Chapter 6: Chromosomes and Cell Reproduction Biology II

Cell Division Cell division occurs at different times in an organism’s life Growth and development Repair Reproduction Gametes – reproductive cells (egg/sperm) When a cell divides, DNA is first copied and then distributed so that each cell ends up with a copy of DNA

Prokaryotic Cell Division Prokaryotes have single circular DNA molecule Reproduce by binary fission Form of asexual reproduction that produces identical offspring Occurs in 2 stages DNA is copied Cell divides by adding a new cell membrane to a point on membrane between the 2 DNA copies

Genes Information encoded in DNA is organized into units called genes Segment of DNA that codes for a protein or RNA molecule Single DNA molecule has 1000s of genes linked together Genes play important role in determining how an organism’s body develops and functions When genes are being used, the DNA is stretched out so that its information can be used to direct protein production

Eukaryotic Cell Division As eukaryotic cell prepares to divide, chromosomes become visible DNA and proteins associated with DNA Before DNA coils, DNA is copied, forming chromatids 2 exact copies of DNA that make up each chromosome Attached at a point called a centromere Chromatids separate during cell division and are placed into each new cell, ensuring that each new cell will have same genetic information as the original cell

Homologous Chromosomes Each human somatic (body) cell has 2 copies of 23 chromosomes (total of 46 chromosomes) Each pair is made up of homologous chromosomes Similar in size, shape, and genetic content Each homologue comes from one of the 2 parents Differ in size, shape, and sets of genes Each contains 1000s of genes

Sets of Chromosomes All cells in the body, other than gametes, are somatic cells Said to be diploid (2n), since they contain 2 sets of chromosomes Symbol “n” is used to represent 1 set of chromosomes (human diploid #: 2n = 46) Gametes are said to be haploid (n), since they contain only one set of chromosomes Fusion of 2 haploid gametes, known as fertilization, forms a diploid zygote Fertilized egg cell

Numbers of Chromosomes Each organism has a characteristic number of chromosomes Number of chromosomes remain constant within each species

Sex Chromosomes Humans have 22 pairs of autosomes and 1 pair of sex chromosomes Autosomes – chromosomes that are not directly involved in determine the gender of an individual Sex chromosomes – contain genes that will determine sex of individual Referred to as X and Y chromosomes in humans and many other organisms Sex of offspring is determined by male XX: Female XY: Male In some insects (grasshoppers), females are XX and males are XO (O = absence of chromosome) In birds, moths, and butterflies, male is XX and female is XO

Karyotypes Presence of normal number of chromosomes is necessary for normal development and function Abnormalities in chromosome number can be detected by analyzing a karyotype Photo of the chromosomes in a dividing cell that shows the chromosomes arranged by size

Trisomy and Nondisjunction Trisomy is a condition in which humans have more than 2 copies of a chromosomes Occurs if one or more chromosomes fail to separate properly, an event known as nondisjunction Trisomy 21 is known as Down’s syndrome Characterized by short stature, round face with upper eyelids that cover inner corners of eyes, and varying degrees of mental retardation More likely in pregnancies of older women because eggs can accumulate increasing amount of damage over time

Change in Chromosome Structure Changes in organism’s chromosome structure are called mutations Breakage of chromosomes can lead to 4 types of mutations Deletion mutation – piece of chromosome breaks off completely Duplication mutation – chromosome fragment attaches to its homologous chromosome Inversion mutation – chromosome piece reattaches to original chromosome but in reverse orientation Translocation mutation – chromosome piece reattaches to a nonhomologous chromosome

Section 6-2 The Cell Cycle

The Cell Cycle Cell division more complicated in eukaryotes because it involves dividing chromosomes, cytoplasm, and other organelles Cell cycle: repeating sequence of cellular growth and division during life of an organism 90% of cell’s time is spent in first of 3 phases, collectively called interphase Cell enters last 2 phases only if it is about to divide

Stages of Interphase First growth phase (G1): cell grows rapidly and carries out routine functions Cells remain in this phase until they prepare to divide Some cells (nerve/muscle cells) never divide Synthesis phase (S): cell’s DNA is copied At end of this phase, each chromosome consists of 2 chromatids attached at a centromere Second growth phase (G2): preparations are made for nucleus to divide and microtubules are rearranged in preparation for mitosis

Stages of Cell Division Mitosis: process during cell division in which nucleus of cell is divided into 2 nuclei Each nucleus ends up with the same number and kinds of chromosomes as original cell Cytokinesis: process during cell division in which cytoplasm divides Recall: Mitosis and cytokinesis produce new cells identical to the original cells Allows organisms to grow, replace damaged tissues, or to reproduce asexually

Control of Cell Cycle Cells know when to divide based on a series of three main checkpoints at which feedback signals from cell can trigger or delay the next phase Cell growth (G1) checkpoint: makes decision whether cell will divide Are conditions favorable for division? Is cell healthy and large enough? If so, certain proteins will stimulate cell to begin synthesis phase DNA synthesis (G2) checkpoint: DNA replication is checked by DNA repair enzymes Mitosis checkpoint: triggers exit from mitosis and signals beginning of G1 phase

Cancer Specific genes contain information to make proteins that regulate cell growth and division If one of these genes is mutated, protein may not function and regulation of cell growth and division may be disrupted Cancer: uncontrolled growth of cells Some mutations cause cancer by overproducing growth-promoting molecules, speeding up cell cycle Others cause cancer by inactivating control proteins that normally act to slow or stop cell cycle

Mitosis and Cytokinesis Section 6-3 Mitosis and Cytokinesis

Mitosis and the Spindle During mitosis, the chromatids of each chromosome are physically moved to opposite sides of dividing cell Structures called spindles are involved in moving these chromosomes Made up of centrioles and individual microtubule fibers

Spindle Formation Organelles that organize the assembly of the spindle are called centrosomes Found at each of the cell’s poles In animals, a pair of centrioles is found inside each centrosome Like spindle fibers, centrioles are made of microtubules Each spindle fiber consists of an individual microtubule Each centriole is made of nine triplets of microtubules arranged in a circle Though plant cells do not have centrioles, they form a spindle almost identical to that of an animal cell

Separation of Chromatids As cell prepares to divide, microtubules in spindle extend out toward opposite poles of cell Once microtubules attach to centromeres and poles, the 2 chromatids (now called chromosomes) can be separated As the paired chromatids separate, they move toward opposite poles along paths described by microtubules to which they are attached Chromatids draw closer to poles of cell as spindle microtubules break down and become shorter Once breakdown of the spindle is complete, each pole has one complete set of chromosomes

Stage 1: Prophase Stage 1 of mitosis is called prophase During this stage: Chromosomes coil up, becoming visible Nuclear envelope dissolves Spindle forms

Stage 2: Metaphase The second stage of mitosis is called metaphase During this stage: Chromosomes move to center of cell and line up along equator Spindle fibers link chromatids of each chromosome to opposite poles

Stage 3: Anaphase The third stage of mitosis is called anaphase. During this stage: Centromeres divide Separated chromatids (now called chromosomes) move toward opposite poles as spindle fibers attached to them shorten

Stage 4: Telophase The last stage of mitosis is called telophase. During this stage: Nuclear envelope forms around chromosomes at each pole Chromosomes (now at opposite poles) uncoil Spindle dissolves as spindle fibers break down and disappear

Stages of Cell Cycle

Cytokinesis As mitosis ends, cytokinesis begins. Cytoplasm of cell is divided in half Cell membrane grows to enclose each cell, resulting in formation of 2 separate but genetically identical cells

Cytokinesis: Plant vs. Animal Cells Cytokinesis varies according to cell type: In animals (lack cell walls), cells are pinched in half by belt of protein threads In plants (have cell walls), vesicles formed by Golgi apparatus fuse at midline of dividing cell to form a cell plate A new cell wall then forms on both sides of cell plate, separating the plant cell into 2 genetically identical cells