Cell Division

• Chromatin is a DNA and histone protein complex found in the nucleus of eukaryotic cells. 
• During cell division, it condenses into chromosomes. 
• They consist of DNA which is made up of many small genes. 
• Chromosomes are found in matching pairs called homologous pairs. 
• During interphase chromosomes are visible as 2 identical chromatids joined at the centromere. 
• Humans have 23 pairs of homologous chromosomes. One comes from the mother and one from the father. 
• The total number of chromosomes in a body cell is called the diploid number. 
• Gametes have half the diploid number os they are haploid. 

• Majority of the cell cycle is interphase
• This is when the cell undergoes DNA replication, organelle production, protein synthesis and ATP synthesis. 
• The mitotic division is the division of the nucleus that proceeds in 4 stages. Prophase Metaphase, Anaphase, Telophase

Chromatin condenses and becomes visible as 2 chromatids joined at the centromere. 

Centrioles move to the opposite poles and microtubules form from each centriole and the spindle develops. 

The nuclear envelope disintegrates and the nucleus disappears and chromosomes move to the equator. 

Prophase is the longest stage of mitosis. 

METAPHASE

Chromosomes arrange themselves at teh equator and attach to spindle finres at the centrioles. 

ANAPHASE

Spindle fibres contract and shorten. Teh centromere splits and chromatids oull apart. The chromatids are drawn to opposite poles. 

Telophase is when the chromosomes uncoil and lengthen. The spindle fires break down and the nucleus reforms. 

Cytokinesis occurs via the constriction of the parent cell from outside inwards to form a cleavage furrow in animals. 

For plants a cell plate forms across the equator and a new cellulose cell wall is laid down. 

In animals, cells are rounded before mitosis but in plants to stay the same. 

Animals have centrioles but plants do not have centrioles. 

The spindle degenerates at telophase in animals but it stays present in plants. 

In animal cells, mitosis occurs in epithelium cells, bone marrow and other sites. In plants, it occurs in the meristems. 

Mitosis produces 2 daughter cells with the same number of chromosomes as the parents. 

Each chromosome is an exact replica so cells are genetically identical which gives genetic stability. 

Mitosis allows growth and replaces dead cells. 

In plants, mitosis takes place in the root tips and the growing point of the shoots. 

In animals, mitosis replaces dead skin cells from underneath. 

The length of the cell cycle is controlled by genes which ensure that mitosis happens when and where it's needed. 

This allows the repair and replacement of adult tissues. 

Carcinogens- Radiation, chemicals and some viruses can mutate then DNA and affect the genes which control the cell cycle. 

Protooncogenes - Control the cell cycle and tell the cell when to divide. If mutated, uncontrollable cell division can occur to form a tumour. 

Produces offspring that are genetically identical to the parents due to mitoss. There is no genetic variation so offspring are clones of parents. 

ADVANTAGES

Offspring are genetically identical to parents. 

Less energy is wasted on finding a partner. 

A faster rate of population growth. 

DISADVANTAGES

Lack of variation means that offspring are not able to adapt to an environmental change. 

Meoisis is used to produce gametes in sexually reproducing organisms. It produces 4 daughter cells that are genetically different. 

Meiosis 1 - Chromosome number is halved. 

Meiosis 2- Haploid cells divide again to form 4 daughter cells. 

Chromosomes condense and become visible. 

Homologous pairs join together as bivalents. 

Chromatids wrap around each other then repel a little but join at a point called the chiasmata where DNA can exchange. This is called crossing over. 

Nucleolus and the nuclear membrane disappear and disintegrate. 

Centrioles move to opposite poles and microtubules from the spindle. 

METAPHASE

Microtubules of the spindle attach to the centromere. Bivalents randomly align at the equation of the spindle. Independent assortment takes place which results in a random combination of chromosomes in each cell. 

ANAPHASE 

Spindle fibres contract and shorten. Bivalents separate and chromosomes are pulled to opposite poles. Each pole receives one homologous pair. There is a mixture of both maternal and paternal chromosomes. 

In some species, the envelope reforms around haploid group of chromosomes and the chromosomes unwind so they are no longer visible. 

But in many species, the chromosomes stay in their condensed form. 

Cytokinesis then occurs to make 2 haploid cells. 

Chromatin condenses and becomes visible as 2 chromatids join at the centromere. 

Centrioles move to opposite poles. 

Protein microtubules form from each centromere and the spindle develops. 

The nuclear envelope disintegrates and the nucleolus disappears as chromosomes move to the equator. 

Prophase is the longest stage. 

Chromosomes align themselves at the equator and attach to spindle fibres at the centromere. 

Independent assortment happens because the chromatids of the chromosomes can face either pole. 

ANAPHASE

Spindle fibres shorten and the centromere splits and cromatids pull apart. The chromatids are drawn to opposite poles. 

Chromosomes uncoil and lengthen. Spindle fibres break down and the nucleolus reforms. 

CYTOKINESIS gives rise to 4 haploid cells. 

Crossing Over 

In prophase 1 chromosomes condense and homologous pairs form a bivalent. This is called a synapsis. The chromatids in the bivalent join at the chiasmata. DNA can be exchanged at the chiasmata. This is known as crossing over. 

INDEPENDENT ASSORTMENT

During metaphase, the chromosomes distribute themselves along the equator of the spindle. This leads to an independent assortment of chromosomes. 

RANDOM FUSION OF HAPLOID GAMETES

At fertilisation, the diploid chromosome number is restored when the nuclei of haploid gametes fuse to form a zygote. Half of the chromosomes come from the mother and half from the father. 

Mitosis is only one cell division while meiosis is two cell divisions.

Mitosis forms 2 daughter cells whereas meiosis forms 4. 

In mitosis, chromosomes don't associate into bivalents but in meiosis they do. 

Mitosis has diploid daughter cells while meiosis has haploid daughter cells. 

Crossing over only takes place in meiosis. 

Daughter cells are identical in meiosis but they arent in mitosis. 

DNA mass doubles as DNA replication takes place during interphase. 

I the halves due to mitotic division to 2 daughter cells. 

It then doubles due to the second interphase and halves again during meiosis 1. This is because of the chromosome numbers halving. 

It halves again during meiosis 2 as the DNA is divided into 4 daughter cells.