CHAPTER 8 CELLULAR REPRODUCTION: CELLS FROM CELLS
WHAT CELL REPRODUCTION ACCOMPLISHES
- 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
; 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
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
; 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
; 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
; 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:
- 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
- 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
- Cytokinesis, in which the cytoplasm is divided in two
; Mitosis consists of four distinct phases:
- 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
- Produces offspring that contain a unique combination of genes from the parents
; 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
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
; 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.