Genetics Cell Cycle Mitosis Meiosis Eukaryotic chromosomes contain

Genetics Cell Cycle Mitosis Meiosis

Eukaryotic chromosomes contain DNA and protein l The chromosomes carry the genetic information

Ø When a cell divides, chromatin fibers are very highly folded, and become visible in the light microscope as chromosomes. Ø During interphase (between divisions), chromatin is more extended, a form used for expression genetic information.

DNA is organized into informational units called genes Chromosomes contain hundreds to thousands of genes

Cell Cycle The cell cycle is a sequence of cell growth and division. The cell cycle is the period from the beginning of one division to the beginning of the next. The time it takes to complete one cell cycle is the generation time.

Cells divide when they reach a certain size NO (nerve, skeletal muscle and red blood cells) Cell division involves mitosis and cytokinesis. Mitosis involves division of the chromosomes. Cytokinesis involves division of the cytoplasm. Mitosis without cytokinesis results in multinucleate cells.

Eukaryotic cell cycle Beginning of one division to beginning of next l Stages in eukaryotic cell cycle l Interphase l First gap phase l Synthesis phase l Second gap phase l l M phase Mitosis l Cytokinesis l

Chromosomes become duplicated during interphase Cells are very active during interphase, synthesizing biological molecules and growing the G 1 (gap) phase The S (synthesis) phase is marked by DNA replication The G 2 (gap) phase occurs between the S phase and mitosis

Despite differences between prokaryotes and eukaryotes, there are several common features in their cell division processes. Replication of the DNA must occur. l Segregation of the "original" and its "replica" follow. l Cytokinesis ends the cell division process. l Whether the cell was eukaryotic or prokaryotic, these basic events must occur.

Hereditary material is passed on to new cells by mitosis or meiosis Cell division, growth, and reproduction Interphase Mitosis Cytokinesis Meiosis

Cell division • Chromosomal packaging of DNA allows efficient distribution of genetic material during cell division • Life cycle requires two distinct types of cell division processes: mitosis and meiosis • Cell division: one cell becomes two cells during an organism’s life cycle

Mitosis is nuclear division plus cytokinesis, and produces two identical daughter cells during the following steps: l l Prophase Metaphase Anaphase Telophase. Interphase is often included in discussions of mitosis, but interphase is technically not part of mitosis, but rather encompasses stages G 1, S, and G 2 of the cell cycle.

Interphase The cell is engaged in metabolic activity and performing its prepare for mitosis (the next four phases that lead up to and include nuclear division). Chromosomes are not clearly discerned in the nucleus, although a dark spot called the nucleolus may be visible. The cell may contain a pair of centrioles (or microtubule organizing centers in plants) both of which are organizational sites for microtubules.

Prophase Ø Chromatin in the nucleus begins to condense and becomes visible in the light microscope as chromosomes. ØThe nucleolus disappears. ØCentrioles begin moving to opposite ends of the cell and fibers extend from the centromeres. ØSome fibers cross the cell to form the mitotic spindle.

Prometaphase The nuclear membrane dissolves, marking the beginning of prometaphase. Proteins attach to the centromeres creating the kinetochores. Microtubules attach at the kinetochores and the chromosomes begin moving.

Metaphase Spindle fibers line the chromosomes along the middle of the cell nucleus. This line is referred to as the metaphase plate. Polar microtubules extend from the pole to the equator, and typically overlap Kinetochore microtubules extend from the pole to the kinetochores This organization helps to ensure that in the next phase, when the chromosomes are separated, each new nucleus will receive one copy of each chromosome.

Anaphase The paired chromosomes separate at the kinetochores and move to opposite sides of the cell. The chromosomes are pulled by the kinetochore microtubules to the poles and form a "V" shape Motion results from a combination of kinetochore movement along the spindle microtubules and through the physical interaction of polar microtubules.

Telophase Chromatids arrive at opposite poles of cell, and new membranes form around the daughter nuclei. The chromosomes disperse and are no longer visible under the light microscope. The spindle fibers disperse, and cytokinesis will start.

Cytokinesis In animal cells, cytokinesis results when a fiber ring composed of a protein called actin around the center of the cell contracts pinching the cell into two daughter cells, each with one nucleus. In plant cells, synthesis of new cell wall between two daughter cells rather than cleavage furrow in cytoplasm

Interphase

Prophase

Prometaphase

Metaphase

Anaphase

Telophase

Cytokinesis

Animated GIF (203 Kb)

Reproduction Asexual reproduction Sexual reproduction

Asexual Reproduction A form of duplication using only mitosis. Example, a new plant grows out of the root or a shoot from an existing plant. Produces only genetically identical offspring since all divisions are by mitosis.

Sexual reproduction Formation of new individual by a combination of two haploid sex cells (gametes). Fertilization- combination of genetic information from two separate cells that have one half the original genetic information Gametes for fertilization usually come from separate parents

Female- produces an egg 2. Male produces sperm 1. Both gametes are haploid, with a single set of chromosomes The new individual is called a zygote, with two sets of chromosomes (diploid). Meiosis is a process to convert a diploid cell to a haploid gamete, and cause a change in the genetic information to increase diversity in the offspring.

Chromosomes in a Diploid Cell Summary of chromosome characteristics Diploid set for humans; 2 n = 46 Autosomes; homologous chromosomes, one from each parent (humans = 22 sets of 2) Sex chromosomes (humans have 1 set) 1. Female-sex chromosomes are homologous (XX) 2. Male-sex chromosomes are non-homologous (XY)

Number of sets of chromosomes in a cell Haploid (n)-- one set chromosomes Diploid (2 n)-- two sets chromosomes Most plant and animal adults are diploid (2 n) Eggs and sperm are haploid (n)

Most cells in the human body are produced by mitosis. These are the somatic (or vegetative) line cells. Cells that become gametes are referred to as germ line cells. The vast majority of cell divisions in the human body are mitotic, with meiosis being restricted to the gonads.

Diploid cells l Characteristic number of chromosome pairs per cell l Homologous chromosomes l Similar in length, shape, other features, and carry similar attributes Haploid cells l Contain only one member of each homologous chromosome pair

Meiosis Diploid cells undergo meiosis to form haploid cells Meiosis potentially produces four haploid cells Meiosis involves two separate divisions

Two successive nuclear divisions occur, Meiosis I (Reduction) and Meiosis II (Division). Meiosis I reduces the ploidy level from 2 n to n (reduction) while Meiosis II divides the remaining set of chromosomes in a mitosis-like process (division).

In meiosis, homologous chromosomes are separated into different daughter cells Meiosis I and meiosis II each include prophase, metaphase, and telophase

The First Division Meiosis I Prophase I is one of the most important stages of meiosis. During this stage, many crucial events occur.

In prophase I, The spindle appears. Nuclear envelopes disappear. The DNA of the chromosomes begin to twist and condense, making the DNA visible to the microscope. Each chromosome actively seeks out its homologous pair (which also has a sister chromatid).

The two replicated homologous pairs find each other and form a synapse. The structure formed is referred to as a tetrad (four chromatids). The point at which the two non-sister chromatids intertwine is called a chiasma. Sometimes a process known as crossing over occurs at this point. This is where two non-sister chromatids exchange genetic material. This exchange does not become evident, however, until the two homologous pairs separate.

Prophase I includes synapsis and crossing over Homologous chromosomes pair and undergo synapsis One member of a pair is the maternal homologue, the other is the paternal homologue Synapsis is the association of four chromatids (two from each homologue)

In metaphase I, the tetrads line up along the equator. Anaphase I results in the separation of homologous pairs. Cells are haploid at this point. Telophase I results in a brief reappearance of nuclear envelopes, and the spindle disappears. The cell waits momentarily during interkinesis. Interkinesis separates meiosis I and II; no DNA synthesis occurs

The Second Division Meiosis II In prophase II, the spindle reappears, and the nuclear membrane fragments. In metaphase II, the chromosomes align at the equator. In anaphase II, sister chromatids separate. In telophase II, the nuclear envelopes reappear, and four haploid cells are the result.

Interphase Prophase I

Metaphase I Anaphase I Telophase I Prophase II

Metaphase II Anaphase II Telophase II

Germ line cells undergo gametogenesis l Spermatogenesis produces sperm l Oogenesis typically produces eggs, or a single ovum and two or more polar bodies