The Cell Cycle, Mitosis & Meiosis

Not all cells can divide

Viruses need a host cell to divide

What cells have a cell cycle?

Eukaryotic cells that can divide

Phases of the cell cycle

-Interphase: G1, S, G2
-M Phase: mitosis and cytokinesis

Interphase

-The cell grows and DNA is replicated
-G1: the cell grows, new organelles and proteins are made
-S: DNA is synthesised
-G2: the cell grows more, before mitotic phases begin

Mitosis

-The division of the cell nucleus to create two identical daughter cells
-Needed for growth and repairing damaged tissue
-A method of asexual reproduction

(Mi) Gap phase 1 + checkpoint

-G1: the cell grows and new organelles and proteins are made

-Checkpoint: The cell checks that the chemicals needed for replication are present + if there is any damage to the DNA

(Mi) Synthesis

-DNA replication occurs in preparation for division by mitosis

(Mi) Gap phase 2 + checkpoint

-The cell continues to grow while proteins required in cell division are created

-Checkpoint: the cell checks the replicate DNA for damage

Mitosis phases

-Prophase
-Metaphase
-Anaphase
-Telophase

(Mi) Prophase

-Chromosomes condense
-Centrioles begin to move to the opposite ends of the cell, making a network of spindle fibres across it
-The nuclear envelope has dissolved
-Chromosomes are free in the cytoplasm

Chromatids

One of two identical "sister" parts of a duplicated chromosome

Centromere

Area where the chromatids of a chromosome are attached

(Mi) Metaphase

-The chromosomes and two chromatids line up in the middle of the cell

(Mi) Anaphase

-The centromeres divide, separating the pair of sister chromatids
-The spindles contract, pulling the chromatids to the opposite ends of the cell by their centromere

(Mi) Telophase

-The chromatids reach opposite ends of the cell on the spindle
-The chromatids reverse their condensation, making them chromosomes again
-A nuclear envelope appears around each group of chromosomes
-There are now 2 nuclei

Cytokinesis

-Division of the cytoplasm during cell division
-Begins during anaphase and finishes in telophase
-SEPARATE TO MITOSIS

Observing the cell cycle

Stain the chromosomes, allowing for visibility under a microscope

Meiosis

-The production of gametes
-Involves reduction division
-Cells produced are all genetically different + are haploid

(MeI) Prophase I

-The chromosomes condense
-They arrange themselves into homologous pairs

• Crossing over occurs
  -Centrioles begin moving to opposite poles, forming spindles
  -The nuclear envelope breaks down

Crossing over

-Where homologous pairs twist around each other and parts of the chromatids swap over
-The chromatids still contain the same genes but now have a different combination of alleles

(MeI) Metaphase I

-The homologous pairs line up across the equator of the cell
-Spindle fibres attach to the centromeres

(MeI) Anaphase I

-The spindles contract
-Homologous pairs separate
-One chromosome goes to each end of the cell
-Cytokinesis starts

(MeI) Telophase I

-A nuclear envelope forms around each group chromosomes
-Cytokinesis finishes
-Two haploid daughter cells are produced with two chromosomes each

(MeII) Prophase II

-The chromosomes do not cross over
-Centrioles begin moving to opposite poles, forming spindle fibres
-The nuclear envelope begins to break down

(MeII) Metaphase II

-The chromosomes line up across the equator of the cell
-Spindle fibres attach to the centromeres

(MeII) Anaphase II

-The chromatids on each chromosome are separated by spindle fibre contraction
-Cytokinesis begins

(MeII) Telophase II

-A nuclear envelope forms over the two chromatids at each of the cell
-Cytokinesis finishes