Cell division

How are chromosomes formed

When DNA is tightly coiled around histones it forms chromosomes (condensed form) and can be seen in a light microscope

How is chromatin formed

When DNA is loosely coiled around histones it forms chromatin (decondensed)form

What are the stages of the cell cycle

• Interphase:G1,S,G2

• Mitosis:prophase,metaphase,anaphase,telophase

• Cytokinesis

What happens during interphase

G1-protein synthesis

S-DNA replication which makes two identical chromatids

G2-replication of organelles and production of centrioles

What happens during prophase

• DNA super coils and condenses so is now visible

• Nuclear envelope breaks down

• Spindle fibres are produced from centrioles which attaches to the centromeres of the chromosomes

What happens in metaphase

Spindle fibres pull chromosomes and they line up in single file along the equator of the cell

What happens during anaphase

Spindle fibres shorten and contract to pull and separate sister chromatids to opposite poles of the cell (can be seen by V shape chromatids)

What happens during telophase

• Nuclear envelopes reform around each group of chromosomes, so each pole receives all genetic information/identical, copies of each chromosome

• Chromosomes decondense and uncoils

What happens during cytokinesis

• Cytoplasm divides and forms 2 identical daughter cells

Describe the part played by spindle fibres in cell division

Spindle fibres shorten and contract

Pull chromosomes to opposite poles of the cell during anaphase

What is cancer

When uncontrolled cell division occurs and there is formation of a tumor

What are the problems with tumors

Physical pressure

Uses up resources so healthy cells cannot e.g blood supply

Stops normal cells functioning

Invades tissue

What cancer treatments target

Directed at controlling the rate of mitosis

How do you calculate how long a specific stage of the cell cycle takes

A) know how long the cycle took in total

B)see a sample of tissue and work out percentage of total cells undergoing that certification stage

-if 5% of the cells are undergoing prophase then 5% of the total cell cycle is spent in prophase

E.g cell cycle takes 10 hours,300 cells observed

240 in interphase,240/300 × 100 to find percentage of cells=80%

find time by doing 80% of 10 hours which is 8 so 8 hours are spent in prophase

What shows evidence that chromosomes are in mitosis based on an image

Chromosomes are visible so Have supercoiled and condensed

Each chromosome is made of 2 sister chromatids because DNA has replicated

Chromosomes are not arranged in homologous pairs which they would be if meiosis