CHAPTER 8 The Cellular Basis of Reproduction and Inheritance Overview: Reproduction: asexual & sexual prokaryotes eukaryotes Mitosis Meiosis Abnormalities

The life cycle of a multicellular organism includes –development –Reproduction: the birth of new organisms Just in the past second, millions of your cells have divided in two Cell division is at the heart of the reproduction of cells and organisms; cell division plays a role in replacement of lost or damaged cells Organisms can reproduce sexually or asexually

Some organisms can also reproduce asexually –This sea star is regenerating a lost arm –Regeneration results from repeated cell divisions Some organisms make exact copies of themselves, asexual reproduction

In asexual reproduction, single-celled organisms reproduce by simple cell division The Reproduction of Organisms

All cells come from cells Cellular reproduction is called cell division –Cell division allows an embryo to develop into an adult –It also ensures the continuity of life from one generation to the next Cells arise only from preexisting cells

Before a parent cell divides, it duplicates its chromosomes Passing On the Genes from Cell to Cell The two resulting “daughter” cells are genetically identical

Sexual reproduction is different –It requires fertilization of an egg by a sperm –Production of egg and sperm is called meiosis

Prokaryotic cells divide asexually –These cells possess a single chromosome, containing genes –The chromosome is replicated –The cell then divides into two cells, a process called binary fission Prokaryotes reproduce by binary fission

An eukaryotic cell has many more genes than a prokaryotic cell –The genes are grouped into multiple chromosomes, found in the nucleus –The chromosomes of this plant cell are stained dark purple The large, complex chromosomes of eukaryotes duplicate with each cell division THE EUKARYOTIC CELL CYCLE AND MITOSIS

A genome –Is the complete set of an organism’s genes –Is located mainly on chromosomes in the cell’s nucleus

Chromosomes Eukaryotic Chromosomes –Are made of chromatin, a combination of DNA and protein molecules –Are not visible in a cell until cell division

Eukaryotic cells that divide undergo an orderly sequence of events called the cell cycle The Cell Cycle

The cell cycle consists of two major phases: –Interphase, where chromosomes duplicate and cell parts are made –The mitotic phase, when cell division occurs

Eukaryotic cell division consists of two stages: –Mitosis –Cytokinesis Mitosis –Is the division of the chromosomes –Is preceded by interphase Interphase

In mitosis, the duplicated chromosomes are distributed into two daughter nuclei –After the chromosomes coil up, a mitotic spindle moves them to the middle of the cell Mitosis consists of four distinct phases:

The sister chromatids then separate and move to opposite poles of the cell –The process of cytokinesis divides the cell into two genetically identical cells

Cytokinesis –Typically occurs during telophase –Is the division of the cytoplasm

In animals, cytokinesis occurs by cleavage –This process pinches the cell apart Cytokinesis differs for plant and animal cells

In plants, a membranous cell plate splits the cell in two

Most animal cells divide only when stimulated, and others not at all In laboratory cultures, most normal cells divide only when attached to a surface –They are anchorage dependent Anchorage, cell density, and chemical growth factors affect cell division

Cells continue dividing until they touch one another –This is called density-dependent inhibition

Growth factors are proteins secreted by cells that stimulate other cells to divide

Proteins within the cell control the cell cycle –Signals affecting critical checkpoints determine whether the cell will go through a complete cycle and divide Growth factors signal the cell cycle control system

The binding of growth factors to specific receptors on the plasma membrane is usually necessary for cell division

Normal plant and animal cells have a cell cycle control system Cancer Cells: Growing Out of Control When the cell cycle control system malfunctions –Cells may reproduce at the wrong time or place –A benign tumor may form What is cancer? What Is Cancer? –Cancer is caused by a breakdown in control of the cell cycle

Cancer treatment Cancer Treatment –Radiation therapy disrupts cell division –Chemotherapy involves drugs that disrupt cell division

Cancer prevention includes changes in lifestyle Cancer Prevention and Survival –Not smoking –Avoiding exposure to the sun –Eating a high-fiber, low-fat diet –Visiting the doctor regularly –Performing regular self-examinations

Somatic cells of each species contain a specific number of chromosomes –Human cells have 46, making up 23 pairs of homologous chromosomes MEIOSIS AND CROSSING OVER Chromosomes are matched in homologous pairs

Different organisms of the same species have the same number and types of chromosomes Homologous Chromosomes A somatic cell –Is a typical body cell –Has 46 chromosomes in a human Humans have –Two different sex chromosomes, X and Y –22 pairs of matching chromosomes, called autosomes

Cells with two sets of chromosomes are said to be diploid Gametes are haploid, with only one set of chromosomes Gametes have a single set of chromosomes

At fertilization, a sperm fuses with an egg, forming a diploid zygote –Repeated mitotic divisions lead to the development of a mature adult –The adult makes haploid gametes by meiosis –All of these processes make up the sexual life cycle of organisms

Humans are diploid organisms –Their cells contain two sets of chromosomes –Their gametes are haploid, having only one set of chromosomes Fertilization –Is the fusion of sperm and egg –Creates a zygote, or fertilized egg

Meiosis, like mitosis, is preceded by chromosome duplication –However, in meiosis the cell divides twice to form four daughter cells Meiosis reduces the chromosome number from diploid to haploid

In the first division, meiosis I, homologous chromosomes are paired –While they are paired, they cross over and exchange genetic information –The homologous pairs are then separated, and two daughter cells are produced

Meiosis II is essentially the same as mitosis –The sister chromatids of each chromosome separate –The result is four haploid daughter cells

For both processes, chromosomes replicate only once, during interphase Review: A comparison of mitosis and meiosis

Each chromosome of a homologous pair comes from a different parent –Each chromosome thus differs at many points from the other member of the pair Independent orientation of chromosomes in meiosis and random fertilization lead to varied offspring

Offspring of sexual reproduction are genetically different from their parents and from one another The Origins of Genetic Variation The large number of possible arrangements of chromosome pairs at metaphase I of meiosis leads to many different combinations of chromosomes in gametes Random fertilization also increases variation in offspring

The human egg cell is fertilized randomly by one sperm, leading to genetic variety in the zygote Random Fertilization The differences between homologous chromosomes are based on the fact that they can carry different versions of a gene at corresponding loci Homologous chromosomes carry different versions of genes

In crossing over Crossing Over –Homologous chromosomes exchange genetic information –Genetic recombination occurs

Crossing over is the exchange of corresponding segments between two homologous chromosomes Genetic recombination results from crossing over during prophase I of meiosis –This increases variation further Crossing over further increases genetic variability

To study human chromosomes microscopically, researchers stain and display them as a karyotype –A karyotype usually shows 22 pairs of autosomes and one pair of sex chromosomes ALTERATIONS OF CHROMOSOME NUMBER AND STRUCTURE A karyotype is a photographic inventory of an individual’s chromosomes A karyotype is an orderly arrangement of chromosomes

This karyotype shows three number 21 chromosomes An extra copy of chromosome 21 causes Down syndrome Connection: An extra copy of chromosome 21 causes Down syndrome

Abnormal chromosome count is a result of nondisjunction –Either homologous pairs fail to separate during meiosis I Accidents during meiosis can alter chromosome number

Nondisjunction can also produce gametes with extra or missing sex chromosomes –Unusual numbers of sex chromosomes upset the genetic balance less than an unusual number of autosomes Connection: Abnormal numbers of sex chromosomes do not usually affect survival

Chromosome breakage can lead to rearrangements that can produce genetic disorders or cancer –Four types of rearrangement are deletion, duplication, inversion, and translocation Connection: Alterations of chromosome structure can cause birth defects and cancer