BIOLOGY UNIT 6 CHAPTER 6: Chromosomes and Cell Reproduction UNIT 6 CHAPTER 7: Meiosisand Sexual Reproduction
Formation of New Cells by Cell Division • About 2 trillion cells are produced by an adult human body every day – about 25 million new cells per second. • The type of cell division differs depending on the organism. • Aprokaryote cell such as bacteria reproduces bybinary fission.
Binary fission is a form ofasexual reproductionthat occurs in bacteria and produces two identical daughter cells of equal size. First, the DNA is copied, then the cell divides.
Eukaryote cellsreproduce bymitotic cell division. • Mitotic cell division, ormitosis, is cell division that produces genetically identical, or cloned,daughter cells. • Mitosis involves a series of steps that results in the formation oftwo identical nuclei. • Mitosis differs in plant cells and animal cells.
There are two main differences between cell division in plant cells and animal cells. • In plant cells, a cell plate forms to divide the two new cells. • A cell plate is a membrane-bound cell wall that forms across the middle of the plant cell to separate the plant cell into two new cells.
Animal cells do not form a cell plate. Instead, animal cells form a cleavage furrow. • A cleavage furrow is an indentation that pinches apart the two new cells. • The second difference is that plant cells do not have centrioles, but they do form a spindle that is almost identical to that of an animal cell.
Formation of New Cells • Gametesare an organism’s reproductive cells, such as spore or egg cells. • A zygote is a fertilized egg cell, the first cell of a new individual. • Ageneis a segment of DNA that carries the code for a protein or RNA molecule. • As a eukaryotic cell begins to divide, thechromosomes– the DNA and the proteins associated with the DNA – becomesvisible.
Achromosomeis one of the structures in thenucleusthat are made up of DNA and protein. • Chromosomesfunction to pass ongenetic information from one generation to the next. • Before cell division, each chromosome is duplicated, or copied. This is known asDNA replication.
half of a chromatid pair • Each replicated chromosome consists of two identical “sisterchromatids, ” which are strands of a chromosome that become visible during mitosis or meiosis. • Each pair of chromatids is attached at an area near the center called the centromere.
Sets of Chromosomes • Homologous chromosomes are chromosomes that are similar in size, shape, and genetic content. • Each homologue in a pair of homologous chromosomes comes from one of the two parents. • The46 chromosomesin human somatic cells(or body cells) are actually23 pair of chromosomes.One set comes from the mother, and one set comes from the father.
Somatic cells, or body cells, are all of the cells in the body other than gametes. • Diploid means a cell containstwo sets of chromosomes, usually one from the mother and one from the father.The diploid number is the total number of chromosomes of the organism. • Human body cells are diploid. • The diploid number can be written as2N = 46, where ‘N’ represents one set of chromosomes.
Haploid means a cell contains one set of chromosomes – or half the total number for the organism. • Humansex cells, orgametes, are haploid. • The haploid number can be written asN = 23.
SEX CHROMOSOMES • Autosomesare chromosomes that are not directly involved in determining the sex, or gender, of an individual. • Thesex chromosomes, one of the 23 pair of chromosomes in humans, contain genes that will determine the sex of the individual.
In human males, thesex chromosomesare made up of oneXchromosomeand oneYchromosome(XY). • The sex chromosomes in human females consists of twoXchromosomes(XX). • Because a female can donate only an X chromosome to her offspring,the sex of an offspring is determined by the male,who can donate either anXor aYchromosome.
Change in Chromosome Number • A karyotype is a photo of the chromosomes in a dividing cell that shows the chromosomes arranged by size. • Karyotypes describe the number of chromosomes, and what they look like under a light microscope. • Attention is paid to their length, the position of the centromeres, banding pattern, any differences between the sex chromosomes, and any other physical characteristics.
Change in Chromosome Number • Karyotypes can be used to find abnormalities in human chromosomes. • A person must have the characteristic number of chromosomes in their cells for normal development and function. • Trisomyis a condition that results in abnormal development when humans have more than two copies of a chromosome.
Down syndrome, ortrisomy 21, results when an individual has an extra copy of chromosome 21. • What events can cause an individual to have an extra copy of a chromosome? • When sperm and egg cells form, each chromosome and its homologue pair separate, an event calleddisjunction.
If one or more chromosomes fail to separate properly – an event occurs callednondisjunction-- one new gamete ends up receiving both chromosomes and the other gamete receives none. • Trisomy occurs when the gamete with both chromosomes fuses with a normal gamete during fertilization, resulting in offspring with three copies of that chromosome instead of two.
Older mothers are more likely to have a baby with Down syndrome because all the eggs a female will ever produce are present in her ovaries when she is born, unlike males who produce new sperm throughout adult life. • As a female ages, her eggs can accumulate an increasing amount of damage. • Because of this risk, a pregnant woman over the age of 35 may be advised to undergo prenatal testing that includesfetal karyotyping.
Types of Mutations • deletion mutation – a piece of chromosome breaks off completely, causing the new cell to lack a certain set of genes after cell division • duplication mutation – a chromosome fragment attaches to it homologous chromosome, which will then carry to copies of a certain set of genes
Types of Mutations • inversion mutation – a chromosome piece reattaches to the original chromosome but in a reverse orientation • translocation mutation – occurs if a piece of chromosome reattaches to a nonhomologous chromosome
TYPES OF CELL DIVISION Cell division, or cell reproduction, is the process through which a cell divides to form two new cells called daughter cells. There are three basic types of cell division: 1.) asexual reproduction, 2.) mitosis, and 3.) meiosis.
TYPES OF CELL DIVISION • 1.) The first type is asexual reproduction,cell division in prokaryotes (bacteria). • 2.) The second type is mitosis, or cell division in eukaryotes for the purpose of growth and repair. • 3.) The third type is meiosis, or cell division that produces gametes, or sex cells.
A S E X U A L
Cell Division in Bacteria • Prokaryotes reproduce through binary fission. • Binary fission is a form ofasexual reproductionthat occurs in bacteria (prokaryotes) and produces two identical daughter cells of equal size. • First, the DNA is copied, then the cell divides. • The two daughter cells are identical to the original parent cell from which they formed.
The Cell Cycle • The cell life cycle has two major periods: interphaseandcell division. • Interphaseis the longer phase where the cell is actively growing and carrying on metabolic activities. • Cell division consists of two events: • 1.) mitosis -- division of the nucleus, and • 2.) cytokinesis -- division of the cytoplasm.
INTERPHASE -- G1, Synthesis, G2 • A cell spends 90% of its time in the G1 cycle, or phase, of interphase. • How does a cell know when to divide? • In eukaryote cells, the cell cycle is controlled by many proteins. • Feedback signals from these proteins occur at key checkpoints (G1 and G2 checkpoints) during interphase, letting the cell know if it is healthyand large enough to divide.
Uncontrolled Growth of Cells • Canceris the uncontrolled growth of cells caused by a mutated protein that does not respond normally to the body’s control mechanisms. • In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. • The cancer may also spread to more distant parts of the body through the lymphatic system or bloodstream.
Tumors • Not all tumors are cancerous. • Benign tumors do not grow uncontrollably, do not invade neighboring tissues, and do not spread throughout the body. • Malignant tumors tend to become progressively worse and to potentially result in death. • A malignant tumor contrasts with a non-cancerous benign tumor in that a malignancy is not self-limited in its growth, and is capable of invading into adjacent tissues, and may be capable of spreading to distant tissues. A benign tumor has none of those properties. • There are over 200 different known cancers that afflict humans.
Interphase cell growth before mitosis begins • Chromosomes are replicated (copied) during the Synthesis phase (S) of Interphase, and the chromosome number doubles. • Chromosomes appear as threadlike coils called chromatin at the beginning of interphase. • Each chromosome and its copy (called sister chromosomes) change to sister chromatidsat the end of this phase. Cell Membrane Nucleus Chromatin
What’s Happening in Interphase? What the cell looks like Animal Cell What’s occurring
Interphase Animal Cell Plant Cell
There are 4 main stages of mitosis: • PROPHASE:occurs when the genetic material in the cell, which is normally loosely bundled, condenses, or thickens, to form chromosomes. Each chromosome has duplicated and now consists of two sister chromatids. • METAPHASE:occurs when the chromosomes align themselves along the cell spindle in the middle of the cell, ready to separate • ANAPHASE:occurs when the sister chromatids separate and move to opposite ends of the cell • TELOPHASE:occurs when the cell prepares to cleave, or separate, into two parts
Animated Mitosis Cycle MITOSIS ANIMATION LINK http://www.cellsalive.com/cell_cycle.htm MITOSIS VIDEO CYCLE http://www.youtube.com/watch?v=NR0mdDJMHIQ&feature=related http://www.youtube.com/watch?v=DD3IQknCEdc http://www.youtube.com/watch?v=K6ZRlJsHu4o
Prophase1st Stage in Mitosis • During prophase, the chromatin condenses, forming the chromosomes. • Pairs of sister chromatids are joined at the centromere. • The nuclear membrane breaks down. • Centriolesappear and begin to move to opposite ends of the cell. • Spindle fibersform between the poles.
Forming the Spindle • The centromere is the part of a chromosome that links sister chromatids. • During mitosis, spindle fibers attach to the centromere • In animal cells, a pair of centrioles is found inside each centrosome. • Centrioles and spindle fibers are both made of hollow tubes of protein called microtubules. • Each spindle fiber is made of an individual microtubule.
Forming the Spindle • Eachcentrioleis made of nine triplets of microtubules in a circle. • Plant cells do not have centrioles, but they form a spindle that is almost identical to that of an animal cell.
Prophase Animal Cell Plant Cell Centrioles Spindle fibers
Metaphase 2nd Stage in Mitosis • Chromatids(or pairs of chromosomes) attach to the spindle fibers. • The pairs of chromosomes cluster and line up on the equator of the cell. • Each chromosomes is attached to a spindle fiber at its centromere.
Metaphase Animal Cell Plant Cell
Anaphase3rd Stage in Mitosis • Chromatids separate and begin to move to opposite ends of the cell. • The centromeres that have held the chromatids together split, and the chromatids become chromosomes again. • Sister chromatids are pulled apart by the spindle fibers towards the opposite poles.
Anaphase Animal Cell Plant Cell