HOW MANY CHROMOSOMES ARE FOUND IN EACH HUMAN BODY CELL?

Section 10.2 Summary – pages Genes do not exist free in the nucleus of a cell; they are lined up on chromosomes.

Section 10.2 Summary – pages In the body cells of animals and most plants, chromosomes occur in pairs. In each pair, one you inherited from your mom, the other from your dad. Diploid Cells From Mom From Dad

Section 10.2 Summary – pages The two chromosomes of a pair are called homologous chromosomes. Each pair of homologous chromosomes has genes for the same traits. Homologous Chromosomes

Section 10.2 Summary – pages A cell with two of each kind of chromosome is called a diploid cell. Diploid Cells

Section 10.2 Summary – pages Body cells (diploid) are different from sex cells. Each person can only pass on ONE of their homologous chromosomes to their young. HUMANS = 46 CHROMOSOMES IN THEIR BODY CELLS Each parent passes 23 to their young, so that when the sperm and egg combine- it will equal 46 Chromosomes.

Section 10.2 Summary – pages So, sex cells CANNOT be diploid. A cell containing only one of each kind of chromosome is called a haploid cell. Haploid Cells

Section 10.2 Summary – pages Sexual Reproduction Zygote

Section 10.2 Summary – pages Chromosome Numbers of Common Organisms OrganismBody Cell Fruit fly 8 Garden pea14 Corn20 Tomato24 Leopard Frog Apple34 Human46 Chimpanzee48 Dog78 Adder’s tongue fern 1260 Sex Cell Diploid and Haploid Cells

In Humans, how many chromosomes would each of these cells have….. CHEEK CELL 46 SPERM CELL 23 EGG CELL 23 NERVE CELL 46

Homologsseparate Sisterchromatidsseparate Diploid Meiosis I Meiosis II Diploid Haploid The kind of cell division that produces sex cells (haploid) containing half the number of chromosomes as a body cell, is called meiosis.

Meiosis: cell division that makes SEX CELLS. It starts with a DIPLOID cell 46 Chromosomes Replicate It ends with FOUR HAPLOID cells (not identical) Click here to see the animation

Section 10.2 Summary – pages These haploid cells are called sex cells— gametes. Male gametes are called sperm. Female gametes are called eggs. Meiosis

Section 10.2 Summary – pages When a sperm fertilizes an egg, the resulting zygote once again has the diploid number of chromosomes. Meiosis

Sect ion 10.2 Summary – pages

Growth period in which the cell replicates its chromatin (DNA). Interphase

Meiosis I: Reduction Division Nucleus Spindlefibers Nuclearenvelope

Prophase I Chromatin coils up. Chromatin coils up. Spindle forms. Spindle forms. Nuclear envelope fragments. Nuclear envelope fragments.

Tetrads Form in Prophase I Homologous chromosomes (each with sister chromatids) Homologous chromosomes (each with sister chromatids) Join to form a TETRAD

Crossing-Over in Prophase I The tetrads pair so tightly that non-sister chromatids from homologous chromosomes can actually break and exchange genetic material in a process known as crossing over.

Crossing-Over Crossing over can occur at any location on a chromosome, and it can occur at several locations at the same time.

Section 10.2 Summary – pages The centromere of each chromosome becomes attached to a spindle fiber. Metaphase I In Metaphase I, the tetrads line up in the middle, or equator.

Anaphase I Homologous chromosomes separate and move to opposite poles. Sister chromatids remain attached at their centromeres attached at their centromeres.

Telophase I Nuclear envelopes reassemble. Spindle disappears. Cytokinesis divides cell into two.

Meiosis II: Reducing Chromosome Number Prophase II Metaphase II Anaphase II Telophase II 4 Identical haploid cells The second division in meiosis is simply a mitotic division of the products of Meiosis I.

Prophase II Nuclear envelope fragments. Spindle forms.

Metaphase II Chromosomes align along equator of cell.

Anaphase II Sister chromatids separate and move to opposite poles Sister chromatids separate and move to opposite poles. Equator Pole

Telophase II Nuclear envelope assembles. Chromosomes decondense. Spindle disappears. Cytokinesis divides cell into two.

Results of Meiosis Gametes (egg & sperm) form Four haploid cells with one copy of each chromosome One allele of each gene Different combinations of alleles for different genes along the chromosome

Section 10.2 Summary – pages The failure of homologous chromosomes to separate properly during meiosis is called nondisjunction. This would occur in Anaphase I, when homologous chromosomes are suppose to separate from each other. Nondisjunction

Section 10.2 Summary – pages When a gamete with an extra chromosome is fertilized by a normal gamete, the zygote will have an extra chromosome. This condition is called trisomy. An example of this in humans is Downs Syndrome Nondisjunction

Downs Syndrome Extra chromosome on 21 1 in every 1100 live births Range from mild to severe physical and mental disabilities

Section 10.2 Summary – pages Although organisms with extra chromosomes often survive, organisms lacking one or more chromosomes usually do not. When a gamete with a missing chromosome fuses with a normal gamete during fertilization, the resulting zygote lacks a chromosome. This condition is called monosomy. An example of this in humans is Turner’s Syndrome. Nondisjunction

Turners Syndrome Absence of one X chromosome Occurs in 1 out of every 2,500 live female births. Physical characteristics No mental disability

Section 2 Check A cell with two of each kind of chromosome is __________. Question 1 D. polyploid C. biploid B. haploid A. diploid

Section 2 Check The answer is A. The two chromosomes of each pair in a diploid cell are called homologous chromosomes. Each has genes for the same traits. Homologous Chromosome 4 aA Terminal Axial Inflated D Constricted d Tall T Short t

Section 2 Check What is the importance of meiosis in sexual reproduction? Question 2 Meiosis Sperm Cell Egg Cell Haploid gametes (n=23) Fertilization Diploid zygote (2n=46) Mitosis and Development Multicellular diploid adults (2n=46)

Section 2 Check Meiosis is cell division that produces haploid gametes. If meiosis did not occur, each generation would have twice as many chromosomes as the preceding generation. Meiosis Sperm Cell Egg Cell Haploid gametes (n=23) Fertilization Diploid zygote (2n=46) Mitosis and Development Multicellular diploid adults (2n=46)

Section 2 Check How does metaphase I of meiosis differ from metaphase of mitosis? (TX Obj 2; 10A) Question 3

Metaphase I Section 2 Check During metaphase of mitosis, sister chromatids line up on the spindle's equator independent of each other. During metaphase I of meiosis, homologous chromosomes are lined up side by side as tetrads. Sister chromatids Centromere