Transcript Cell Division

Cell Reproduction
Chapter 8
• Henrietta Lacks
– cervical cancer
– HeLa Cells
Cell Division
• Mitosis
– How body cells divide
– Makes exact copy – clone - of parent cell
– Identical daughter cells
• Meiosis
– Makes sex cells – eggs and sperm
– Contain only one chromosome of a pair
– Half the normal number of chromosomes
Prokaryotic cell division
• No nucleus or organelles, and only one
circular chromosome
• Process is called binary fission
– Not mitosis since there is no nucleus to divide
http://www.anselm.edu/homepage/jpitocch/genbio/binfission.JPG
Eukaryotic cell division
• More complicated
• More DNA
• Chromosomes come in pairs:
– homologous chromosomes
Cell cycle
•
All the processes from one division to the
next:
1. Interphase – “resting stage”
2. Mitosis – nuclear division
3. Cytokinesis – division of the cytoplasm
Interphase
• Cell is busy doing its job – just not dividing
– Nerve cells spend most of life in interphase
– Cancer cells spend little time in interphase
• Three parts:
– G1 or Gap 1 or growth phase
– S phase : Synthesis of DNA ; cell is committed
to division
– G2 or Gap 2
Mitosis
• Refers specifically to division of the nucleus
• Four phases:
– Prophase
– Metaphase
– Anaphase
– Telophase
PMAT
Prophase
• Chromosomes wind up or condenses
– 2 meters / cell
– Linked by centromere
– Sister chromatids
• Nucleoli disappear; mitotic spindle forms
from the centrioles
Prometaphase or late prophase
• Transition phase – to cell it is all part of
one process
• Nuclear membrane disappears
• Chromatids attach to fibers of mitotic
spindle by means of specialized structure
called a kinetochore
Metaphase
• Chromosomes line up in the center of the
cell at the metaphase plate – a disc
• Remain this way for about an hour
Anaphase
• Centromere splits
• Microtubles of mitotic spindle pull
members of each pair of duplicate
chromosomes to opposite sides of the cell.
Telophase
• Mitotic spindle disappears
• Nuclear membranes form
• Chromosomes unwind
Cytokinesis
• Division of the cytoplasm
• Accompanies mitosis
• Begins in anaphase and finishes after
telophase
• Animal cells form a contractile ring using
actin – one of the proteins found in
muscles.
• Indentation is called the cleavage furrow
• Pinches the cells apart
Plant cells
• Mitosis occurs in plants as in animal cells,
but cytokinesis is different because of the
plant cell wall.
• A disc of new membrane called the cell
plate forms between the two cells during
telophase and expands until it reaches the
edges of the cell membrane.
• The new cells then make cellulose fibers
to form new cell walls.
Control of cell division
• Cell senescence – cells reproduce only a
certain number of times and then stop.
– May be due to telomeres –repeating series of
bases at the end of chromosomes that
decrease with each division
– Telomerase
• Apoptosis – programmed cell death –
”suicide genes”
http://www-ermm.cbcu.cam.ac.uk/fig001nkg.gif
Contact inhibition
• Cells divide until they establish contact
with other cells on all sides.
Other factors
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Cell size
hormones
Growth factors
Cyclins
Genes:
Oncogenes – want these turned off
– Oncology – the study of cancer
• Tumor suppressor genes – want these
turned on
Tumors - neoplasms
• Cells have lost control over cell division
• Benign tumors grow only in one area
• Cancers invade local tissues (look like a
crab) and can metastasize or spread to
other areas of the body through the
vascular or lymphatic systems