Chapter 9 – Cell reproduction

Sizes of living things

Surface area represents ability to take in/get rid of materials. Volume represents needs of the cell Rather than grow bigger, cells divide to increase in number

Small cube 1 mm tall Surface area = 6 mm 2 Volume = 1 mm 3 Surface area to volume ratio = 6:1 Larger cube 2 mm tall Surface area = 24 mm 2 Volume = 8 mm 3 Surface area to volume ratio = 3:1

Why are cells so small? Cells take in nutrients and expel wastes across the plasma membrane, which surrounds the cell. Staying small ensures that the cell can more efficiently take in nutrients and expel wastes Movement of substances within the cell are better managed with small cell than large Substances move by diffusion or through movement of the cytoskeleton – occurs too slowly if cell is too big Cells communicate better when small Movement of signaling proteins within the cell can only work in smaller cells

Cell Increase and Decrease Cell division increases the number of somatic (body) cells Two parts of cell division: Mitosis (division of nucleus) Cytokinesis (division of cytoplasm) Apoptosis (cell death) decreases the number of cells.

Cell division occurs when: Body growth Maintenance and repair Fighting infection Replacing worn/dead cells Apoptosis occurs when: Tail of tadpole disappears  frog Skin between human fingers and toes dies during development Death of cells leading to leaves falling from trees in fall Both cell increase and apoptosis occur during normal development and growth – example of homeostasis.

The Cell Cycle An orderly sequence of events that occurs from the time a cell is first formed until it divides into two new cells.

Most of the cell cycle is spent in interphase: G 1 stage – cell growth, cell doubles its organelles (cell structures), prepares for DNA replication S stage – DNA replication occurs G 2 stage – cell makes proteins needed for cell division Amount of time spent in interphase varies – average for adult mammals is 20 hours Nerve cells and muscle cells exit the cell cycle  G 0 phase

Following interphase is the M stage, including mitosis and the C stage, when cytokinesis occurs (definitions slide #5). During mitosis, two copies of DNA made during replication are separated, and become the nuclei of the two daughter cells – takes about 4 hours. The cell cycle ends when cytokinesis, the splitting of the cytoplasm, is complete.

Chromosome Structure In a non-dividing cell – genetic material is in the form of chromatin (DNA & protein) In a dividing cell, chromatin undergoes coiling to form chromosomes Proteins called histones package the DNA so it can fit into the nucleus (2 meters of DNA fit into nucleus that is 5 micrometers)

After replication, there are 2 identical sister chromatids, held together by a centromere

Each species has a set number of chromosomes: Humans – 46 Crayfish – 200 Corn – 20 Adder’s tongue fern – 1262 Chimpanzee - 48 Sand dollar – 52 Dog – 78 Cat - 32

Body cells contain the diploid (2n) number of chromosomes – 2 chromosomes of each kind (1 from each parent) Sex cells (eggs and sperm) contain only 1 chromosome of each kind – haploid (n) number of chromosomes

Mitosis – occurs in body cells – diploid cells divide to produce diploid cells – daughter cells are genetically identical to parent cells Animation of mitosis

Mitosis overview

1.Centriole 2.Chromatin 3.Nucleolus (in yellow) 4.Nuclear membrane 5.Spindle fibers 6.Chromosome (replicated) 7.Centromere 8.Sister Chromatids (each half of replicated chromosomes) 9.Daughter Chromosomes (once the replicated chromosome splits) 10.Cell membrane 11.Cleavage furrow 12.Asters 13.Centrosome (=aster + centriole)

Late Interphase Centrosomes (which contain pair of centrioles and an aster – which are short microtubules) duplicate Chromatin condenses into chromosomes

Early Prophase (sometimes referred to as prophase) Chromosomes become visible Centrosomes move to opposite ends of the cell Nucleolus disappears

Late Prophase (sometimes referred to as prometaphase) Nuclear membrane disappears Spindle fibers form Chromosomes become attached to spindle fibers – centromere attaches to spindle fibers

Metaphase Chromosomes line up at metaphase plate – equidistant from poles

Anaphase Centromeres holding sister chromatids divide Sister chromatids separate, becoming daughter chromosomes, and move toward opposite ends of cell

Telophase Spindle disappears Nuclear membrane reappears Chromosomes turn into chromatin Nucleolus reappears

Mitosis in Plant Cells Same phases as in animal cells Have centrosome and spindle, but no centrioles or asters

Cytokinesis, or division of cytoplasm, accompanies mitosis. Cleavage of the cytoplasm begins in anaphase, but is not completed until just before the next interphase. Newly-formed cells receive a share of organelles made during interphase.

Cytokinesis in Animal Cells A cleavage furrow (indentation of membrane where cell will divide) begins at the end of anaphase. A band of actin and myosin filaments, called the contractile ring, slowly forms a constriction between the two daughter cells. A narrow bridge between the two cells is apparent during telophase, then the contractile ring completes the division.

Cytokinesis in animal cells

Cytokinesis in Plant Cells The rigid cell wall surrounding plant cells cannot form a cleavage furrow. Instead, a cell plate forms from vesicles released by the Golgi apparatus (a part of the cell that processes proteins) New plant cell walls form and are later strengthened by cellulose fibers.

Cytokinesis in plant cells

Cell Division in Prokaryotes The process of asexual reproduction in prokaryotes is called binary fission. The two daughter cells are identical to the original parent cell, each with a single chromosome. Following DNA replication, the two resulting chromosomes separate as the cell elongates. Cell divides without cell structures seen in plants & animals

Animation of binary fission

Meiosis Produces sex cells (gametes) – eggs & sperm Reduces the chromosome number so that egg or sperm cells each have only one of each kind of chromosome (2n  1n). The process ensures that the next generation will have: 1)the diploid number of chromosomes 2)a combination of traits that differs from that of either parent.

Overview of meiosis

Meiosis involves two cell divisions and produces four haploid cells. Humans have 23 pairs of homologous chromosomes (chromosomes with the same genes), or 46 chromosomes total. Prior to meiosis I, DNA replication occurs.

In many organisms, haploid daughter cells mature into gametes (sex cells – eggs and sperm) Fertilization (fusion of egg and sperm) - restores the diploid number of chromosomes

Phases of Meiosis The same four phases seen in mitosis – prophase, metaphase, anaphase, and telophase – occur during both meiosis I and meiosis II. The period of time between meiosis I and meiosis II is called interkinesis. No replication of DNA occurs during interkinesis Animation Meiosis square dance

Meiosis I Prophase I Nuclear memebrane & nucleolus disappear Spindle forms Homologous chromosomes pair during synapsis; pair of homologous chromosomes are referred to as a tetrad Metaphase I Homologous chromosomes line up at metaphase plate

Anaphase I Homologous chromosomes separate & move to opposite poles Telophase I Nuclear membrane and nucleolus reappear Cytokinesis occurs Interkinesis Period of time between meiosis I and II

Meiosis II Prophase II Spindle reappears, nucleolus and nuclear membrane disappear Chromosomes attach to spindle Metaphase II Chromosomes line up at metaphase plate

Anaphase II Sister chromatids separate, becoming daughter chromosomes Telophase II Spindle disappears, nuclear membrane and nucleolus reappear Cytokinesis divides the cells

Genetic Recombination Genetic variation occurs in several ways: 1)Crossing-over of nonsister chromatids – occurs during prophase I

2) Independent assortment of homologous chromosomes – separate in a random manner – 2 23 or 8,388,608 possible combinations of the 23 pairs of chromosomes

3)Combining of chromosomes of genetically different gametes during fertilization Between crossing over, combinations of gametes produced, and random combining of sperm and egg, variation is endless in the human population

Meiosis vs. Mitosis Mitosis DNA replication occurs only once during interphase. One cell division Two diploid daughter cells – genetically identical to parent Meiosis DNA replication occurs only once during interphase. Two cell divisions. Four haploid daughter cells – genetically different from parent

Mitosis Daughter cells are identical to each other Occurs in all somatic cells for growth and repair Meiosis Daughter cells are different from each other Occurs only in the reproductive organs for the production of gametes

Meiosis compared to mitosis

The Human Life Cycle Requires both mitosis and meiosis. In males, meiosis occurs as spermatogenesis and produces 4 haploid sperm. In females, meiosis occurs as oogenesis and produces 1 egg cell. Mitosis is involved in the growth of a child and repair of tissues during life.

Life cycle of humans

Spermatogenesis Diploid primary spermatocytes undergo meiosis I to produce haploid secondary spermatocytes. Secondary spermatocytes divide by meiosis II to produce 4 haploid spermatids. Spermatids mature into sperm with 23 chromosomes.

Spermatogenesis

Oogenesis Diploid primary oocyte undergoes meiosis I to produce one haploid secondary oocyte and one haploid polar body. Secondary oocyte begins meiosis II, stops at metaphase II, and is released from the ovary. Meiosis II will be completed only if sperm are present. Following meiosis II, there is one haploid egg cell with 23 chromosomes and up to three polar bodies. Polar bodies serve as a dumping ground for extra chromosomes – will disintegrate.

Oogenesis

In humans, both sperm cells and the egg cell have 23 chromosomes each (1n) Following fertilization of the egg cell by a single sperm, the zygote has 46 chromosomes, the diploid number (2n) found in human somatic cells. The 46 chromosomes represent 23 pairs of homologous chromosomes. Cell differentiation occurs during development resulting in a variety of cell types

Control of the cell cycle Regulated by proteins called cyclins which bind to enzymes called cyclin- dependent kinases Different combinations of these at different stages of the cell cycle control different activities

Three checkpoints: 1.During G 1 prior to the S stage – if DNA is damaged, apoptosis occurs 2.During S and G 2 stage – will not proceed if DNA is damaged or not copied 3.During the M stage prior to the end of mitosis – if chromosomes are not properly aligned

Abnormal cell cycle Cancer – the uncontrolled growth and division of cells Results in a tumor, an abnormal mass of cells. Cancer cells crowd out normal cells

Carcinogenesis, the development of cancer, is a gradual process – could take decades. Apoptosis - programmed cell death Angiogenesis - the formation of new blood vessels to bring additional nutrients and oxygen to a tumor; cancer cells stimulate angiogenesis Metastasis - the invasion of other tissues by establishment of tumors at new sites

A patient’s prognosis is dependent on the degree to which the cancer has progressed; Whether tumor has invaded surrounding tissues Whether there is any lymph node involvement Whether there are metastatic tumors in distant parts of body Early diagnosis and treatment is critical to survival.

Information on staging of cancer

Causes of cancer: Mutations in segments of DNA that control the production of proteins, such as those that regulate the cell cycle Environmental factors such as carcinogens – agents that can cause cancer Ex: radiation, tobacco, chemicals