CELL REPRODUCTION
Chapter 8
1. DNA molecules twist into tightly packed chromosomes. These chromosomes
are made up of segments called genes. Chromosomes direct all chemical
reactions that determine inherited traits in organisms.
2. Each species is unique due to number of chromosomes & arrangement of genes
on the chromosomes.
3. Chromosome structure:
a) thin strands of DNA called chromatin twist into thick strands called
chromatids & can be easily seen with a microscope
b) chromosome is formed when 2 chromatids are held together by a
centromere
4. Body cells are called somatic cells & are diploid because they contain both
chromosomes of each homologous pair.
5. Sex cells (sperm & egg) are haploid & contain only 1 chromosome from each
homologous pair.
6. For growth & repair, cell numbers are constantly increasing. This occurs in a
continuous series of events called the cell cycle.
a) most of cell’s life is spent in interphase:
1) G1 Phase - rapid growth in size & organelle number
2) S Phase - DNA replication
3) G2 Phase - growth & preparation for division
b) M Phase - mitosis - nucleus divides
c) cytokinesis - splitting of the cell into 2 new cells identical to original cell
7. Once body cells reach maximum size, the phases of mitosis begin:
1st - prophase:
1) chromatin coils tightly into chromosomes
2) nucleolus & nuclear membrane breakdown & disappear
3) centrioles appear & move toward poles
4) spindle fibers develop & extend between centrioles
2nd - metaphase: chromosomes line up along spindle fibers at the equator
3rd - anaphase: centromere of each chromosome divides & chromatids
separate & move toward opposite poles
4th - telophase:
1) identical sets of chromatids cluster at opposite poles
2) centrioles & spindle fibers disappear while nuclear membrane &
nucleolus reappear
3) cleavage furrow forms as membrane pinches inward at cell’s
equator to separate the 2 new nuclei & other organelles
8. Gametes (sex cells - sperm & egg) form in a process called meiosis when the
number of chromosomes is reduced by half to allow sperm & eggs to contain
only 1 chromosome from each homologous pair. When fertilization occurs,
homologous pairs join & produce inherited characteristics of the offspring. The
zygote formed when egg is fertilized by a sperm is a diploid cell.
9. Meiosis is 2 different divisions involving of prophase, metaphase, anaphase, &
telophase. 1 diploid cell becomes 4 haploid cells:
1st - meiosis I: parent cell produces 2 daughter cells that each contains 1
chromosome from each homologous pair - because of replication, each
chromosome has twice the amount of original DNA
2nd - meiosis II: does not involve replication - within nuclei of each cell,
chromatids separate & 4 daughter cells with half the original number
of chromosomes are created from original parent cell
10. A mistake called nondisjunction may occur during meiosis when chromosomes
fail to separate correctly. 1 daughter cell receives an extra chromosome &
the other daughter cell is missing a chromosome. Organisms with extra
chromosomes often survive, but those lacking chromosomes usually do not.
11. Every species of living things must reproduce or it will face extinction.
Depending on the species, offspring may be produced by asexual or sexual
reproduction.
a) asexual reproduction does not involve the union of sperm & eggs – many
offspring can be produced in a short time, but the offspring are
genetically identical to the parent
1) binary fission – cell division (bacteria & protists)
2) budding – an outgrowth of the parent (hydra & sponges)
3) fragmentation – breaking & regrowth (starfish)
b) sexual reproduction produces offspring through the union of sperm &
eggs – this increases genetic variety among species & enables them to
more easily adjust to changing environments