Cell Reproduction

Chapter 8

Chromosomes = rod-shaped structures made of DNA and proteins

Histone = protein that helps maintain the shape of chromosomes and aid in the tight packing of DNA

Chromatid = each half of the chromosome, exact copies that have been replicated

Centromere = point where 2 chromatids are attached

Chromatin = DNA that is less tightly coiled

Sex chromosomes = determines the sex of an organism; humans have either an X or Y

Autosomes = all of the other chromosomes

o Every organism is produced by sexual reproduction has 2 copies of each autosome.

Homologous chromosome = homologues

= same size, shape, and carry genes for same traits

Karotype = photomicrograph of chromosomes = 46 chromosomes exist as 22

homologous pairs of autosomes and 2 sex chromosomes

Genetic Disorder

A disease or condition caused by an absent or defective gene or by a chromosomal aberration

Diploid = cells having 2 sets of chromosomes (2n)

Haploid = cell having 1 set of chromosomes (1n) = sperm and egg cells

approximately 2 trillion cells (25 million cells per second) are produced by an adult human body everyday

Cell division = process by which cells produce offspring cells

Binary Fission = division of a prokaryote cell into 2 offspring cells

Asexual reproduction = production of offspring from one parent

Mitosis = cell division in Eukaryote cells; new cells with genetic material that is identical to the first (original cell)

- Occurs in organisms undergoing growth development, repair or asexual reproduction

Cell Cycle

Interphase- time between cell divisions; cell spends most of its time

G1 phase – grow to mature size

S phase – DNA copied (synthesized)

G2 phase – prepares for cell division

Go phase – usually after G1; does not copy DNA, does not divide

Mitosis – nucleus divided Cytokinesis – cytoplasm divides

Stages of Mitosis

Prophase shortening & tight coiling of DNA into rod-shaped chromosomes nucleolus & nuclear envelope breaks down centrosomes & centrioles appear in animal cells only centrosomes appear in plant cells spindle fibers = made of microtubules radiate from centrosomes in preparation of the next phase mitotic spindle = array of spindle fibers; serves to equally divide chromatids between 2 offspring cells

made up of kinetochore fibers – attached to a disk shaped protein and to one of the chromosomes and polar fibers extend from centrosome to centrosome

Kinetos means moving choros means place

Metaphase kinetochore fibers move chromosomes to center (equator)

Anaphase chromatids separate at centromere & slowly move toward opposite poles

Telophase spindle fibers disassemble; chromosomes return to a less tightly coiled chromatin state nuclear envelope forms around each set of chromosomes; as well as a nucleolus

Cytokinesis (in animals)

begins with the cell membrane pinching inward in the middle between 2 poles (cleavage furrow)

Cytokinesis (in plants) vesicles from Golgi apparatus join together at the midline to form cell plate cell wall will eventually form from cell plate

Checkpoints

Meiosis- process of division that reduces the number of chromosomes in new cells to half the number in the original cell

synapsis – pairing of homologous chromosomes

Crossing-over = process where portions of the chromosome may break off and attach to adjacent chromatids on the homologous chromosomes

genetic recombination = a new mixture of genetic material is created

Meiosis I

Prophase I

• DNA tightly coils into chromosomes• Spindle fibers appear• Nucleolus and nuclear membrane disassemble• Every chromosome lines up next to its homologue

(the pairing is called synapsis and each pair is called a tetrad)

Metaphase I• The tetrads line up randomly along the dividing

cell’s midline• Spindle fibers from one pole attach to the

centromere of one homologous chromosome (spindle fibers from opposite pole attached to the other homologous chromosome of the pair)

Anaphase I

• Each homologous chromosome (consisting of two chromatids attached by a centromere) moves to an opposite pole of the dividing cell (random separation of homologous chromosomes= independent assortment)

Telophase I and Cytokinesis I

• Chromosomes reach the opposite ends of the cell

• Cytoplasm splits• New cells contain haploid

number of chromosomes• Meiosis I result: 2 new cells,

containing one chromosome from each homologous pair but 2 copies of each because of the “sister” chromatids”

Meiosis I

Meiosis II

Prophase II• Spindle fibers form and begin to move

chromosomes toward the midline of the dividing cell

Metaphase II

• The chromosomes move to the midline of the dividing cell, with each chromatid facing opposite poles of the dividing cell

Anaphase II• Chromatids separate and move toward

opposite poles of the cell

Telophase II and Cytokinesis II

• A nuclear envelope forms around the chromosomes in each of the four new cells

• Cytoplasm splits in each of the two dividing cells resulting in 4 cells that contain half of the original cell’s number of chromosomes

Meiosis II

Where are gametes formed in humans??

• Males: in the testes– In SPERMATOGENESIS: a diploid reproductive cell

divides meiotically to form FOUR haploid spermatids, each of which develop into a mature sperm cell (AKA spermatozoa)

• Females: in the ovaries– In OOGENESIS: a diploid reproductive cell divides

meiotically to produce ONE mature egg cell (ovum)• Why not four?

– During cytokinesis I and II, the cytoplasm of the cell is divided unequally between new cells….one cell receives most of the cytoplasm and therefore it can become the egg. The other three products that will eventually degenerate are called polar bodies

Sexual Reproduction

• The production of offspring through meiosis and the union of a sperm and an egg

forms a zygote (fertilized egg)• Offspring produced by sexual

reproduction are genetically different from the parents because genes are combined in new ways in meiosis