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Ch. 8 Cellular Reproduction-Cells from Cells

By Bonnie Sims,2014-07-08 11:05
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Ch. 8 Cellular Reproduction-Cells from Cells

    CHAPTER 8 CELLULAR REPRODUCTION: CELLS FROM CELLS

    WHAT CELL REPRODUCTION ACCOMPLISHES

    ; Reproduction

    - May result in the birth of new organisms

    - Occurs much more often at the cellular level

    ; Any of your body’s cells reproducing themselves, e.g., skin cells reproducing themselves

    ; When a cell undergoes reproduction, or cell division, two “daughter” cells are produced that are

    genetically identical to each other and to the “parent” cell

    ; Before a parent cell splits into two cells, it duplicates its chromosomes, the structures that contain most

    of the organism’s DNA

    ; During cell division, each daughter cell receives one set of chromosomes ; Cell division plays a role in

    - The replacement of lost or damaged cells

    - Cell reproduction

    - Growth

; In asexual reproduction:

    - Single-celled organisms reproduce by simple cell division

    - There is no fertilization of an egg by a sperm

    - Some multicellular organisms, such as sea stars, can grow new individuals from fragmented pieces.

    - Growing a new plant from a clipping is another example of asexual reproduction.

; Sexual reproduction is different.

    - It requires fertilization of an egg by a sperm

    - Production of egg and sperm is called meiosis

    - Thus, sexually reproducing organisms use meiosis for reproduction and mitosis for growth and

    maintenance

    THE CELL CYCLE AND MITOSIS

    ; In a eukaryotic cell:

    - Most genes are located on chromosomes in the cell nucleus

    - A few genes are found in DNA in mitochondria and chloroplasts

    Eukaryotic Chromosomes

    ; Each eukaryotic chromosome contains one very long DNA molecule, typically

    bearing thousands of genes

    ; The number of chromosomes in a eukaryotic cell depends on the species ; Chromosomes

    - Are made of chromatin, a combination of DNA and protein molecules

    - Are not visible in a cell until cell division occurs ; The DNA in a cell is packed into an elaborate, multilevel system of coiling and

    folding.

    ; Histones are proteins used to package DNA in eukaryotes.

    ; Nucleosomes consist of DNA wound around histone molecules.

    ; Before a cell divides, it duplicates all of its chromosomes, resulting in two copies called sister

    chromatids.

    ; Sister chromatids are joined together at a narrow “waist” called the centromere.

; When the cell divides, the sister chromatids separate from each other

    ; Once separated, each chromatid is:

    - Considered a full-fledged chromosome

    - Identical to the original chromosome

    The Cell Cycle

    ; A cell cycle is the orderly sequence of events that extend from the time a cell is first formed from a

    dividing parent cell to its own division into two cells. ; The cell cycle consists of two distinct phases:

    - Interphase

    - The mitotic phase

    ; Most of a cell cycle is spent in interphase. ; During interphase, a cell:

    - Performs its normal functions

    - Doubles everything in its cytoplasm

    - Grows in size

    Mitosis and Cytokinesis

    ; Mitosis

    - Is the division of the chromosomes

    - Is preceded by interphase

    ; The mitotic (M) phase includes two overlapping processes:

    - Mitosis, in which the nucleus and its contents divide evenly into two daughter

    nuclei

    - Cytokinesis, in which the cytoplasm is divided in two

    ; Mitosis consists of four distinct phases:

    - Prophase

    - Metaphase

    - Anaphase

    - Telophase

; Cytokinesis

    - Typically occurs during telophase

    - Is the division of the cytoplasm

    - Is different in plant and animal cells

    Cancer Cells: Growing Out of Control

    ; Normal plant and animal cells have a cell cycle control system that consists of specialized proteins,

    which send “stop” and “go-ahead” signals at certain key points during the cell cycle.

; What Is Cancer?

    - Cancer is a disease of the cell cycle.

    - Cancer cells do not respond normally to the cell cycle control system.

    - Cancer cells can form tumors, abnormally growing masses of body cells.

    - The spread of cancer cells beyond their original site of origin is metastasis.

    - Malignant tumors can:

    ; Spread to other parts of the body

    ; Interrupt normal body functions

    - A person with a malignant tumor is said to have cancer.

; Cancer Treatment

    - Cancer treatment can involve

    ; Radiation therapy, which damages DNA and disrupts cell division.

    ; Chemotherapy, which uses drugs that disrupt cell division.

; Cancer Prevention and Survival

    - Cancer prevention includes changes in lifestyle:

    ; Not smoking ; Eating a high-fiber, low-fat diet

    ; Exercising adequately ; Visiting the doctor regularly

    ; Avoiding exposure to the sun ; Performing regular self-examinations

    MEIOSIS, THE BASIS OF SEXUAL REPRODUCTION

    ; Sexual reproduction depends on

    - Meiosis

    - Fertilization

    - Produces offspring that contain a unique combination of genes from the parents

    Homologous Chromosomes

    ; Different organisms of the same species have the same number and types of chromosomes

    ; A human somatic cell

    - Is a typical human body cell

    - Has 46 chromosomes

    ; A karyotype is an image that reveals an orderly arrangement of chromosomes. ; Homologous chromosomes are matching pairs of chromosomes that can possess different versions of

    the same genes.

    ; Humans have

    - Two different sex chromosomes, X and Y

    - Twenty-two pairs of matching chromosomes, called autosomes

    Gametes and the Life Cycle of a Sexual Organism

    ; The life cycle of a multicellular organism is the sequence of stages leading from

    the adults of one generation to the adults of the next.

    ; Humans are diploid organisms (2n=46)

    - Their cells contain two sets of chromosomes

    - Their gametes are haploid, having only one set of chromosomes (n=23)

; In humans, a haploid sperm fuses with a haploid egg during fertilization to form a diploid zygote

    ; Sexual life cycles involve an alternation of diploid and haploid stages ; Meiosis produces haploid gametes, which keeps the chromosome number from doubling every

    generation

The Process of Meiosis

    ; In meiosis,

    - Haploid daughter cells are produced in diploid organisms

    - Interphase is followed by two consecutive divisions occur, meiosis I and meiosis II

    - Crossing over occurs

Review: Comparing Mitosis and Meiosis

    ; In mitosis and meiosis, the chromosomes duplicate only once, during the preceding interphase

    ; The number of cell divisions varies:

    - Mitosis uses one division and produces two diploid cells

    - Meiosis uses two divisions and produces four haploid cells ; All the events unique to meiosis occur during meiosis I

    The Origins of Genetic Variation

    ; Offspring of sexual reproduction are genetically different from their parents and from one another. ; Independent Assortment of Chromosomes

    - When aligned during metaphase I of meiosis, the side-by-side orientation of each homologous pair

    of chromosomes is a matter of chance.

    - Every chromosome pair orients independently of the others during meiosis.

    - For any species the total number of chromosome combinations that can appear in the gametes due to

    independent assortment is:

    ; 2n where n is the haploid number.

    - For a human:

    ; n = 23

    ; 223 = 8,388,608 different chromosome combinations possible in a gamete

; Random Fertilization

    - A human egg cell is fertilized randomly by one sperm, leading to genetic variety in the zygote.

    - If each gamete represents one of 8,388,608 different chromosome combinations, at fertilization,

    humans would have 8,388,608 × 8,388,608, or more than 70 trillion, different possible chromosome

    combinations.

; Crossing Over

    - In crossing over,

    ; Homologous chromosomes exchange genetic information

    ; Genetic recombination, the production of gene combinations different from those carried by

    parental chromosomes, occurs

    When Meiosis Goes Awry

    ; What happens when errors occur in meiosis?

    ; Such mistakes can result in genetic abnormalities that range from mild to fatal. ; How Accidents During Meiosis Can Alter Chromosome Number

    - In nondisjunction, the members of a chromosome pair fail to separate during anaphase, producing

    gametes with an incorrect number of chromosomes

    - Nondisjunction can occur during meiosis I or II

    - If nondisjunction occurs, and a normal sperm fertilizes an egg with an extra chromosome, the result

    is a zygote with a total of 2n + 1 chromosomes

    - If the organism survives, it will have an abnormal number of genes

; Down Syndrome: An Extra Chromosome 21

    - Down Syndrome:

    ; Is also called trisomy 21

    ; Is a condition in which an individual has an extra chromosome 21

    ; Affects about one out of every 700 children

    - The incidence of Down Syndrome increases with the age of the mother.

; Abnormal Numbers of Sex Chromosomes

    - Nondisjunction can also affects the sex chromosomes.

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