AS OCR Biology - Chapter 3 - Cell Division

Cell Cycle -

• Most animals start life as zygote - formed when two gametes fuse. Set of chromosomes from mother and set of chromosomes from father.

• Process of growing and dividing is called the cell cycle; made up of two main phases called interphase and mitosis.

• G1 phase,  then S phase (DNA replication), then G2 phase, then mitosis then cytokinesis.

• 95% of cell cycle spent in interphase - just under 5% spent in mitosis, and a small part spent in cytokinesis.

Interphase -

• DNA copied exactly. The copies remain attached at the centromere in the middle.

• Even the smallest mutation in the DNA sequence could have harmful complications for the cell.

• Proteins go along DNA molecules checking for errors and if found, try to correct them.

• The centrioles duplicate.

Mitosis -

• Nucleus of the cell divides into two nuclei.

• The two chromatids that make up each chromosome break apart. They each go into a nucleus - the nuclei will be identical now.

• Four stages; Prophase, Metaphase, Anaphase, and Telophase. There are no breaks between the stages.

You can remember the stages of mitosis in the right order by ( S H ) I P M A T ( E ).

I for Interphase.

P for Prophase.       ETC.

Prophase -

• Currently, the chromosomes are very long and very thin threads that can't be seen with a light microscope.
• The chromosomes become visible because the DNA molecules coil into thicker threads that, if stained, can be seen with a light microscope.
• The chromosomes made of two chromatids held together at the centromere, they contain identical sequences of DNA.
• The nucleolus disappears, and the spindle (made of microtubules) begins to form.
• Centrioles start to move to opposite ends of the cell where they organise the formation of the microtubules.

Metaphase -

• Nuclear envelope breaks down, so now the whole cell can be used for cell division. By this point, many of the microtubules have attached to the centromeres.

• Each centromere is grabbed by two microtubules, one on each side. The microtubules pull in opposite directions meaning that the chromosomes lie at the  equator of the cell.

• Centromeres of chromosomes are now lined up quite evenly through the equator.

Anaphase -

• The spindle starts to retract towards each side of the cell, and this splits the centromeres so that half the chromatids go one way and half go the other.

You can remember anaphase by the triangular A shape the chromatids make as they are being pulled to the other end of the cell.

Telophase -

• Chromatids have now arrived at each side of cell and each group contains a complete set of chromatids; now called chromosomes.

• Spindle is broken down and the nuclear envelope starts to build up around the chromosomes again. Chromosomes uncoil and become thinner again.

• There is also a centriole on each side with a nucleus.

Cytokinesis -

• The cytoplasm now divides surrounding each nucleus with it's own cytoplasm.

• Two new cells have been formed each contains new organelles, a nucleus containing a set of chromosomes and a centriole.

Differentiation -

• A zygote divides to form a blastocyst, this continues to divide to form an embryo.
• Specialisation of a cell to carry out a particular function is DIFFERENTIATION.
• After differentiation, there's usually no going back to being a stem cell.
• Stem cells are unspecialised, and they can; divide repeatedly to form new cells, and differentiate into many types of specialised cells.
• Totipotent - stem cells that can differentiate into any type of specialised cell.
• Erythrocytes (RBC's) - made from stem cells in bone marrow.
• Leucocytes (WBC's) - made by the same stem cells as RBC's. Especially those found in the ribs, vertebrae, pelvis and skull. E.g. Neutrophils

Specialised Plant Cells -

• Meristems - cells able to divide are found here.
• Cambium - found in meristem - form xylem vessels or phloem sieve tube elements.
• Palisade cells - main type of photosynthetic cell - highly specialised, contains many chloroplasts.
• Root hair cells - specialised epidermal cells - cells that cover the outside of a plant organ. 

Tissues -

• A tissue is a group of similar cells with a common function or common functions.
• E.g. Epithelial tissues; squamous epithelium, and ciliated epithelium.

Organ -

• An organ is a group of tissues working together.

Organ System -

• An organ system is a group of organs working together with particular function.