M3: Mitosis & Meiosis

Genome

Collection of all genetic information of a cells

Chromosomes

DNA strands are packaged into structures

Histones

DNA is wrapped around proteins

chromatin

• less condensed form

• complex of DNA strands and associated proteins

somatic cells

• 46 chromosomes

• half of chromosomes belong from each parent

• dIploid

Gametes

• 23 chromosomes only

• haploid

Sister chromatids

duplicated chromosomes during cell division

Centromere

• “waist portion” of of chromosomes

• where chromatids are joined together

Mitosis

division of the genetic material in the nucleus

Cytokinesis

division of the cytoplasm

Interphase

cell cycle: much longer stage

G1

S

G2

Three phases of interphase

G1

• “first gap”

• increased cell growth and metabolic activity

S

• “synthesis”

• duplication of the chromosomes

G2

• “2nd gap”

• completing the necessary preparations for mitotic phase

G0

• non-dividing state

• resting or quiescent phase

• the cell is not replicating its genetic material

• cells: mature nerve and muscle cells

Prophase

• chromatin fibers condense to form discrete chromosomes

• mitotic spindle begins to form with the centrosomes moving away from each other as the microtubules lengthen

Metaphase

• nuclear envelope fragments

• kinetochore forms at the centromeres of each chromatid

• centrosomes are now found at opposite poles of the cell

• chromosomes are now aligned at the metaphase plate

Anaphase

• two sister chromatids separate into a separate set of chromosomes

• the number of chromosomes doubles

Telophase

• the cell separated into two daughter cells with identical material

• the nuclear envelope and nucleolus arise and reappear, chromosomes became less condensed

Cytokinesis

• not part of mitosis

• the formation of cleavage furrow

• pinches the cell into two separate cells

• daughter cells are genetically identical & same number of chromosomes as the parent cell

Meiois

• Gametes contain only one set (n) of chromosomes. Therefore, called haploid cells

• it is also preceded by the interphase

• there are two consecutive cell divisions: meiosis I & meiosis II

Prophase I

• subdivided into 4 stages: Leptotene, Zygotene, Pachytene, Diplotene

Leptotene

• “thin thread”

• chromatings are stretched and sperad thinly

• chromosomes cannot be observed at this stage

• DNA replication has already occured

• strands are already organized into paired chromatids

Zygotene

• “yoked thread”

• homologous chromosomes are lined up side by side forming a synapsis

• only observed in meiosis requiring the presence of ladder-like proteinaceous ribbon called “synaptonemal complex”

• four chromatids are found in the complex, and known as tetrad

Pachytene

• “thick thread”

• chromatids thicken & shorten

• can now be observed under the microscope

• crossing-over to exchange genetic material

Diplotene

• “double threads”

• two homologous chromosomes separate but still attached to certain points called chiasmata

Metaphase I

• starts with diakinesis

• “moving apart” of the chromosomes

• Homologous chromosomes are aligned instead of individual chromosomes

Anaphase I

• homologous chromosomes separate & move towards opposite poles

• sister chromatids are still attached in the centromere

Telophase I and Cytokinesis

• observed by the reduction in chromosome number

• the parent cell separates into 2 daughter cells forming 2 haploid cells

• a cleavage furrow forms in animal cells thus, pinching the cell into two

Meiosis II

• identical to mitosis in terms of the process

• consists of Prophase II, Metaphase II, Anaphase II, and Telophase II

• at the end of this, there will be 4 daughter cells that are haploid due to 2 rounds of cell division

• Due to crossing-over during Prophase I, the daughter cells are different from the parent cell