Chapter 9 The Continuity of Life: Cellular Reproduction

Why Do Cells Divide? Cells reproduce by cell division, in which a parent cell normally gives rise to two daughter cells Each daughter cell receives a complete set of hereditary information from the parent cell and about half its cytoplasm The hereditary information is usually identical with that of the parent cell All cells come from cells – critical importance of cellular reproduction.

Why Do Cells Divide? Cell division transmits hereditary information to each daughter cell deoxyribonucleic acid (DNA) - hereditary information Polymer of nucleotides Phosphate a sugar (deoxyribose) one of four bases adenine (A), thymine (T), guanine (G), cytosine (C) Chromosome - DNA in a double helix

The Structure of DNA phosphate nucleotide base T A A sugar C G G C C C The specific sequence of nucleotides in genes spell out the instructions for making the proteins in a cell. Like the letters of the alphabet. For cell survival, it must have a complete set of genetic instructions. So it can’t just give half to a daughter cell. It must duplicate its DNA first. A T T A T Fig. 9-1 (a) A single strand of DNA (b) The double helix

Why Do Cells Divide? Cell division is required for growth and development Mitotic cell division - The cell division of eukaryotic cells by which organisms grow or increase in number After cell division, the daughter cells may differentiate, becoming specialized for specific functions Cell cycle - The repeating pattern of divide, grow, and differentiate, then divide again Most multicellular organisms have three categories of cells Stem cells Other cells capable of dividing Permanently differentiated cells … this division continues every day in many organs. … such as muscle cells, fighting infections

Why Do Cells Divide? Stem cells Two important characteristics: self-renewal retain the ability to divide one daughter remains a stem cell, thus continuing the line; the other daughter undergoes several divisions the ability to differentiate into a variety of cell types Stem cells include most of the daughter cells formed by the first few cell divisions of a fertilized egg, as well as a few adult cells 1a. Perhaps for the entire life of the organism. b. when a stem cell divides… 2. Eventually resulting in differentiation into a variety of cells. Some stem cells in early embryos can produce any of the specialized cell types of the entire body

Why Do Cells Divide? Other cells capable of dividing typically differentiate into only one or two different cell types Dividing liver cells, for example, can only become more liver cells

Why Do Cells Divide? Cell division is required for sexual and asexual reproduction Sexual reproduction - eukaryotic organisms occurs when offspring are produced by the fusion of gametes (sperm and eggs) from two adults Asexual reproduction - Reproduction in which offspring are formed from a single parent, without having a sperm fertilize an egg 2. Clones – genetically identical to the parent and to each other.

Cell Division Enables Asexual Reproduction Bacteria and single-celled eukaryotic organisms (Paramecium) Some multicellular organisms, like Hydra by budding Many plant and fungi reproduce both asexually and sexually Groves of aspen may be produced asexually by shoots growing up from the root system of a single parent tree or reproduce sexually from seeds Fig. 9-2

The Prokaryotic Cell Cycle The DNA is contained in a single, circular chromosome about a millimeter or two in circumference Contained in a nucleoid Both prokaryotic and eukaryotic cells have cell cycles However, they have major structural and functional differences One of the most pronounced differences is the organization of their DNA, particularly the respective structure, size, number, and location of their chromosomes

The Prokaryotic Cell Cycle (a) The prokaryotic cell cycle cell wall plasma membrane circular DNA attachment site The circular DNA double helix is attached to the plasma membrane at one point. The DNA replicates and the two DNA double helices attach to the plasma membrane at nearby but separate points. cell division by prokaryotic fission cell growth and DNA replication 1 2 (b) Prokaryotic fission New plasma membrane is added between the attachment points, pushing them farther apart. The plasma membrane grows inward at the middle of the cell. The parent cell divides into two daughter cells. 3 4 5 Long period of growth and replication. Binary fission = prokaryotic fission (binary fission also applies to single-celled eukaryotes) 3. As the cell grows … 4. When the cell has about doubled in size … Two identical DNA molecules -> two identical daughter cells to each other and the parent. Under ideal conditions, E. coli can go through the cell cycle in about 20 minutes. Fig. 9-3

DNA in Eukaryotic Chromosomes Differ from prokaryotic chromosomes: membrane-bound nucleus always have multiple chromosomes (2-1200) longer and have more DNA (human chromosomes are 10 to 80 times longer and have 10 to 50 times more DNA) Eukaryotic chromosomes are separated from the cytoplasm by a … … smallest number being 2 (females of a species of ant) most have dozens, some ferns have 1200 These differences account for the complexity of eukaryotic cell division 1. … if relaxed and extended, 0.6–3 in (15-75mm) long each; one cell would contain 6ft (1.8m) of DNA.

DNA in Eukaryotic Chromosomes histone proteins protein scaffold DNA double helix DNA wound around histone proteins Folded chromosome, fully condensed in a dividing cell Coiled DNA/histone beads Loops attached to a protein scaffold; this stage of partial condensation typically occurs in a nondividing cell 1 2 3 4 5 A linear DNA double helix bound to proteins Each human chromosome contains a single DNA double helix, about 50 million to 250 million nucleotides long Most of the time, the DNA in each chromosome is wound around proteins called histones 5. During cell division, chromosomes are 10x shorter than during resting (4)

DNA in Eukaryotic Chromosomes Genes are segments of the DNA of a chromosome Units of inheritance Sequences of DNA from hundreds to thousands of nucleotides long Each gene occupies a specific place, or locus (plural, loci) on a chromosome May be 70 to over 3,000 genes on 1 chromosome.

DNA in Eukaryotic Chromosomes In addition to genes, every chromosome has specialized regions that are crucial to its structure and function: Two telomeres Protective caps on each end of a chromosome Essential for chromosome stability One centromere Temporarily holds two daughter DNA double helices together after DNA replication Is the attachment site for microtubules that move the chromosomes during cell division Telomeres – chromosomes at the ends would be lost during DNA replication. also keep chromosomes from fusing with each other Centromeres – two principal functions:

The Principal Features of a Eukaryotic Chromosome During Cell Division gene loci centromere telomeres (a) A eukaryotic chromosome before DNA replication sister chromatids duplicated chromosome (two DNA double helices) (b) A eukaryotic chromosome after DNA replication independent daughter chromosomes, each with one identical DNA double helix (c) Separated sister chromatids become independent chromosomes At the end of DNA replication, a duplicated chromosome (b) consists of 2 identical chromatids. Fig. 9-5

DNA in Eukaryotic Chromosomes Eukaryotic chromosomes usually occur in pairs with similar genetic information Karyotype - an entire set of stained chromosomes from a single cell sex chromosomes Both members of each pair are the same length and shape, and have the same staining pattern

DNA in Eukaryotic Chromosomes These similarities occur because each chromosome in a pair carries the same genes arranged in the same order Chromosomes that contain the same genes are called homologous chromosomes, or homologues Cells with pairs of homologous chromosomes are called diploid, which means “double” 2n Egg and sperm cells don’t have both pairs of chromosomes: called haploid n

DNA in Eukaryotic Chromosomes Homologous chromosomes are usually not identical Diploid = 23 pairs of chromosomes, for a total of 46 Autosomes - 22 pairs of chromosomes Sex chromosomes - 1 pair and are different in the male and the female Female - two X chromosomes that usually look similar Male - an X and a Y chromosome that appear very different However, in a male, the X and Y chromosomes behave as a pair during meiotic cell division 1. Haploid would have 23 total 2. Autosomes have similar appearance and similar DNA sequences, and are paired in diploid cells of both sexes

The Eukaryotic Cell Cycle G2: cell growth and preparation for cell division S: synthesis of DNA; chromosomes are duplicated G1: cell growth and differentiation prophase metaphase anaphase cytokinesis telophase and interphase mitotic cell Interphase, a cell grows in size, replicates its DNA, and often differentiates Three phases: G1 (growth phase 1) Growth Differentiates Decides to divide (growth factors) S (synthesis phase) G2 (growth phase 2) Cell cycle consists of Interphase and Cell division. Most eukaryotic cells spend the majority of their time in interphase Human skin cells spend 22 hours in interphase and 2 hours dividing. G1 – newly formed daughter cell carries out one or more of 3 activities: a. almost always - is a time for acquisition of nutrients and growth to proper size c. is stimulated by growth factors to enter S S - is characterized by DNA synthesis, during which every chromosome is replicated G2 - includes completion of cell growth and preparation for division by making proteins

The Eukaryotic Cell Cycle During interphase, a cell grows in size, replicates its DNA, and often differentiates Permanently differentiated cells are stuck in interphase. Without enough cell divisions at the right time and in the right organs, development falters or body parts fail to replace worn-out or damaged cells With too many cell divisions, cancers may form 1. Heart and brain cells never divide. Liver cells are called out of differentiated state

The Eukaryotic Cell Cycle There are two types of cell division in eukaryotic cells Mitotic cell division (mitosis) Meiotic cell division (meiosis)

The Eukaryotic Cell Cycle Mitotic cell division During mitosis (nuclear division), the nucleus of the cell and the chromosomes divide Each daughter nucleus receives one copy of each of the replicated chromosomes of the parent cell During cytokinesis (cytoplasmic division), the cytoplasm is divided roughly equally between the two daughter cells, and one daughter nucleus enters each of the daughter cells 2a. Single-celled organisms (yeast, amoebas, paramecium) multicellular organisms (hydra, aspens)

Mitotic Cell Division Mitotic cell division takes place in all types of eukaryotic organisms It is the mechanism of asexual reproduction Mitotic cell division followed by differentiation of the daughter cells allows a fertilized egg to grow into an adult with perhaps trillions of specialized cells It allows organisms to maintain, repair, and even regenerate body parts It is the mechanism whereby stem cells reproduce

Mitotic Cell Division Four phases followed by cytokinesis Prophase sister chromatids centromere duplicated chromosome (two DNA double helices) (b) A eukaryotic chromosome after DNA replication Four phases followed by cytokinesis Prophase Metaphase Anaphase Telophase Cytokinesis After S phase, this is what the chromosomes look like. Then starts Prophase …

Mitotic Cell Division Prophase Spindle microtubules form Pairs of centrioles, which serve as loci from which spindle microtubules form, begin to migrate to opposite sides of the cell, to regions called spindle poles The spindle microtubules radiate from the poles, both toward the nucleus, forming a basket around it and outward toward the plasma membrane The nuclear envelop disintegrates, releasing the duplicated chromosomes After the duplicated chromosomes condense … In animal cells, … centrioles not required for proper cell division since only in animal cells. in other organisms, spindle poles are still formed.

Mitotic Cell Division in an Animal Cell INTERPHASE MITOSIS nuclear envelope chromatin nucleolus centriole pairs beginning of spindle formation kinetochore microtubules spindle pole condensing chromosomes spindle (a) Late Interphase Duplicated chromosomes are in the relaxed uncondensed state; duplicated centrioles remain clustered. (b) Early Prophase Chromosomes condense and shorten; spindle microtubules begin to form between separating centriole pairs. (c) Late Prophase The nucleolus disappears; nuclear envelope breaks down; some spindle microtubules attach to the kinetochore (blue) of each sister chromatid. (d) Metaphase Kinetochore microtubules line up the chromosomes at the cell's equator. c. Nucleolus disappears because there is no ribosome synthesis during this time. d. Metaphase (middle stage) chromosomes line up at the equator.

Mitotic Cell Division in an Animal Cell (g) Cytokinesis The ring of microfilaments contracts, dividing the cell in two; each daughter cell receives one nucleus and about half of the cytoplasm. (h) Interphase of daughter cells Spindles disappear, intact nuclear envelopes form, and the chromosomes extend completely. INTERPHASE chromosomes extending nuclear envelope re-forming nucleolus reappearing (e) Anaphase Sister chromatids separate & move to opposite poles of the cell; polar microtubules push the poles apart. polar microtubules (f) Telophase One set of chromosomes reaches each pole & begins to decondense; nuclear envelopes start to form; nucleoli begin to reappear; spindle microtubules begin to disappear; microfilaments form rings around the equator. Anaphase – sister chromatids separate and move to the poles. each pole contains one identical copy from the parent. Telophase – chromosomes at poles. Spindle microtubules disintegrate. Nucleus is put back together. mitosis can stop here producing cells with multiple nuclei (skeletal muscle cells) Cytokinesis – apart of telophase. Microfilaments attached to cell membrane form ring around equator and contracts until the cell is split in two.

Cytokinesis in a Plant Cell Stiff plant cell walls prevent the “pinching off” of cytokinesis seen in animal cells, which only have a plasma membrane This is called a cell plate The plasma membranes of the plate fuse with the plasma membrane of the cell, forming two cells, with the carbohydrate in between becoming part of the cell wall As in animals, plant cells enter G1 of interphase following cytokinesis, thus completing the cell cycle Fig. 9-10

How Is the Cell Cycle Controlled? The cells of some tissues, such as skin and intestines, divide frequently throughout the lifespan of an organism Cell division occurs rarely or not at all in other tissues, such as brain, heart, and skeletal muscles Cell division in eukaryotes is driven by enzymes and controlled at specific checkpoints 1. To repair and maintain. 3. Example if you cut your skin. Uses growth factors (hormones-like) and enzymes controlled to prevent mutations or uneven daughter cells that may become cancerous.

Why Do So Many Organisms Reproduce Sexually? Sexual reproduction is the prevalent form of reproduction Asexual reproduction by mitosis produces genetically identical offspring Sexual reproduction by meiosis shuffles the genes to produce genetically unique offspring Two parents, each with a different advantageous trait (allele), can combine those traits in one individual (their offspring) through sexual reproduction Some organisms are quite successful with only asexual reproduction. But a lot of those organisms can still reproduce sexually. 2. Combines useful, genetically determined traits to be combined so that the offspring may survive better than either of the parents. May get a completely new trait

Why Do So Many Organisms Reproduce Sexually? Genetic variability among organisms is essential for survival in a changing environment Mutations produce new variation but are relatively rare occurrences The genetic variability that occurs from one generation to the next results almost entirely from meiosis and sexual reproduction Gametes from two humans could produce about 64 trillion different combinations Better adapt organisms to changing environments! Every sperm and egg is genetically unique. There will never be anyone just like you!

The Eukaryotic Cell Cycle Meiotic cell division occurs in animal ovaries and testes prerequisite for sexual reproduction in all eukaryotic organisms Meiotic cell division involves a specialized nuclear division called meiosis, and two rounds of cytokinesis Two divisional steps produce four daughter cells that can become haploid gametes Each gamete receives one homologue of each pair of chromosomes Meiosis means to diminish

Meiosis Is a Reduction Division That Halves the Number of Chromosomes sister chromatids homologous chromosomes Meiosis separates homologous chromosomes, producing haploid daughter nuclei consists of one round of DNA replication, followed by two rounds of nuclear divisions One round of DNA replication produces two chromatids in each duplicated chromosome Because diploid cells have pairs of homologous chromosomes, with two chromatids per homologue, a single round of DNA replication creates four chromatids for each type of chromosome (a) Replicated homologues prior to meiosis (b) After meiosis I (c) After meiosis II Fig. 9-13

Meiotic Cell Division Fusion of gametes keeps the chromosome number constant between generations meiotic cell division fertilization diploid parental cells fertilized egg haploid gametes 2n n Meiosis reduces the chromosome number by half, producing haploid (n) gametes (eggs and sperm) Fusion of the gametes (fertilization) combines the two haploid chromosome sets to produce a diploid (2n) zygote

Meiotic Cell Division Prophase I, homologous chromosomes pair up and exchange DNA Crossing over If the exchanged segments carry different traits, genetic recombination has occurred Have the same stage names as mitosis with a I or II following to distinguish the nuclear division. Before this begins, the chromosomes have already been duplicated during interphase (S) and attached to a centromere. Prophase I – line up side by side to exchange segments of DNA (crossing over) there may be two or three segments exchanged. Random locations. The genes on one homologue are thus combined with an allele from the other homologue, and the combination may be totally new Spindle microtubules begin to assemble. Near end, nuclear envelope breaks down.

Meiotic Cell Division in an Animal Cell MEIOSIS I (a) Prophase I Duplicated chromosomes condense. Homologous chromosomes pair up and chromatids of homologues exchange parts by crossing over. The nuclear envelope disintegrates, and spindle microtubules form. (b) Metaphase I Paired homologous chromosomes line up along the equator of the cell. One homologue of each pair faces each pole of the cell and attaches to the spindle Microtubules. (c) Anaphase I Homologues separate, one member of each pair going to each pole of the cell. Sister chromatids do not separate. (d) Telophase I Spindle microtubules disappear. Two clusters of chromosomes have formed. Cytokinesis commonly occurs at this stage. There is little or no interphase between meiosis I and meiosis II. paired homologous chromosomes recombined chromatids spindle microtubule kinetochores chiasma Fig. 9-15a, b, c, d Metaphase I – chromosomes line up at the equator. Random in alignment. this random alignment and crossing over is responsible for genetic variability. Anaphase I – homologous chromosomes separate. Telophase I – spindle microtubules disappear. Cytokinesis occurs. No interphase before Meiosis II starts.

Meiotic Cell Division in an Animal Cell MEIOSIS II (e) Prophase II Spindle microtubules re-form and attach to the sister chromatids. (f) Metaphase II The chromosomes line up along the equator. (g) Anaphase II The chromatids separate into independent daughter chromosomes, one chromatid moving toward each pole. (h) Telophase II Nuclear envelopes re-form, and the chromosomes decondense. (i) Four haploid cells result from Cytokinesis each containing one member of each pair of homologous chromosomes. Fig. 9-15e, f, g, h, i Looks very similar to mitosis but ends with 4 haploid cells instead of 2 diploid cells. Prophase II – re-condense if need be. Metaphase II – line up at equator Anaphase II – sister chromatids separate and move to the poles. Telophase II – everything reforms and Cytokinesis occurs dividing each cell equally into 2 haploid daughter cells (4 total).

The Three Types of Eukaryotic Life Cycles multicellular diploid adult mitotic cell division and growth or asexual reproduction mitotic cell division, differentiation, and growth haploid mitotic cell division, differentiation, and growth adults zygote n meiotic cell division meiotic cell division (b) Diploid life cycle (animals) (c) Alternation of generations (plants) 2n haploid (n) stages diploid (2n) stages gametes (a) Haploid life cycle (protists, algae, fungi) fusion of spore Fig. 9-17