Cell Reproduction

 

Biology and Society: Virgin Birth of a Dragon

           In 2002, zookeepers at the Chester Zoo were surprised to discover that their Komodo Dragon laid eggs.

          The female dragon had not been in the company of a male.

          The eggs developed without fertilization, in a process called parthenogenesis.

          DNA analysis confirmed that her offspring had genes only from her.

           A second European Komodo dragon is now known to have reproduced

          asexually, via parthenogenesis, and

          sexually.

 

WHAT CELL REPRODUCTION ACCOMPLISHES

           Reproduction

          may result in the birth of new organisms but

          more commonly involves the production of new cells.

           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, it duplicates its chromosomes, the structures that contain most of the cell’s DNA.

           During cell division, each daughter cell receives one identical set of chromosomes from the lone, original parent cell.

           Cell division plays important roles in the lives of organisms.

           Cell division

          replaces damaged or lost cells,

          permits growth, and

          allows for reproduction.

           In asexual reproduction,

          single-celled organisms reproduce by simple cell division and

          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.

           In asexual reproduction, the lone parent and its offspring have identical genes.

           Mitosis is the type of cell division responsible for

          asexual reproduction and

          growth and maintenance of multicellular organisms.

           Sexual reproduction requires fertilization of an egg by a sperm using a special type of cell division called meiosis.

           Thus, sexually reproducing organisms use

          meiosis for reproduction and

          mitosis for growth and maintenance.

           In a eukaryotic cell,

          most genes are located on chromosomes in the cell nucleus and

          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, fibers composed of roughly equal amounts of DNA and protein molecules and

          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 containing identical genes.

           Two sister chromatids are joined together tightly at a narrow waist called the centromere.

           When the cell divides, the sister chromatids of a duplicated chromosome separate from each other.

           Once separated, each chromatid is

          considered a full-fledged chromosome and

          identical to the original chromosome.

 

The Cell Cycle

           A cell cycle is the ordered 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 and

           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, and

          grows in size.

           The mitotic (M) phase includes two overlapping processes:

          mitosis, in which the nucleus and its contents divide evenly into two daughter nuclei and

          cytokinesis, in which the cytoplasm is divided in two.

 

Mitosis and Cytokinesis

           During mitosis the mitotic spindle, a football-shaped structure of microtubules, guides the separation of two sets of daughter chromosomes.

           Spindle microtubules grow from structures within the cytoplasm called centrosomes.

           Mitosis consists of four distinct phases:

           Prophase

           Metaphase

           Anaphase

           Telophase

           Cytokinesis usually

          begins during telophase,

          divides the cytoplasm, and

          is different in plant and animal cells.

           In animal cells, cytokinesis

          is known as cleavage and

          begins with the appearance of a cleavage furrow, an indentation at the equator of the cell.

           In plant cells, cytokinesis begins when vesicles containing cell wall material collect at the middle of the cell and then fuse, forming a membranous disk called the cell plate.

 

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.

           If the abnormal cells remain at the original site, the lump is called a benign tumor.

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

           Malignant tumors can

          spread to other parts of the body and

          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, and

          chemotherapy, the use of drugs to disrupt cell division.

 

Cancer Prevention and Survival

           Certain behaviors can decrease the risk of cancer:

          not smoking,

          exercising adequately,

          avoiding exposure to the sun,

          eating a high-fiber, low-fat diet,

          performing self-exams, and

          regularly visiting a doctor to identify tumors early.

 

Sexual Reproduction

           Sexual reproduction

          depends on meiosis and fertilization and

          produces offspring that contain a unique combination of genes from the parents.

 

Homologous Chromosomes

           Different individuals of a single species have the same

          number and

          types of chromosomes.

           A human somatic cell

          is a typical body cell and

          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, and

          22 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 with

          body cells containing two sets of chromosomes and

          haploid gametes that have only one member of each homologous pair of chromosomes.

           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, meiosis I and meiosis II, and

          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 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
all of the others at metaphase I.

           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.

          With n = 23, there are 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.

           So we see that the random nature of fertilization adds a huge amount of potential variability to the offspring of sexual reproduction.

 

Crossing Over

           In crossing over,

          nonsister chromatids of homologous chromosomes exchange corresponding segments and

          genetic recombination, the production of gene combinations different from those carried by parental chromosomes, occurs.

 

The Process of Science: Do All Animals Have Sex?

           Observation: No scientists have ever found male bdelloid rotifers, a microscopic freshwater invertebrate.

           Question: Does this entire class of animals reproduce solely by asexual means?

           Hypothesis: Bdelloid rotifers have thrived for millions of years despite a lack of sexual reproduction.

           Prediction: Bdelloid rotifers would display much more variation in their pairs of homologous genes than most organisms.

           Experiment: Researchers compared sequences of a particular gene in bdelloid and non-bdelloid rotifers.

           Results:

          Non-bdelloid sexually reproducing rotifers had a nearly identical homologous gene, differing by only 0.5% on average.

          The two versions of the same gene in asexually reproducing bdelloid rotifers differed by 3.5–54%.

           Conclusion: Bdelloid rotifers have evolved for millions of years without any sexual reproduction.

           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 at 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 karyotype and

           probably a syndrome of disorders caused by the 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, and

          affects about one out of every 700 children.

           The incidence of Down syndrome in the offspring of normal parents increases markedly with the age of the mother.

 

Abnormal Numbers of Sex Chromosomes

           Nondisjunction in meiosis

          can lead to abnormal numbers of sex chromosomes but

          seems to upset the genetic balance less than unusual numbers of autosomes, perhaps because the Y chromosome is very small and carries relatively few genes.

 

Evolution Connection: The Advantages of Sex

           Asexual reproduction conveys an evolutionary advantage when plants are

          sparsely distributed and unlikely to be able to exchange pollen or

          superbly suited to a stable environment.

           Asexual reproduction also eliminates the need to expend energy

          forming gametes and

          copulating with a partner.

           Sexual reproduction may convey an evolutionary advantage by

            speeding adaptation to a changing environment or

            allowing a population to more easily rid itself of harmful genes.