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How Cells Reproduce. Chapter 9. Impacts, Issues Henrietta’s Immortal Cells. Henrietta Lacks died of cancer at age 31, but her cells ( HeLa cells) are still growing in laboratories. Henrietta Lacks.
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How Cells Reproduce Chapter 9
Impacts, IssuesHenrietta’s Immortal Cells • Henrietta Lacks died of cancer at age 31, but her cells (HeLa cells) are still growing in laboratories
Henrietta Lacks • Cancer cells used in research to identify viral strains, investigate cancer, study the effects of radiation on cells, develop techniques for polio vaccine.
Mitosis, Meiosis, and the Prokaryotes • Eukaryotic cells • Mitosiscopies DNA and divides a nucleus, producing two identical daughter cells; asexual reproduction (clones); exception identical twins • Meiosisis a nuclear division that produces haploid gametes for sexual reproduction (two parents involved) • Prokaryotic cells reproduce asexually by prokaryotic fission
A comparision of Mitosis and Meiosis Mitosis – Greek Mitos = thread Meiosis A type of division that reduces the chromosome number from diploid to haploid Occurs in germ cells (gametes – egg and sperm) Haploid chromosomes (23) - a single set • Division of eukaryotic cell or mother cell into two identical cells (daughter cells – receives copies of the original chromosome. • Occurs in somatic cells • Diploid chromosomes (46) – two sets
Mitosis Meiosis Variation in traits Two nuclear divisions (meiosis I and meisosis II Meiosis II resembles mitosis Parent = 46 and egg/sperm = 23 • Clones • One nuclear division • Parent = 8 chromosomes and the daughter cells = 8
Why do cells divide (Mitosis) • Growth • Repair of damaged tissue (ex. Healing of a wound) • Replacement of worn out cells (RBC’s) • Asexual reproduction (single celled organism producing a clone or exact copy of itself)
Key Points About Chromosome Structure • Each species has a characteristic number of chromosomes that differ in length and shape • Each consists of one double strand of DNA • After duplication, each consists of two double strands (sister chromatids) that remain attached to each other at a centromere until late in nuclear division – Study Figure 9.2
Key Points About Chromosome Structure • A chromosome consists of DNA that is wrapped around proteins (histones) and condensed • Each histone and the DNA wrapped around it make up a nucleosome
9.2 Introducing the Cell Cycle • Cell cycle • A sequence of three stages (interphase, mitosis, and cytoplasmic division) through which a cell passes between one cell division and the next
Interphase • Interphase consists of three stages, during which a cell increases in size, doubles the number of cytoplasmic components, and duplicates its DNA • G1: Interval of cell growth and activity; before the onset of DNA replication • S: Interval of DNA replication, synthesis or duplication (two DNA double helix) • G2: Interval when the cell prepares for division; the stage before cytoplasmic division
Interphase and the Life of a Cell • Most cell activities take place during G1 • Control mechanisms work at certain points in the cell cycle; some can keep cells in G1 (muscle and nerve cells) • Loss of control may cause cell death or cancer
Mitosis and the Chromosome Number • Mitosis produces two diploid nuclei/cells with the same number and kind of chromosomes as the parent or mother cell ( each new cell receives copies of all the original chromosomes) • Chromosome number • The sum of all chromosomes in a type of cell • Human cells have 46 chromosomes paired in 23 sets - Study Figure 9.5a • Pairs have the same shape and information about the same traits
9.3 A Closer Look at Mitosis • When a cell undergoes mitosis, the daughter cells have identical chromosomes, and the daughter cell has chromosomes identical to those of the mother cell that produced it. • There are four main stages/sequences of mitosis: prophase, metaphase, anaphase, and telophase
Prophase • Prophase • Chromosomes condense • Microtubules form a bipolar spindle or spindle apparatus (becomes visible) • Nuclear envelope/membrane breaks up and disappears • Microtubules attach to the chromosomes
Metaphase and Anaphase • Metaphase (meta = between) • All duplicated chromosomes line up at the spindle equator (midway) • Anaphase • Microtubules separate the sister chromatids of each chromosome and pull them to opposite spindle poles
Telophase • Telophase • Two clusters of chromosomes reach the spindle poles • A new nuclear envelope or membrane reforms • Two new cells are formed, each with the same chromosome number as the parent cell
9.4 Cytoplasmic Division Mechanisms • In most kinds of eukaryotes, the cell cytoplasm divides between late anaphase and the end of telophase, but the mechanism of division differs • Cytokinesis • The process of cytoplasmic division or distribution • Plants cytokinesis leads to cell plate formation • Animals cytokinesis leads to the formation of a contractile ring (cleavage furrow)
9.5 When Control is Lost • Sometimes, controls over cell division are lost • Cancer may be the outcome
Cell Cycle Controls • Checkpoints in the cell cycle (at G1, G2, and M) and allow problems to be corrected before the cycle advances • Proteins produced by checkpoint genes interact to advance, delay, or stop the cell cycle • Kinases – stops the cycle if DNA is damaged or kills the cell • Growth factors – stimulates a cell to grow and divide (ex repairs injuries)
Checkpoint Failure and Tumors • When all checkpoint mechanisms fail, a cell loses control over its cell cycle and may form a tumor(abnormal mass of cells) in surrounding tissue • Usually one or more checkpoint gene products are missing in tumor cells • Tumor suppressor gene products inhibit mitosis • Proto oncogene products stimulate mitosis
Neoplasms • Carcinogens – chemicals that cause cancer • Neoplasms AKA tumors • Abnormal masses of cells that lack control over how they grow and divide • Benign neoplasms (such as ordinary skin moles) stay in one place and are not cancerous; do not pose a threat to surrounding tissues • Malignant neoplasms are cancerous; cells may break away, spread and invade distant tissues (metastasis)
