Biology Review Ch 6: Cell Division & Reproduction Review Ch: 6 Cell Division & Chromosomes

Biology Review Ch 6: Cell Division & Reproduction  About 2 trillion cells are produced by an adult human body every day. That is about 25 million new cells per second. These new cells are formed when older cells divide.  Cell division, also called cell reproduction, occurs in humans and other organisms at different times in life.  Regardless of the type of cell division, all of the information stored in the molecule DNA must be present in each of the resulting cells.  When a cell divides, the DNA is first copied.  Each cell ends up with a complete copy of the DNA.

Biology Review Ch 6: Cell Division & Reproduction  Prokaryotes reproduce by a type of cell division called binary fission.  Binary Fission is a form of asexual reproduction that produces identical offspring.  In asexual reproduction, a single parent passes exact copies of all it’s DNA to it’s offspring.  Binary Fission occurs in two stages: first, the DNA is copied and second, the cell divides.

Biology Review Ch 6: Cell Division & Reproduction  The vast amount of information encoded in DNA is organized into units called genes.  A gene is a segment of DNA that carries the code to create a protein or RNA molecule.  As eukaryotic cells prepare to divide, the chromosomes (the DNA and associated proteins)coil tighter as the cell prepares to divide.  Before the DNA coils up, the DNA is copied.  The two exact copies of DNA that make up each chromosome are called chromatids

Biology Review Ch 6: Cell Division & Reproduction  The two chromatids of a chromosome are attached at a point called a centromere.  The chromatids become separated during cell division and placed into each new cell.  Somatic cells are any cell other than a sperm or egg cell.  Each human somatic cell normally has two copies of 23 different chromosomes for a total of 46 chromosomes.  The 23 chromosomes differ in shape, size, and sets of genes.

Biology Review Ch 6: Cell Division & Reproduction  Each of the 23 pairs of chromosomes consists of two homologous chromosomes.  Homologous chromosomes are chromosomes that are similar in shape, size, and genetic content.  Each homologue in a pair comes from one of the two parents.  Thus, the two sets of 46 chromosomes in human somatic cells are actually two sets of 23 chromosomes.  One set comes from the mother, the other from the father.  When a cell, such as a somatic cell, contains two sets of chromosomes, it is said to be diploid.

Biology Review Ch 6: Cell Division & Reproduction  When a cell, such as a gamate cell (sperm or egg cell) contains only one set of chromosomes, it is said to be a haploid.  The fusion of two haploid gametes, a process called fertilization, forms a diploid zygote.  A zygote is a fertilized egg cell, the first cell of a new individual.  The number of chromosomes within a cell is constant within that species. Humans always have 46 and dogs always have 78.

Biology Review Ch 6: Cell Division & Reproduction  Of the 23 chromosomes in human somatic cells, 22 pairs are called autosomes.  Autosomes are chromosomes that are not directly involved in determining the sex (gender) of an individual.  The 23rd pair contains the genes that will determine the gender of the new organism and is called the sex chromosome.  This pair of chromosomes are called the X and Y chromosomes. (XY)

Biology Review Ch 6: Cell Division & Reproduction  The genes that cause a fertilized egg to develop into a male are located on the Y chromosome.  In human males, the sex chromosomes are made up of one X chromosome and one Y chromosome (XY).  The sex chromosomes in human females consist of two X chromosomes (XX)  Because a female can donate only an X chromosome, the sex of an offspring is determined by the male who can donate either an X or a Y.

Biology Review Ch 6: Cell Division & Reproduction  Each of a persons 46 chromosomes has thousands of genes. The presence of all 46 chromosomes is essential for normal development to occur.  Sometimes a human will have more than two copies of a chromosome in a cell which will result in abnormal development.  Abnormalities in chromosome number can be detected using a karotype, a photo of the chromosomes in a dividing cell that shows the chromosomes arranged by size.

Biology Review Ch 6: Cell Division & Reproduction  The life of a eukaryotic cell is often shown as a cycle. The cell cycle is a repeating sequence of cellular growth and division during the life of an organism.  The 5 phases of cell growth: First growth (G1) phase Synthesis (S) phase Second Growth (G2) phase Mitosis Cytokinesis

Biology Review Ch 6: Cell Division & Reproduction Stage 1: G1 phase (first growth)  During the G1 phase, a cell grows rapidly and carries out it’s routine functions. Cells that are not dividing stay in the G1 phase. For most organisms, this phase occupies the major portion of the cell’s life. Stage 2: Synthesis (S) phase  A cell’s DNA is copied during this phase. At the end of this phase, each chromosome consists of two chromatids attached at the centromere.

Biology Review Ch 6: Cell Division & Reproduction Stage 3: Second Growth (G2)  In the G2 phase, preparations are made for the nucleus to divide. Hollow protein fibers called microtubules are rearranged during G2 in preparation for mitosis. Stage 4: Mitosis: nucleus divides  The process during cell division in which the nucleus of a cell is divided into two nuclei is called mitosis. Each nucleus ends up with the same number and kinds of chromosomes as the original cell.

Biology Review Ch 6: Cell Division & Reproduction Stage 5: Cytokinesis  The process during cell division in which the cytoplasm divides is called cytokinesis.  At this point, the cell divides entirely into two separate cells, each now with a copy of the DNA in it’s nucleus.  Mitosis and cytokinesis produce new cells that are identical to the original cells. This allows organisms to grow, replace damaged tissues, and, in some organism, reproduce asexually.

Biology Review Ch 6: Cell Division & Reproduction When Control is Lost: Cancer  Certain genes control the information that trigger the proteins that regulate cell growth and division.  If one of these genes is mutated, the protein may not function and regulation of cell growth and division can be disrupted and malfunction.  Cancer, the uncontrolled growth of cells, may result from this.  Cancer cells do not respond normally to the body’s control mechanisms. Cancer cells keep dividing uncontrollably with no signal to stop.

Biology Review Ch 6: Cell Division & Reproduction  Every second about 2 million new blood cells are produced in your body by cell divisions occurring in the bone marrow. These cells have received the signal to divide.  The cells advance beyond the G2 phase and enter into the last two phases; mitosis and cytokinesis.  During mitosis, the nuclei divide to form two nuclei, each containing a complete set of the cell’s chromosomes with the DNA carrying all the genes.

Biology Review Ch 6: Cell Division & Reproduction  During mitosis, the chromatids on each chromosome are physically moved to opposite sides of the dividing cell with the help of the spindle.  Spindles are cell structures made up of both centrioles and individual microtubule fibers that are involved in moving chromosomes during cell division.

Biology Review Ch 6: Cell Division & Reproduction  Although mitosis, the division of the nucleus, is a continuous process, biologists traditionally divide it into four stages Step 1: Prophase Step 2: Metaphase Step 3: Anaphase Step 4: Telophase

Biology Review Ch 6: Cell Division & Reproduction Step 1: Prophase  Chromosomes coil up and become visible during prophase. The nuclear envelope dissolves and a spindle forms. Step 2: Metaphase  During metaphase, the chromosomes move to the center of the cell and line up along the equator. Spindle fibers link the chromatids of each chromosome to opposite poles.

Biology Review Ch 6: Cell Division & Reproduction Step 3: Anaphase  Centromeres divide during anaphase. The two chromatids (now called chromosomes) move toward opposite poles as the spindle fibers attached to them shorten. Step 4: Telophase  A nuclear envelope forms around the chromosomes at each pole. Chromosomes, now at opposite poles, uncoil and the spindle dissolves. The spindle fibers break down and disappear. Mitosis is complete. The cell’s nucleus has now divided into two parts, each with a copy of the DNA.

Biology Review Ch 6: Cell Division & Reproduction  As mitosis ends with the cell nucleus being divided, cytokinesis begins.  During cytokinesis, the cytoplasm of the cell is divided in half, and the cell membrane grows to enclose each cell, forming two separate cells as a result.  The end result of mitosis and cytokinesis is two genetically identical cells where only one cell existed before. Cytokinesis: the cytoplasm divides

Biology Review Ch 6: Cell Division & Reproduction  In plant cells and other cells that have rigid cell walls, the cytoplasm is divided in a different way.  In plant cells, vesicles formed by the Golgi Apperatus fuse at the midline of the dividing cell and form a cell plate.  A cell plate is a membrane bound cell wall that forms across the middle of the plant cell. A new cell wall than forms on both sides of the cell plate. When complete, the cell plate separates the plant cell into two new plant cells. Cell plate