Biology 12

Cell division

Needed for:

• reproduction in unicellular organisms

• growth in multicellular organisms

• replacement of damaged or dying cells

• repair of damaged organs or tissues

Mitosis

Involves:

• replication of DNA

• controlled division of the nuclear material so new each cell has the same DNA

• division of cytoplasm and organelles so each new cell has similar contents

DNA replication• Occurs in interphase – before

mitosis starts• One set of enzymes split the

strands• Another set of enzymes join new

nucleotides to each strand• Nucleotides match up by the

base pairing rule (C – G, A - T)• The end result is two identical

strands, joined at a point called the centromere

DNA replication 2

Chromosomes

• Chromosomes are only visible during cell division

• They consist of two strands (chromatids) of tightly coiled DNA joined at the centromere

Cell cycle

Stages of mitosis

Interphase

• Phase of growth and normal metabolism

• Nucleus and nucleolus visible

• DNA replication occurs just before cell division occurs

Prophase

• First stage of division• Nucleus and

nucleolus disappear• Spindle starts to form• DNA condenses and

becomes visible as chromosomes

• Centrioles move to opposite poles (animal cells)

Metaphase

• Chromosomes line up on spindle at the equator of the cell

Anaphase

• Chromatids pull apart at the centromere

• One chromatid from each chromosome moves to each end of the cell

Telophase

• Chromosomes uncondense and a new nucleus forms around them

• The cytoplasm splits (cytokinesis) to form two new cells identical to the original except smaller

Interphase

• Phase of growth and normal metabolism• Nucleus and nucleolus visible

Identify the stages of mitosis

telophase

anaphase

metaphase

early prophase

interphase

early prophase

metaphase

early prophase

Identify the stages of mitosis 2telophase

metaphase

anaphase

late anaphase/early telophase

prophase

interphase

early prophase

late anaphase/early telophase

Designing experiments• Pose a question

• Suggest some answers

• Define your hypothesiseg using pesticides will increase yield

What shall I change?

• Select one thing to test

• That is the independent variableeg pesticides

How do I know if it has an effect?• What will I measure?• How will I measure it? • That is the dependent variable

• What will I compare it to?• That is the control

eg yieldMeasured by weight, height or volume

eg group without pesticides

How do I know my results are true?

• Define your controlled variables

• Define how you will increase reliability

eg same soil type, same type & number of seeds/plants, same water, sun,temperature, humidity, weed control, fertiliser, etc

The language of experimental design

• Hypothesis

• Independent variable

• Dependent variable

• The control

• Controlled variables

• Reliability

Ways to increase reliability

• Use accurate measurements • Increase number of measurements• Run experiment for longer• Repeat the experiment• Increase the sample size• Get someone else to replicate the

experiment• Share the results and get a review