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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)
pg 235
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).