Because of crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical • The kinetochores of sister chromatids attach to microtubules extending from opposite poles • Anaphase II: Sister chromatids separate • The sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles

38. Chapter 13 Meiosis cont • Telephase II: The chromosomes arrive at opposite poles • Nuclei form, and chromosomes begin decondensing • Cytokinesis separates the cytoplasm • Four daughter cells • each with a haploid set of unreplicated chromosomes • Each daughter cell is genetically distinct from the others and from the parent cell

39. Chapter 13 Meiosis cont • Mitosis vs. Meiosis: • Mitosis • conserves the number of chromosome sets • producing cells that are genetically identical to the parent cell • Meiosis • reduces the number of chromosomes sets from two (diploid) to one (haploid) • producing cells that differ genetically from each other and from the parent cell • The mechanism for separating sister chromatids is virtually identical in meiosis II and mitosis

40. Chapter 13 Meiosis cont • All three occur in meiosis l: • Synapsis and crossing over in prophase I: • Homologous chromosomes physically connect and exchange genetic information • At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes • At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate

41. Chapter 13 Meiosis cont • Mutations: changes in an organism’s DNA • Are the original source of genetic diversity • Create different versions of genes called alleles • Reshuffling of alleles during sexual reproduction produces genetic variation • Crossing over: begins very early in prophase I • produces recombinant chromosomes, which combine genes inherited from each parent • as homologous chromosomes pair up gene by gene • Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome

42. Chapter 13 Meiosis cont • Three mechanisms contribute to genetic variation: • Independent assortment of chromosomes • Crossing over • Random fertilization • The number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid number • For humans (n = 23) • there are more than 8 million (223) possible combinations of chromosomes