Biology

1. ATENEO de DAVAO UNIVERSITY
Senior High School
STEM- Pre-Science- Biology
E. Jacinto Street, Davao City, Philippines
GENERAL BIOLOGY 1
1st Grading
Session 6: The Cell Cycle
THE CELL CYCLE
• Begins when two new cells are formed
by the division of a parental cell
• Ends when one of these cells divide
again into two cells
• Divided into:
Interphase – includes cell growth and
synthesis of DNA
M Phase – cell division
A. INTERPHASE
• Gap 1 (G1) stage
- Interval between mitosis and
initiation of DNA replication
- The cell is metabolically
active and continuously
grows.
• Synthesis (S) stage
- DNA replication takes place
- The amount of DNA per cell
doubles.
- There is no increase in
chromosome number.
• Gap 2 (G2) stage
- Proteins are synthesized in
preparation for mitosis.
- Cell growth continues.
• Quiescent (G0) stage
- Occurs in some cell types
- Inactive stage
- Cells are metabolically active
but no longer proliferate
unless called on to do so
B. M PHASE: MITOSIS
• In animals: seen only in diploid
somatic cells
• In plants: occurs in haploid and
diploid cells
• Stage 1: Prophase
- Initiation of chromatin
condensation.
- The centrioles begin to move
towards the opposite poles
of the cell.
- The mitotic spindle begins to
form.
- The nuclear envelope breaks
down.
- Prometaphase:
Complete disintegration of
the nuclear envelope
Condensation of
chromosomes is completed
• Stage 2: Metaphase
- Spindle fibers attach to
kinetochores at the
centromere of the
chromosome
- Chromosomes get aligned
along the metaphase plate.

2. ATENEO de DAVAO UNIVERSITY
Senior High School
STEM- Pre-Science- Biology
E. Jacinto Street, Davao City, Philippines
• Stage 3: Anaphase
- Centromeres split
- Chromatids separate and
move to opposite poles.
• Stage 4: Telophase
- Chromosomes cluster at
opposite spindle poles
- The chromosomes
decondense
- Nuclear envelope
reassembles
• Cytokinesis
- The division of the cytoplasm
to form two new cells
- In animal cells: formation of
the cleavage furrow
- In plant cells: formation of the
cell plate
C. M PHASE: MEIOSIS
• Produces gametes and spores
• Results to 4 haploid daughter
cells
• Divided into:
Meiosis I – reduces the number
of chromosomes from diploid to
haploid
Meiosis II – produces four
haploid daughter cells
MEIOSIS I
• Stage 1: Prophase I
- compaction of chromosomes
(leptotene)
- synapsis and tetrad/bivalent formation
(zygotene)
- crossing-over (pachytene)
- formation of chiasmata (diplotene)
- assembly of meiotic spindle; the
nuclear envelope breaks down
(diakinesis)
• Stage 2: Metaphase I
- microtubules from the opposite poles
attach to the pair of homologous
chromosomes
- tetrads align on the metaphase plate
• Stage 3: Anaphase I

3. ATENEO de DAVAO UNIVERSITY
Senior High School
STEM- Pre-Science- Biology
E. Jacinto Street, Davao City, Philippines
- the homologous chromosomes
separate and move towards the
opposite poles
- Sister chromatids remain associated
at their centromeres
• Stage 4: Telophase I
- The chromosomes decondense
- The nuclear membrane and nucleolus
reappears
• Cytokinesis
- The cytoplasm of the parental divides
into two daughter cells
• Interkinesis
- A period of rest in preparation for
meiosis II
- no DNA replication takes place
MEIOSIS II
• Stage 1: Prophase II
- compaction of chromosomes
- the nuclear envelope breaks down
• Stage 2: Metaphase II
- microtubules attach to the kinetochore
- chromosomes align at the equator
• Stage 3: Anaphase II
- splitting of centromere
- the chromatids separate and move
towards the opposite poles
• Stage 4: Telophase II
- The chromosomes decondense
- The nuclear membrane and nucleolus
reappears
• Cytokinesis
- The cytoplasm of the parental divides
into two daughter cells