Lecture 2
Cell Structure, Mitosis and Meiosis
Study Questions - Lecture 2
1) Describe the major components of a typical animal cell and their
function(s).
2) What is the cell cycle? At which stage is the genetic material replicated?
What events take place at each phase?
3) Discuss the concepts of homologous chromosomes, diploidy, and haploidy.
What characteristics are shared between two chromosomes considered to
be homologous?
4) What is mitosis? Describe the events that characterize each stage of
mitosis.
5) Describe how chromosomes are named on the basis of their centromere
placement.
6) What is meiosis? Describe the events that characterize each stage of
meiosis.
7) Contrast mitosis and meiosis. Explain why meiosis leads to significant
genetic variation while mitosis does not.
CELLS
Cells - “fundamental unit of life”
Prokaryotes
-single celled
-no nucleus
-no organelles
-DNA = single, circular
molecule
- e.g., bacteria
Eukaryotes
-single or multicellular
-nucleus with nuclear
membrane
-DNA packaged into
chromosomes
-organelles present
Biological Kingdoms: Monera, Protista, Fungi, Plantae, Animalia
Cellular Components
Cell membrane
Endoplasmic reticulum
Ribosomes
Golgi apparatus
Lysosomes
Mitochondria
Microtubules (centriole)
Nucleus (nucleolus)
Chromatin/Chromosomes
Fluid-Mosaic Model of Membrane Structure - the thing
surrounding the internal components of the cell
Bilayer (2 layers)- of proteins and lipids
Endoplasmic Reticulum-set of
interconnected membranes. 2
types.
Rough ER (studded with
Ribosomes) - involved in
production and modification
of proteins
Smooth ER- (no ribosomes) involved in production of
lipids, detoxification of molecules,
and calcium storage in muscle cells
*Ribosomes formed in nucleolus*
Golgi Apparatus
After proteins and
lipids are made by the
Endoplasmic Reticulum
they may be modified
further and/or stored
by the GA.
Lysosome - formed by GA, contain enzymes that break down
cellular debris and foreign substances brought into the cell
Mitochondria
Aerobic respiration produces ATP
Has own genome
Microtubule
Cell shape, motion,
and growth.
Microtubule- composed of tubulin.
Both cilia and flagella have the same
Structure of microtubules: 9+2.
Forms spindle during mitosis & meiosis
Nuclear Membrane or Envelope - two membranes which form
the nucleus, is porous. Allows RNA to leave nucleus.
Chromosome = DNA (deoxyribonucleic acid) + associated
proteins (mainly histones) = “packaged” DNA
Cell Cycle: replication of genetic material (=DNA) and nuclear
division (= mitosis + cytokinesis).
Cell Cycle: Interphase
•
•
•
•
•
Before mitosis
Time of high metabolic activity
DNA replicated and synthesized
Three phases: G1, S, and G2
G1(gap 1)- longest stage of cell cycle, RNA,
protein sysnthesis
• S (synthesis)- DNA replicated , 2 chromatids per
chromosome, chromatids genetically identical
• G2(gap 2)- RNA synthesis, not well understood
Cell Cycle: Mitosis
• Process of cell division(nuclear division) which
produces daughter cells genetically identical to the
parent cell
• Four Phases (P-M-A-T): prophase, metaphase,
anaphase, and telophase.
• Upon completion of the phases of mitosis (nuclear
division) the cell “officially” divides into two by a
process called cytokinesis - division of cytoplasm
Interphase
Not part of mitosis
DNA is replicated
chromosomes start to condense
Chromosome duplication
Prophase
*Chromosomes coil and condense further.
*Nucler membrane breaks down/ disappears.
*Microtubules increase in number, spindle apparatus
forms.
Metaphase
*Nuclear membrane completely disappeared
*Chromosomes move to equator of cell - begin to line up
*Chromosomes attach to spindle via kinetochore
Centromeric Region
Anaphase
*Movement of chromosomes via microtubules
to opposite sides of the cell. One chromatid to one end the other
Chromatid to the opposite end
Telophase
*Genetically identical info at each pole
*Spindle fibers disappear
*Chromosomes uncoil
*Nuclear envelope reforms around
Cytokinesis - is separate from mitosis,
= pinching of cell/divison of cytoplasm.
Mitosis + Cytokinesis result in two
identical daughter cells.
Mitosis in a plant cell
Mitosis in an onion root
Chromosomes and Chromatids
During Mitosis
Begin
Interphase
After
Interphase
After
Prophase
After
Metaphase
After
Anaphase
After
telophase
# of
Chromosomes
4
4
4
4
8
4
# of
Chromatids
4
8
8
8
8
4
Meiosis
• Cell division which results in halpoid “sex” cells
(i.e., egg and sperm)
• One replication of the genetic material (DNA)
during interphase, but two nuclear divisions
(meiosis I and meiosis II).
• Results in haploid (N) cells (= gametes in animals)
from an initial diploid (2N) cell
• Very similar to mitosis except that the cells
produced are not genetically identical.
The human life cycle
Overview of meiosis: how meiosis reduces chromosome number
Independent Assortment
The stages of meiotic cell division: Meiosis I
Crossing-Over/Synapsis
A Tetrad from the Grasshopper
The stages of meiotic cell division: Meiosis I
The stages of meiotic cell division: Meiosis II
Oogenesis, 2n = 4
Spermatogenesis, 2n = 4
SEM of sea urchin sperm fertilizing egg
Chromosomes and Chromatids
During Meiosis I
Begin
Interphase
After
Interphase
After
Prophase
I
After
Metaphase
I
After
Anaphase
I
After
Telophase
I
# of
Chromosomes
4
4
4
4
4
2
# of
Chromatids
4
8
8
8
8
4
Chromosomes and Chromatids
During Meiosis II
After
Prophase
II
After
Metaphase
II
After
Anaphase
II
After
Telophase
II
# of
Chromosomes
2
2
4
2
# of
Chromatids
4
4
4
2
A comparison of mitosis and meiosis
A comparison of mitosis and meiosis: summary