Cell Growth & Division

Cell Size Take 20 minutes to do pages 5-6 in your Unit 6 packet. Ms. Allen will use the random student selector as we discuss some of the answers to the questions when the time is up.

Why do cells divide?

Why do cells divide? Exchanging Materials What types of things needs to move in and out of cells? Oxygen, CO2, Water, Food Molecules What types of things need to enter and exit the cell? What did our lab today tell us about surface to volume ratio?

Why do cells divide? Exchanging Materials Why might it be less efficient for a cell to have a large volume? Things can’t move through the cell efficiently. Things can’t move between organelles efficiently

Why do cells divide? Problems with DNA DNA, which is found in the nucleus, would not be able to reach all parts of a cell if it grew beyond a normal size. The cell would not know how to function without having these “instructions”. How might DNA be affected by cell size? Where is the DNA located in a cell? Why is DNA important?

Cell Division Cell’s do grow but before they get too big they DIVIDE.

Cell Division Cell Division – The process by which a cell divides into two identical daughter cells. DNA is copied before division so that each cell has a copy of the genetic material. Solves the problem of increasing cell size by decreasing volume and increasing surface area.

Cell Division Mitosis – The first stage of cell division. Refers to the division of the nucleus. Cytokinesis – The second stage of cell division, Refers to the division of the cytoplasm.

Chromosomes Chromosomes consist of DNA and proteins. Before cell division, each chromosome is copied to make two identical “sister” chromatids. They are attached at the centromere, located near the middle of the chromatids.

The Cell Cycle Cell Cycle: the series of events that cells go through as they grow and divide. Happens in 3 parts. Interphase – includes G1 phase, S phase and G2 phase of the cell cycle Mitosis Cytokinesis

Interphase G1 Phase: Cells increase in size and synthesize new proteins and organelles. S Phase: Chromosomes are replicated, DNA is synthesized. G2 Phase: Organelles and molecules required for cell division are produced. Usually the shortest phase.

Mitosis – M Phase Prophase Chromatin condenses into chromosomes. Spindle begins to form. Nuclear envelope breaks down.

Mitosis Metaphase Chromosomes line up in the middle. Each chromosome connects to the spindle at the centromere.

Mitosis Anaphase Sister chromatids separate into individual chromosomes and move towards the poles.

Mitosis Telophase Chromosomes gather at opposite ends and nuclear envelope forms around them.

Mitosis Cytokinesis Cytoplasm pinches in half. Two identical daughter cells form.

Regulating the Cell Cycle Cells stop dividing when they come in contact with other cells. Petri Dish Example and Healing example (cells near injury are stimulated to begin dividing, process slows as you are almost healed)

Cell Cycle Regulators Internal Regulators External Regulators Proteins that respond to events INSIDE the cell. Allow the cell cycle to proceed when certain processes have happened inside the cell (like chromosome replication) Control the Checkpoints External Regulators Proteins that respond to events OUTSIDE the cell. Direct cells to speed up or slow down. Ex. Growth factors (stimulate growth) A broken bone would trigger this.

Cell Cycle Regulators Scientists discovered the protein CYCLIN (internal regulator) The amount of cyclin in a cell fluctuates throughout the cell cycle. Cyclins regulate the timing of the cell cycle in eukaryotic cells.

Cancer Cancer: uncontrolled cell growth. Cancer cells do not respond to the signals that regulate the growth of most cells. Divide uncontrollably and form tumors. What causes cancer? Mutations in specific genes. Cell stops responding to external growth regulators. Cells fail to produce internal regulators.

Cancer P53 gene – a gene, which normally halts the cell cycle until all chromosomes have been replicated. Most cancer cells have a defect in the p53 gene.

Cancer

Chromosomes How many chromosomes do humans typically have? 23 Pairs or 46 in total. Each pair consists of a chromosome from mom and a chromosome from dad.

Meiosis Homologous chromosomes: a pair of chromosomes where one is inherited from the mom and one from dad but they may contain different genetic information. Example: Maternal chromosome: blond hair Paternal chromosome: black hair

Mitosis Review Where does mitosis take place in the human body? Somatic or body cells. Recall that before these cells divide they duplicate their DNA and the result after mitosis is two identical daughter cells that contain 23 pairs or 46 chromosomes. We refer to this as diploid.

Chromosome number A cell that contains both sets of homologous chromosomes (one chromosome from each parent) in it’s nucleus is referred to as DIPLOID (2N). In humans: 23 pairs or 46 chromosomes.

Chromosome number A human baby begins with the combination of sperm and an egg. How many chromosomes must each of these cells have in order to result in a baby with 46 chromosomes? Half the number. So 23 chromosomes. A cell that contains only one of each of the homologous pairs (i.e. egg and sperm cells) in the nucleus is referred to as HAPLOID (N).

Sex Cells In order to make sperm and eggs, a different type of cell division must occur so that the sperm and egg are HAPLOID (contain half the amount of DNA). Meiosis is a process of cell division that reduces the number of chromosomes by half in each cell by separating homologous chromosomes. This process involves the same phases of Mitosis but requires two divisions (1 2  4)

Comparing Meiosis and Mitosis Similarities: Same phases Types of cell division Start with the same number of chromosomes.

Comparing Meiosis and Mitosis Differences Meiosis: Results in 4 non-identical daughter cells. Reproductive cells Results in haploid cells. (Half the number of chromosomes at the end.) More phases Mitosis Results in 2 identical daughter cells. Body (somatic) cells. Results in diploid cells Fewer phases.

Your Task On the next page of your packet there is a blank diagram of the phases of meiosis. Your job is to use online resources or your book to draw pictures for each of the phases. You will notice a lot of similarities between Meiosis and Mitosis but make sure that you’re looking at MEIOSIS images. Don’t forget to include labels and a short description under each of the phases. I highly suggest you use colored pencils so you can show where each chromosome is by the end of the process.