CHROMOSOMES AND CELL REPRODUCTION Chapter 6

HOW DO NEW CELLS FORM? cell reproduction occurs in humans and other organisms When a cell divides, the DNA is first copied and then distributed Remember Cell Theory? Cells come from other cells

BINARY FISSION asexual reproduction that produces identical offspring Prokaryotes reproduce this way single parent passes exact copies of all of its DNA to its offspring Notice how the chromosomes in the bacteria to the right are attached to the cell wall and are circular

Binary Fission has 2 stages: 1.DNA is copied (so that each new cell will have a copy of the genetic information) 1.the cell divides

Eukaryotic Cell Reproduction In preparation for division, cellular DNA and the proteins associated with the DNA coil into a structure called a chromosome When genes in DNA are being used, the DNA is stretched out so that the information it contains can be used to direct the synthesis of proteins. A gene is a segment of DNA that codes for a protein or RNA molecule.

DNA IS A DOUBLE HELIX Nucleotides  A, G, T, C Sugar and phosphate form the backbone Bases lie between the backbone Held together by H-bonds between the bases  A-T – 2 H bonds  G-C – 3 H bonds

H - BONDS Base-pairing rules  A  T only (A  U if DNA-RNA hybrid)  G  C only DNA strand has directionality – one end is different from the other end 2 strands are anti-parallel, run in opposite directions  Complementarity results  Important to replication

HELICAL STRUCTURE

DNA TEMPLATE template Each strand of the parent DNA is used as a template to make the new daughter strand DNA replication makes 2 new complete double helices each with 1 old and 1 new strand

DNA REPLICATION Semiconservative Semiconservative Daughter DNA is a double helix with 1 parent strand and 1 new strand Found that 1 strand serves as the template for new strand

REPLICATION ORIGIN Site where replication begins  1 in E. coli  1,000s in human Strands are separated to allow replication machinery contact with the DNA  Many A-T base pairs because easier to break 2 H-bonds that 3 H-bonds Note anti-parallel chains

DNA POLYMERASE An enzyme that catalyzes the addition of a nucleotide to the growing DNA chain Nucleotide enters as a nucleotide tri- PO 4 3’–OH of sugar attacks first phosphate of tri-PO 4 bond on the 5’ C of the new nucleotide  releasing pyrophosphate (PP i ) + energy

PROOFREADING

Eukaryotic Cell Reproduction Chromatids:  two exact copies of DNA that make up each chromosome  are attached at a point called a centromere.  become separated during cell division  placed into each new cell  ensure that each new cell will have the same genetic information as the original cell

HOW CHROMOSOME NUMBER AND STRUCTURE AFFECT DEVELOPMENT Sets of Chromosomes Homologous chromosomes :chromosomes that are similar in size, shape, and genetic content. Each homologue comes from one of the two parents Humans possess 46 chromosomes (2 sets 23 homologues) in somatic cells

Sets of Chromosomes Diploid: When a cell, such as a somatic cell, contains two sets of chromosomes Haploid: When a cell, such as a gamete, contains one set of chromosomes Fertilization: fusion of two haploid gametes forms a diploid zygote(a zygote is a fertilized egg cell)

CHROMOSOME NUMBER OF VARIOUS ORGANISMS

Sex Chromosomes Autosomes: chromosomes that are not directly involved in determining the sex (gender) of an individual. sex chromosomes: contain genes that will determine the sex of the individual. In humans and many other organisms, the two sex chromosomes are referred to as the X and Y chromosomes.

WHAT IF??? Change in Chromosome Number Humans who are missing even one of the 46 chromosomes do not survive Humans with more than two copies of a chromosome, a condition called trisomy, will not develop properly Abnormalities in chromosome number can be detected by analyzing a karyotype a photo of the chromosomes in a dividing cell that shows the chromosomes arranged by size

KARYOTYPE Section 1 Chromosomes Chapter 6

Change in Chromosome Structure Mutations: Changes in an organism’s chromosome structure Breakage of a chromosome can lead to 4 types of mutations: 1. deletion 2. duplication 3. inversion 4. translocation

THE LIFE OF A EUKARYOTIC CELL The Cell Cycle repeating sequence of cellular growth and division during the life of an organism 90 percent of its time is spent in the first three phases of the cycle, which are collectively called interphase. Section 2 The Cell Cycle Chapter 6

The Cell Cycle 5 Phases: 1. First growth (G 1 ) phase cell grows rapidly and carries out its routine functions (aka GAP 1) 2. Synthesis (S) phase cell’s DNA is copied during this phase. 3. Second growth (G 2 ) phase preparations are made for the nucleus to divide (aka GAP2) 4. Mitosis cell division in which the nucleus of a cell is divided into two nuclei 5. Cytokinesis cytoplasm divides is called cytokinesis.

CONTROL FREAK!!! Why don’t cells continue to grow and grow and grow? ANSWER: key checkpoints (inspection points) at which feedback signals from the cell can trigger the next phase of the cell cycle (green light) Other feedback signals can delay the next phase to allow for completion of the current phase (yellow or red light).

3. principal checkpoints: 1. Cell growth (G 1 ) checkpoint makes the decision of whether the cell will divide. 2. DNA synthesis (S) checkpoint DNA replication is checked at this point by DNA repair enzymes. 3. Mitosis checkpoint triggers the exit from mitosis.

When Control Is Lost: Cancer Cancer: the uncontrolled growth of cells Genes that contain the information necessary to make the proteins that regulate cell growth and division may be mutated

CHROMATID SEPARATION IN MITOSIS During mitosis, the chromatids on each chromosome are physically moved to opposite sides of the dividing cell with the help of the spindle. Spindles made up of both centrioles and individual microtubule fibers that are involved in moving chromosomes during cell division. When a cell enters the mitotic phase, the centriole pairs start to separate, moving toward opposite poles of the cell. As the centrioles move apart, the spindle begins to form.

CHROMATID SEPARATION IN MITOSIS, CONTINUED Forming the Spindle When a cell enters the mitotic phase, the centriole pairs start to separate, moving toward opposite poles of the cell. As the centrioles move apart, the spindle begins to form. Section 3 Mitosis and Cytokinesis Chapter 6

Separation of Chromatids by Attaching Spindle Fibers chromatids are moved to each pole of the cell As soon as the chromatids separate from each other they are called chromosomes.

MITOSIS AND CYTOKINESIS Step 1 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. Step 3 Anaphase Centromeres divide during anaphase. Step 4 Telophase A nuclear envelope forms around the chromosomes at each pole. Mitosis is complete Section 3 Mitosis and Cytokinesis Chapter 6

STAGES OF MITOSIS Section 3 Mitosis and Cytokinesis Chapter 6