GENERAL BIOLOGY Sister chromatids – identical structures
Cell Division (Mitosis and Meiosis) that result from chromosome
replication, formed during S phase.
Why do cells divide?
- Growth
- Repair
- Reproduction
Mitosis
Organisms grow by the addition
of cells
In multicellular organism some of
these cells perform functions
different from other cells.
Differentiation – process of a cell How do cells divide?
becoming different - Cell cycle – sequence of
Under normal conditions once an phases in the life cycle of a
animal cell becomes specialized it cell
can NO longer form an entire
organism, however plant cells are
totipotent and any cell can form
an entire plant.
When do cells divide?
- most limiting factor in size is the
size of the cell membrane.
Cells must obtain nutrients
As volume increases, cell Cell cycle has two parts:
surface area does not - Growth and preparation
increase as greatly (interphase)
Larger cells require a layer - Cell division
surface area for survival Mitosis (nuclear division)
Cytokinesis (cytoplasm
Cytokinesis – actual division of the cell division)
into two new cells Interphase
Mitosis – division of the nucleus of the - Occurs between divisions
cell into two nuclei - Longest part of the cycle
- Has 3 stages
NOTE: Sometimes cells go through
mitosis without going through
cytokinesis.
Chromatin – thin fibrous form of DNA
and proteins
Interphase
- G1 or Gap 1
The cell just finished dividing
so in Gap 1 the cell is
recovering from mitosis
- S or Synthesis Stage
DNA replicates
- G2 or Gap 2
Preparation for mitosis
Organelles are replicated
More growth occurs
Anaphase
- Centromeres divide
- Spindle fibers pull one set of
chromosomes to each pole
- Precise alignment is critical to
Prophase
division
- Chromosome condenses
- Microtubules form
- Nuclear envelope breaks down
Metaphase
- Chromosomes are pulled to the center
of the cell
- Lined up along the “metaphase plate”
Telophase
- Nuclear envelope from
chromosomes
- Chromosomes uncoil
- Cytokinesis
Animals – pinching of MEIOSIS I
plasma membrane Prophase I
Plants – elongates and the
cell plate form (future cell
wall and cell membrane
- Chromosomes condense
- Homologous chromosomes pair
with each other
- Each pair contains four sister
chromatids – tetrad
- Nuclear envelope breaks down
What is Meiosis? - Crossing-over occurs
- Division of the nucleus that
reduces chromosome number by Metaphase I
half
Important in sexual
reproduction
Involves combining the
genetic information of one
parent with that of the other
parent to produce a - Tetrads or homologous
genetically distinct individual chromosomes move to the center
Chromosome Pairing of the cell
- Homologous pair - Pairs of homologous
each chromosome in pair are chromosomes move to the
identical to the other (carry equator of the cell
genes for same trait)
Only one pair differs – sex Anaphase I
chromosomes X or Y
Phases of Meiosis
- A diploid cell replicates its
chromosomes
- Two stages of meiosis
Meiosis I and Meiosis II - Homologous chromosomes move
Only 1 replication to the opposite poles of the cell
Synapsis – pairing of homologous
chromosomes forming a tetrad.
Crossing over – chromatids of tetrad
exchange parts
Telophase I or Cytokinesis Anaphase II
- Daughter nuclei formed
- These are haploid (1n)
- Chromosomes gather at the poles - The centromeres split and the
of the cell. The cytoplasm divides former chromatids (now
chromosomes) are segregated
MEIOSIS II into opposite sides of the cell.
- Daughter cells undergo a second
division; much like mitosis Telophase II
- NO ADDITIONAL REPLICATION
OCCURS
Prophase II
- Telophase II is identical to
Telophase of mitosis. Cytokinesis
separates the cells.
Mitosis and Meiosis
- Both are forms of nuclear division
- Both involve replication
- Spindle fibers form again - Both involve disappearance of the
nucleus, and nucleolus, nuclear
Metaphase II membrane
- Both involve formation of spindle
fibers
Differences
- Meiosis produces daughter cells
that have ½ the number of
chromosomes as the parent. Go
from 2n to 1n
- Sister chromatids move to the
center
- Daughter cells produces by
meiosis are NOT genetically
identical to one another
- In meiosis cell division takes place
twice but replication occurs only
once
Value of Variation
- Variation – differences between
members of a population
- Meiosis results in random
separation of chromosomes in
gametes
- Causes diverse populations that
over time can be stronger for
survival.
