Cell Growth “All living things grow and develop”

Cell Size Limitations 1.Over burdened DNA 1.DNA = nucleic acid that provides instructions to ribosomes to produce protein 2.DNA controls activities of the cell within the nucleus 2.Surface area to volume ratio 1.Surface area increases slowly 2.Volume increases quickly 3.Just can’t get enough or get rid of enough 1.Cell membrane is site of material exchange 2.Diffusion, facilitated diffusion, osmosis too slow

DNA Overload There’s too much work!!! Not enough information to go around in the cell As a cell grows, it has more organelles and conducts more activities Cellular activities require enzymes Enzymes are proteins; therefore, must be produced through protein synthesis Protein synthesis requires information from DNA

Just Can’t Get Enough Can’t get enough oxygen, food and water across the cell membrane to supply the cell Can’t get rid of waste products quickly enough to prevent self poisoning Process of diffusion/osmosis is too slow

Surface Area to Volume Ratio The surface area increases slower than the volume... OR... The volume increases faster than the surface area!! Therefore, the surface area to volume ratio decreases!! More volume = more demands because the rate of consumption increases Surface area can’t keep up with demands

Surface Area

Surface Area vs. Volume Surface area to volume ratio: 6:1 or 6/1 = 6 Surface area to volume ratio: 24:8 or 24/8 = 3 Surface area to volume ratio: 96:64 or 96/64 = 1.5 Surface area = 96 mm 2 Volume = 64 mm 3 As a cell grows, notice the value of the surface area to volume ratio! It decreases!

So... What can cells do about that? Cell division – the process used by cells to divide Dividing rectifies the problem of putting too much demand on DNA Dividing increases the surface area to volume ratio Dividing reduces the size of the cell making it possible to distribute nutrients and remove waste!

Replication DNA makes copies of itself Review: –DNA unzips –Complementary nucleotides attach –Two new strands of identical DNA are produced –Each new strand has one strand from the original DNA must be copied so that each new cell has a complete set of DNA of the original

Reproduction Reproduction is the production of offspring or a new organism There are two general methods/strategies of reproduction: –Asexual reproduction – production of offspring that requires only one parent –Sexual reproduction – production of offspring that requires the combination of two different cells

Asexual Reproduction Organisms produce offspring through one parent Offspring have the same genetic information (information for making proteins) as the parent They are, in essence, clones!!! Unicellular organisms, like bacteria, usually reproduce asexually Some multicellular organisms reproduce asexually

Asexual Reproduction & Multicellular Organisms Organisms such as plants and protists can reproduce asexually Hydra reproduce through budding! Plants can reproduce asexually –Bulbs –Cuttings –Rhizomes –Roots/shoots/runners

Budding in Hydra

Binary Fission of Amoeba

Runners of Strawberry Plants

Asexual Reproduction in Plants

Asexual Reproduction = Clones

Sexual Reproduction Sexual reproduction is a process where two cells (gametes) from different parents join together to form a new organism Offspring of sexual reproduction have different genetic information from both parents Most plants and animals and some unicellular organisms reproduce sexually

Sexual Reproduction

Asexual vs. Sexual Reproduction Asexual No need to find mate; therefore, quicker No genetic diversity; therefore, vulnerable Sexual Must find a mate; therefore, slower Genetically diverse; therefore, healthy

Chromosome A chromosome is a rod-shaped structure that is made of coiled up strands of chromatin and proteins that only forms before cell division.

DNA is a long fine strand of nucleotides with a sugar-phosphate backbone arranged in a double helix (twisted ladder). (a) DNA is wrapped around proteins called histones. Eight histones form a nucleosome. (b) Nucleosomes are bunched/stacked together to form chromatin. (c) Chromatin initially form loops that form coils. (d) The coils of chromatin form more coils. (e) The supercoils of chromatin form only one side of a replicated chromosome, the chromatid.

DNA to Chromosome

Chromosome Parts Chromatid Centromere Sister Chromatids

Chromosomes in Prokaryotes vs. Eukaryotes The cell of a prokaryote does not have a nucleus DNA is found in the cytoplasm; therefore, chromosomes are found in the cytoplasm. Chromosome forms into a circular structure DNA in a eukaryotic cell is found in the nucleus Chromosomes form inside the nucleus right before cell division Chromosomes that form are replicated chromosomes (aka duplicated) Most form familiar X shape

Chromatin Chromatin is a strand of DNA wrapped around proteins called histones. DNA is found in the form of chromatin in the nucleus during most of a cell’s life. Very fine and thread-like; therefore, very difficult to distinguish using a compound light microscope Coils to form chromosomes (visible) right before cell division

Cell Cycle 4 Phases of the Cell Cycle: G1 phase S phase G2 phase M phase

Interphase G1 – Cell growth and activity Most time spent here S – DNA replication G2 – Preparation for mitosis All organelles are copied Centrioles are produced

Cell Division Once a cell reaches a certain size, it divides Why? If you can’t answer this, go back to the beginning of this presentation! In order for the cell to produce a working cell, several things must occur before division: –DNA must be copied (replication) –Other organelles must be duplicated –Structures for division must be produced (centrioles)

Replication DNA is copied so there are two sets of identical DNA in the cell Important: when cell divides, both cells will have an exact copy of DNA Cell identity is also transferred –Cheek cells produce other cheek cells –Muscle cells produce other muscle cells

M Phase M phase is the mitotic phase 2 phases within M Phase: –Mitosis –Cytokinesis Mitosis = division of the nucleus Cytokinesis = division of the cytoplasm

Mitosis Prophase Metaphase Anaphase Telophase

Prophase Chromatin coils to form chromosomes Nuclear membrane disappears Centrioles take position and spindle fibers form

Metaphase Chromosomes line up along equatorial plane Spindle fibers attach to centromeres

Anaphase Chromatids are separated and pulled to opposite ends of the cell

Telophase Chromatids start to uncoil Nuclear membrane reappears

The Cell Cycle in Onion Root Cells

Cytokinesis Separation of cytoplasm Starts at during anaphase with formation of cleavage furrow – pinching in of the cell membrane Continues through telophase Cell membrane from opposite sides fuse and pinch off forming two new cells

Differences between plant cells and animal cells Recall: plant cells have cell wall, animal cells do not Animal cells simply go through cytokinesis and cytoplasm is divided Plant cells form a cell plate in between which later develops into the cell wall

Formation of a Cell Plate Cell Plate

Mitosis in a Plant Cell

The centrosome is located in the cytoplasm attached to the outside of the nucleus. Just before mitosis, the centrosome duplicates. The two centrosomes move apart until they are on opposite sides of the nucleus. As mitosis proceeds, microtubules grow out from each centrosome with their plus ends growing toward the metaphase plate. These clusters of microtubules are called spindle fibers. Centrioles and Centrosomes

Centrioles Each centrosome contains a pair of centrioles. Centrioles are built from a cylindrical array of 9 microtubules, each of which has attached to it 2 partial microtubules.

Made of microtubules which are made of actin and tubulin. Actin and tubulin are structural / globular proteins.

The Process of Meiosis A different form of cell division

Chromosome Number Review: –Chromosomes are supercoils of chromatin which are made of DNA –When chromosomes form during prophase, they are replicated chromosomes. Organisms have specific number of chromosomes In sexually reproducing organisms, chromosomes are found in pairs.

Diploid vs. Haploid Cells that have chromosomes in pairs are called diploid. (2N) Cells that only have one of each chromosome are called haploid. (N) The pairs of chromosomes in diploid cells are similar in the genes they carry; they are called homologous pairs. One chromosome from the pair is from the male and the other chromosome is from the female.

Diploid vs. Haploid Regular cells in the body (somatic cells) are diploid. Sex cells (gametes) are haploid. During sexual reproduction, the male sex cell (sperm) fertilizes the female sex cell (ovum).

Sexual Reproduction Sexual reproduction involves the fusion of two cells (gametes). During fertilization, the chromosomes from the sperm cell combines with the chromosomes in the ovum. If two normal body cells combine, the number of chromosomes would double. There is a necessity for a process that would not only reduce the number of chromosomes in half, but also to make sure only one of each pair of chromosomes is in the gamete.

Meiosis Meiosis is the process that produces gametes. –Separates homologous chromosomes –Makes diploid cells into haploid cells –Reduces the number of chromosomes in half –Produces new genetic combinations in the gametes.

Phases of Meiosis Meiosis is divided into two parts: –Meiosis I –Meiosis II Meiosis I has prophase I, metaphase I, anaphase I, and telophase I. Meiosis II has prophase II, metaphase II, anaphase II, and telophase II.

Meiosis I Separates homologous chromosomes Converts diploid cells to haploid cells Prophase I – nuclear membrane disappears, chromosomes appear and match up with their pair (forms tetrads), crossing over occurs Metaphase I – homologous chromosomes line up and spindle fibers attach to replicated chromosomes

Meiosis I Anaphase I – homologous chromosomes are separated and pulled to the poles of the cell. Telophase I – nuclear membrane reappears, chromosomes uncoils to chromatin, centrioles are produced Cytokinesis produces two haploid cells, each cell still has two copies of DNA

Meiosis II Separates sister chromatids of replicated chromosomes Prophase II – chromatin condenses to form chromosomes, nuclear membrane disappears, centrioles produce spindle Metaphase II – chromosomes line up and spindle fibers attach to centromeres of each replicated chromosome

Meiosis II Anaphase II – sister chromatids are separated and pulled to the poles of the cell Telophase II – individual chromosomes uncoil; nuclear membrane reappears Cytokinesis occurs to produce a total of 4 genetically different haploid cells.

Gametes to Zygotes Meiosis produces gametes in organisms that reproduce sexually –Gametes = sex cells –Male = sperm –Female = ovum (egg) Gametes are produced in the gonads –Male = testes –Female = ovaries

Gametes to Zygotes Upon fertilization, the gametes come together to form a zygote. Gametes are haploid – no homologous chromosomes. Zygote are diploid – homologous chromosomes are restored by fertilization. The zygote enters the regular cell cycle and goes through interphase. Eventually, it will go through cell division via mitosis.

Mitosis vs. Meiosis Mitosis Division occurs once Produces diploid cells Genetically identical 2 daughter cells are produced Occurs in somatic cells to produce more cells Meiosis Division occurs twice Produces haploid cells Genetically different Up to 4 cells are produced Occurs in gonads to produce gametes