Chapter 8How Cells Reproduce Part 4

From Gametes to OffspringIn plants, there are two multicellular life

cycle stages.Sporophytes are diploid multicellular

stages that produce spores.These spores consist of one or a few

haploid cells that undergo mitosis to produce a haploid stage called the gametophyte.

The gametophyte is also a multicellular life cycle stage in which gametes form.

This is called alternation of generations.

Plant Life Cycle Stages

From Gametes to OffspringFor example, large sequoia trees

are sporophytes.Male and female gametophytes

develop in different types of cones that form on the tree.

In flowering plants, the plant itself is the sporophyte and the gametophytes form from the flower.

From Gametes to OffspringThe gametes of animals are produced

through mitosis of diploid germ cells.Specifically, in males, a germ cell

develops into a primary spermatocyte that then undergoes meiosis to produce to produce four haploid cells that develop into spermatids.

These four spermatids then mature into four sperm.

This process is called spermatogenesis.

Spermatogenesis

From Gametes to OffspringIn females, a germ cell becomes a primary

oocyte (an immature egg).The primary oocyte undergoes meiosis.During Meiosis I, there is an unequal division of

the cytoplasm so that one very small cell is produced (called a polar body) and the other cell is very large (called the secondary oocyte).

The secondary oocyte then undergoes Meiosis II.Again, there is an unequal division of the

cytoplasm so that another, very small polar body is produced and the second, much larger cell matures into the female gamete, the ovum.

This process is called oogenesis.

Oogenesis

From Gametes to OffspringThus during production of sex

cells in males, four sperm result from Meiosis, whereas, in females, only one egg results.

From Gametes to Offspring: FertilizationAt fertilization, the fusion of the

two gametes from the two parents produces a zygote.

Fertilization restores the diploid chromosome number.

Fertilization also contributes to variation among offspring produced by sexual reproduction.

From Gametes to Offspring: Fertilization

Meiosis, Fertilization, and VariationConsider this:Cells that give rise to human gametes have

twenty-three pairs of homologous chromosomes.Each time a human germ cell goes through

meiosis, the gametes that are produced have one of 8,388,608 (or 223) possible combinations of homologous chromosomes.

The chromosomes assort independently during Metaphase I of meiosis so that which one of these 8, 388, 608 possible arrangements of chromosomes contained within any given gamete is totally random, never predetermined.

Meiosis, Fertilization, and Variation

Meiosis, Fertilization, and VariationIn addition, any number of genes may occur as

different alleles on the maternal and paternal chromosomes.

Add to that the fact that crossing over makes “mosaics” of genetic information contained on these homologous chromosomes.

Finally, out of all the male and female gametes that form, which two get together during fertilization is also totally random.

All of these factors taken together explain how such different and fascinating combinations of traits show up among the generations of any family tree.

Meiosis, Fertilization, and Variation

Checkpoint Genes and TumorsCertain proteins, which are the

products of “checkpoint” genes, can monitor whether a cell’s DNA has been copied completely, if the DNA is damaged, and even if there are enough nutrients and energy to support cell growth and division.

These proteins then will act to allow the cell cycle to proceed, be arrested, or be cancelled.

Checkpoint Genes and TumorsIf a mutation alters a checkpoint gene so that its

protein product is nonfunctional, it could result in the cell skipping Interphase and meiosis happening over and over again with no rest period.

Damaged DNA may also be replicated, introducing new mutations into the DNA of daughter cells or mutations may cause signals that would result in the cell committing suicide to fail.

No matter the mistake, when mutations cause checkpoint mechanisms to fail, the cell loses its control over the cell cycle and daughter cells produced from this cell may form tumors.

Checkpoint Genes and TumorsTumor suppressor genes are

genes whose protein products inhibit mitosis.

They get their name from the fact that, when these genes are missing, tumors form.

The BRCA genes that we have previously discussed whose mutations are believed to cause breast and ovarian cancers are examples of tumor suppressor genes.

Checkpoint Genes and Tumors

The BRCA genes help to regulate the expression of genes that code for enzymes that repair broken DNA.

Thus, when they are mutated, broken DNA does not get repaired, which can lead to the development of cancer.

The HPV (or human papilloma) virus causes cells to make proteins that interfere with checkpoint gene proteins.

This can cause non-cancerous growths (warts) to grow and can lead to cervical cancer.

CancerMoles and other tumors are

called neoplasms, which are abnormal masses of cells that have lost control over cell division.

Benign neoplasms are called this because they grow very slowly and they retain their cell surface recognition proteins that keep them in their home tissue.

CancerMalignant neoplasms grow quickly and

lose their cell surface recognition proteins so that they may leave their home tissue and invade other tissues in the body.

Malignant neoplasms are dangerous to health.

They can slip in and out of blood vessels and lymph vessels and travel throughout the body, invading and disrupting other tissues physically and metabolically.

CancerCancer cells typically have three

identifying characteristics:◦They grow and divide abnormally◦They have an altered plasma membrane and

cytoplasm. The cell membrane may be leaky and have altered or missing proteins, The cytoskeleton may be shrunken, disorganized or both. Cell metabolic may be altered.

◦They have a weakened ability to adhere. Because they have altered or missing recognition proteins, they lose the ability to adhere to cells in their home tissue, so they can metastasize.

Cancer

HeLa CellsResearchers used HeLa cells to test the

effects of the drug, taxol, on cancer cells.Taxol works to stop the division (mitosis) of

cancer cells by blocking the microtubules of the spindle apparatus from disassembling.

If spindle fibers cannot disassemble, mitosis stops.

Since cancer cells divide more frequently than healthy cells, it makes them much more vulnerable to this poison than healthy cells.

Taxol