Download File

PART 6: Cell Reproduction
1. Epidermal cells: skin cells. They die very quickly and have to be replaced just
as quickly.
2. Cell cycle: life cycle of a cell; it has two stages – interphase (Vocab #3) and
cellular division (Vocab #4).
G1
Phase
S Phase
Cellular
Division
G2
Phase
3. Interphase: the “resting” stage of a cell, meaning whenever a cell is growing
and/or performing its purpose. There are three stages of interphase:
 G1 phase: growing phase of the cell when it produces proteins and
enzymes needed for DNA replication.
 S Phase: the packaging and replication of a cell’s entire DNA sequence.
They form chromatids with then form a chromosome (sister chromatids) by
DNA replication. However, for now the chromosomes stay uncoiled as
chromatin.
 G2 Phase: similar to G1, the cell grows and completes its regular tasks.
4. Cellular division: the processes of how cells reproduce by splitting themselves
in two. This stage varies in different types of cells; there is binary fission, mitosis,
and meiosis.


Binary fission: The reproduction of prokaryotic cells. The circular DNA
molecule is replicated; then the cell splits into two identical cells, each
containing an exact copy of the original cell's DNA.
Mitosis: the reproduction of non-sexual (somatic) eukaryotic cells. The
purpose of this is for tissues to maintain themselves as their cells produce
genetically identical copies of themselves. There are 4 stages of mitosis:
48
o Prophase: dissolving of the nucleolus, thickening and coiling of
chromatin into chromosomes, spreading of the centrioles,
lengthening of microtubules (called spindle fibres) from the
centrioles, and joining of these spindle fibres to centromeres of
chromosomes by kinetochore links.
o Metaphase: the moving of chromosomes into a straight line by the
spindle fibres. This line is called the metaphase plate.
o Anaphase: the splitting of sister chromatids (chromosomes) into
single chromatids, pulled to opposite sides of the cell by the spindle
fibres.
o Telephase: the forming of nuclear membranes around the two sets
of chromatids, and the beginning of cytokinesis. After this has
completed, the cell returns to interphase to begin another cycle.
 Cytokinesis: the splitting of cell membranes to form two
separate cells. In ...
 animal cells, the membrane pinches until the
membranes break apart. This pinching is called a
cleavage furrow.
 plant cells, a cell wall forms down the middle of a cell.
The structure is called a cell plate.
 Meiosis: the reproduction of sexual eukaryotic cells. Before learning the
stages of meiosis, there are some terms that need to be reviewed first.
o Gametogenesis: another name for meiosis. In males, it is called
spermatogenesis, and in females it is called oogenesis. In oogenesis,
3 of the 4 cells produced dengenerate (they are called polar bodies).
This is so the surviving egg, the ovum, has the most cytoplasm for the
potential growth of an embryo.
o Diploid cell: a cell that has two full sets of chromosomes, one from
each parent (2n). The pairs created here by the same type of
chromosomes are called homologous chromosomes.
o Haploid cell: a cell that has one full set of chromosomes (n).
o Gonads: sex organs where meiosis takes place. In males, they are
the testes and in females, they are the ovaries.
o Gametes: haploid sex cells which join to form diploid embryos that
form a new organism; also known as gem cells; in males, they are
sperm cells and in females they are egg cells.
49
*Meiosis has two rounds of division:
 Meiosis I: the first four stages of meiosis.
o Prophase I: nuclear membrane disappears, chromosomes coil and
thicken, centrioles move to opposite ends of cell, spindle fibres join
to the chromosomes by kinetochores, homologous chromosomes
line up side by side (this is called synapsis) into tetrads (meaning four
homologous chromatids). This is where crossing-over takes place.
 Crossing-over: when large sections of DNA are traded
between chromatids, causing genetic variation. The regions
where this crossing happens are called chiasmata.
o Metaphase I: the lining up of tetrads along a metaphase plate,
pulled by spindle fibres.
o Anaphase I: the splitting of tetrads into chromosomes as pulled by
spindle fibres.
o Telephase I: nuclear membrane forms and cytokinesis takes place
(same as in mitosis telephase)
 Meiosis II: identical process to mitosis, except that the final cells are
haploid.
50
Main Differences in Cellular Divisions
Binary Fission
Mitosis
Occurs in prokaryotes
Identical
Diploid
Diploid
1 becomes 2
1 division
Occurs in somatic eukaryotes
Identical
Diploid
Diploid
1 becomes 2
1 division
Meiosis
Occurs in sex cell eukaryotes
Gametes
Diploid
Haploid
1 becomes 4
2 divisions
5. Mutations: as discussed in Part 5, #7, this is an unintentional change in the
genetic code. One other type of mutation that was not previously discussed (that
happens during cell reproduction) is disjunction.
 Nondisjunction: having one too many or one too few of a specific type of
chromosome. The effects of this are severe in a human. For example,
trisomy 21 (more well known as Down’s Syndrome) is caused by an extra
21st chromosome.
o Monosomy: only one chromosome where there should be two.
o Trisomy: three chromosomes where there should be two.
51
6. Gene regulation: the regulation of gene expression.
 Operon: region of bacterial DNA that regulates gene expression.
o lac Operon: operon controlling the expression of enzymes that break
down lactose. As lactose levels get high, the operon turns on
transcription to produce enzymes to break it down. People with
lactose intolerance have a problem with this operon. Lactose is the
inducer as it induces the reaction.
o trp Operon: Operon controlling the expression of the production of
an amino acid called tryptophan. This operon is always on and
producing tryptophan until it senses high levels of tryptophan and
shuts down production.
 Structural gene: the gene that codes for production of amino acids, some
to form enzymes.
 Promoter gene: region where RNA polymerase binds to begin
transcription.
 Regulatory gene: a gene that codes for a repressor protein. The repressor
can attach to the operator to stop transcription if needed.
52