Cell Reproduction Chapter 8

Cell Division A basic feature of living things is the ability to reproduce themselves Unicellular organisms produce a new individual Multicellular organisms create new members Cell division can occur for several reasons Create new individuals, reproduction Replace worn out cells Repair damage

Eukaryotes and prokaryotes differ Prokaryotes with simpler structure: binary fission Cell divides into two Eukaryotes: the nucleus must divide: Mitosis Makes exact copy – clone - of parent cell; Identical daughter cells

Binary fission 1st, DNA must all be copied so each cell gets the same blueprint. Then the cell splits into two cells http://www.anselm.edu/homepage/jpitocch/genbio/binfission.JPG

Eukaryotic cell division More complicated More DNA, packaged in pieces: chromosomes Chromosomes come in pairs: homologous chromosomes Nuclear membrane in the way Microtubules used to pull chromosomes around

Homologous chromosomes: Same, but not identical. Karyotype analysis

Cell cycle The cell doesn’t suddenly decide to divide and poof, mitosis happens. The activities of the cell can be described as a cell cycle. Overview of the cell’s activity: Interphase – “resting stage” Mitosis – nuclear division Cytokinesis – division of the cytoplasm

The Cell Cycle

Interphase Cell is busy doing its job – just not dividing Three parts: Nerve cells spend most of life in interphase Cancer cells spend little time in interphase Three parts: G1 or Gap 1 or growth phase S phase : Synthesis of DNA ; cell is committed to division G2 or Gap 2

Normally, DNA is wrapped around proteins (histones) to make nucleosomes (beads on a string). This is wound up to make a solenoid which is looped around in the nucleus, appears as chromatin. For mitosis, DNA has to be wrapped up even tighter so it can be easily divided up.

Mitosis Refers specifically to division of the nucleus Four phases: Prophase Metaphase Anaphase Telophase We look for things that are happening to break mitosis into steps, but it is one continuous process. PMAT

Centrioles: the puppet masters. Themselves made of microtubules, they organize the microtubules that attach to the chromosomes and pull them to where they belong.

Prophase Chromosomes wind up or condense 2 meters / cell Linked by centromere Sister chromatids: pairs of identical chromosomes Nucleoli disappear; mitotic spindle forms from the centrioles Mitotic spindle: web of microtubules that attaches to DNA to control where it goes.

Late prophase (also called Prometaphase) Nuclear membrane disappears Nuclear membrane would just be in the way. Chromatids attach to fibers of mitotic spindle by means of specialized structure called a kinetochore Review: centromere is where sister chromatids attach to each other; kinetochore is thepart of the centromere where the microtubules attach to the sister chromatids. Chromatids are identical pieces of DNA made during S phase of the cell cycle.

Metaphase Chromosomes line up in the center of the cell at the metaphase plate – a disc The next step of mitosis doesn’t happen until ALL the chromosomes are lined up and ready to go.

Anaphase Centromere splits Microtubles of mitotic spindle pull members of each pair of duplicate chromosomes to opposite sides of the cell.

Telophase Mitotic spindle disappears Nuclear membranes form Chromosomes unwind In other words, everything does back to the way it was except now there are two nuclei, and the rest of the cell is dividing too.

Cytokinesis Division of the cytoplasm Accompanies mitosis Begins in anaphase and finishes after telophase Animal cells form a contractile ring using actin – one of the proteins found in muscles. Indentation is called the cleavage furrow Pinches the cells apart

Plant cells Mitosis occurs in plants as in animal cells, but cytokinesis is different because of the plant cell wall. A disc of new membrane called the cell plate forms between the two cells during telophase and expands until it reaches the edges of the cell membrane. The new cells then make cellulose fibers to form new cell walls.

What controls whether a cell divides? Cell size hormones Growth factors Cyclins: proteins that increase and decrease during the cell cycle

Cancer is unregulated cell growth Much of what we know about how and why a cell divides was learned from trying to understand why cells lose control. Cancer is unregulated cell growth If we know what normally controls cell division, we can find out how it goes bad. Genes involved in cell regulation named from cancer studies: Oncogenes – genes that turn on division Oncology – the study of cancer Tumor suppressor genes – prevent unwanted cell division.

Tumors - neoplasms Cells have lost control over cell division Multiply, often without stopping. Benign tumors grow only in one area Cancers invade local tissues (look like a crab) and can metastasize or spread to other areas of the body through the vascular or lymphatic systems

Cancer cells lose contact inhibition.