Cell Reproduction

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 division3. 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