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• 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