think back to what you have learned from previous classes. what is a chromosome? how many...

Think back to what you have learned from previous classes. What is a chromosome? How many chromosomes do humans have? Why are chromosomes usually in pairs?

Cells need to replace themselves In an organism, all cells must have the

same genetic material

How could a human cell with 46 chromosomes divide into two cells? What must cells undergo before they divide

Understand why DNA must duplicate before a cell can split

Know each phase of the cell cycle Know all the stages of mitosis and be

able to give an overview of what is happening at each stage

Chromosomes replicate to create sister chromatids (identical chromosomes) attached at the “waist”

This “waist” is called the centromere

The cell cycles between phases of; DNA synthesis, gap phases, and a mitotic phase

During gap phases, the cell grows and undergoes normal functioning

Mitosis - The division of the nucleus Cytokinesis – The division of the

cytoplasm

The mitotic phase includes both mitosis and cytokinesis

Interphase – In between mitosis Prophase – Getting ready for mitosis

(chromosomes condensing) Prometaphase – Getting ready for mitosis

cont. (microtubules extending and nuclear envelope fragments)

Metaphase – Chromosomes line up Anaphase – Chromosomes pull apart Telophase and Cytokinesis – The cells split

apart

Chromosomes not condensed

Bound by nuclear envelope

Nucleolus present Two centrosomes

have formed

Chromosomes condense

Nucleoli disappear Mitotic spindle begins

to form (Mitotic spindle includes microtubules, centrosomes and asters Centrosomes move

away from each other

Nuclear envelope fragments

Microtubules start to attach to the kinetochore (Protein located at the centromere)

Nonkinetochore microtubules interact with each other

Chromosomes line up on metaphase plate

Each microtubule attached to one sister chromatid kinetochore

Centrosomes are at opposite sides of the cell

Sister chromatids detach

Each chromosome moves to opposite ends of the cell

The cell elongates as non kinetochore microtubules lengthen

Telophase Nuclear envelopes

form to create new nuclei

Chromosomes become less condensed

Cytokinesis Division of

cytoplasm

ANIMAL CELLS PLANT CELLS

Cleavage – Pinching off by contractile ring of actin microfilaments Like pulling a draw string

Vesicles from the golgi move to the middle of the two new cells and merge to produce a cell plate (the vesicles carry the material needed to produce the cell wall)

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Mitosis

Signals from inside and outside the cell give the “go ahead” if the cell can continue to divide

Why might this be important? So cells don’t continue to

divide if they are not needed

Most mature human cells are in a nondividing state G0

So damaged cells don’t continue to divide

Cyclin-dependent kinases CDK’s activate many

enzymes to start or continue cell division

Kinases are always in the cell

Kinases need cyclin to work

Cyclin “cycles” in its concentration within the cell

MPF stands for maturation promoting factor

Signals from outside the cell can signal the cell to divide or not. Examples: Cells fail to divide if they lack a nutrient Growth Factors such as platelet-derived

growth factor (made by blood platelets) continue the cell cycle at the G1 phase

Density dependent inhibition – cell surface proteins bind to receptors on nearby cells to inhibit growth

Anchorage dependence – only divide when they are able to anchor

Vestigial wings Wild type

Eye color Red (wild type) White sepia

What is Heredity – the transmission of traits from one generation to the next

Gametes: reproductive cells such as egg and sperm

Zygote: The fertilized egg

We can look at chromosomes under a microscope and arrange them according to their size and banding patterns

Karyotype: The display of chromosomes

Homologous chromosomes – chromosomes that carry the same genes, but may contain different variations of that gene (one of the pair comes from the mother and one from the father)

Ex. Eye color is positioned at the same locus on both chromosomes of a homologous pair Locus (plural loci): fancy word for

location (used when dealing with chromosomes)

Autosomes – everything except the sex chromosomes

Sex chromosomes – the chromosomes that determine the individuals sex In humans

XX – Female XY - Males

Sex chromosomes

HAPLOID DIPLOID

Now, what has to happen in meiosis if chromosomes from one diploid parent mix with chromosomes from another diploid parent (Hint: the baby should also be diploid)

Meiosis only happens in germ cells (cells that produce gametes)

Mitosis happens in both somatic (cells other than germ cells) and germ cells.

Prophase I Metaphase I Anaphase I Telophase I and cytokinesis Prophase II Metaphase II Anaphase II Telophase II and cytokinesis

1. Crossing over

2. Homologs on metaphase plate

3. Separation of homologs

3 main differences in meiosis

Independent assortment Crossing over Random Fertilization

Draw a cell of a diploid organism that has three chromosomes. (you may want to use different colored pens for chromosomes from the mother and father)

Now, draw a picture of them lined up during metaphase I of meiosis.

Look at your neighbors. Are their pictures different from yours?

Each homologous pair is positioned independently of other pairs

Possible combinations are 2n, where n is the number of chromosomes in a single set

So… for humans n=23, there are about 8.4 million possible combinations resulting from independent assortment!

To further increase variation, chromosomes cross over during prophase I They exchange

genetic information during this phase

Furthermore, if we account that one of 8.4 million possibilities of eggs merge with one of 8.4 million possibilities of sperm, then we know that there are about 70 trillion (223 x 223) different combinations of zygotes that two humans could make (without including the variation brought about by crossing over).