Transcript Body cells
Genes are on chromosome pairs • Inheritance happens through o Sexual reproduction: a cell containing genetic information from the mother and a cell containing genetic information from the father combine into a completely new cell, which becomes the offspring • Genes are on chromosome pairs Processes are coded for by genes: a unit of heredity that occupies a specific location on a chromosome and code for a particular product o Heredity is the passing of genes from parents to offspring o Genes code for the expression of traits o Traits are not inherited: the gene code for the trait is. Many genes can code for one trait Genes are on chromosome pairs • Cells contain pairs of chromosomes • One chromosome of each pair coming from each of two parents • Homolog: chromosomes in a pair o Sites where specific genes are located Homologous Chromosomes There is a gene code for a trait at a specific site on a chromosome EX: plant height may be at site A on a pair of homologs Both homologs have the gene for height at site A, but the genes may be different (tall vs short) EX: flowers color may be at site B on a pair of homologs Both homologs have the gene for color at site B, but the genes may be different (purple vs white) Chapter 4: Patterns of Heredity 4.1 Living things inherit traits in patterns 4.2 Patterns of heredity can be predicted 4.3 DNA is divided during meiosis 4.4 Cells use DNA and RNA to make proteins 4.3 Before you learned: Mitosis produces two genetically identical cells In sexual reproduction, offspring inherit traits from both parents Genetic traits are inherited in predictable patterns 4.3 Now, you will learn: Why meiosis is necessary for sexual reproduction How cells and chromosomes divide during meiosis How meiosis differs from mitosis Meiosis is necessary for sexual reproduction body cells vs reproductive cells • Cells containing genetic information from each the mother and father combine to form a completely new cell: becomes the offspring – The offspring has some genetic material from each parent • Body cells (most human cells) contain 46 chromosomes (23 pairs) – this is a 2n cell, or a diploid cell • n tells you the # of chromosomes in one set • Diploid means two sets of chromosomes: 2 x 23 – Humans: 23 sets of chromosomes, 2 x 23 = 46 total – Fruit fly: 4 sets of chromosomes, 2 x 4 = 8 Meiosis is necessary for sexual reproduction body cells vs reproductive cells • Gamete (reproductive cell): cells that contain half the usual number of chromosomes (46/2 = 23) – one from each pair: this is a 1n cell, or a haploid cell – Only found in the reproductive organs of plants and animals – Just the female gamete: egg cell = 1n – Just the male gamete: sperm cell = 1n – Fertilized egg cell = 2n develops into an offspring – Ex: gamete of human: n cells, 23 unpaired chromosomes – Ex: gamete of fruit fly: n cells, 4 unpaired chromosomes Fertilization Two gametes combine to become a 2n cell that can grow into a new offspring. humans: egg (23 chromosomes) + sperm (23 chromosomes) = fertilized egg cell (46 chromosomes) Body cells (most human cells) contain 46 chromosomes: this is a 2n cell, or a diploid cell Gamete: cells that contain half the usual number of chromosomes: this is a 1n cell, or a haploid cell Just the female gamete - egg cell = 1n Just the male gamete – sperm cell = 1n Fertilized egg cell = 2n develops into an offspring Meiosis • Body cells divide by mitosis – Produces two daughter cells, each containing exact copies of the chromosomes in the parent cell – Each daughter cell is a standard diploid cell: 2n To produce gametes (haploid), need a different kind of division: Meiosis •Produces haploid, 1n, cells •A single cell goes through two divisions: •Meiosis I •Meiosis II •Only in the reproductive tissues of an organism • • Meiosis I BEFORE Meiosis: DNA in each chromosome of the parent cell is copied • a cell ready to divide has twice the usual amount of DNA…therefore, for a cell to have half the amount of DNA, two divisions are necessary Four steps in Meiosis I: 1. Prophase I: Chromosomes pair up with their partner • Two sets of each of the chromosome pairs in the parent cell • Each chromosome pair is attached together in sets of doubled homologs 2. Metaphase I: each set of chromosome pairs lines up along the center of the cell 3. Anaphase I: The two copies of one homolog are pulled apart from the two copies of the other homolog 4. Telophase I and Cytokinesis: A new cell membrane forms at the center of the cell; parent cell divides two daughter cells Meiosis II • In Meiosis I, two daughter cells formed – Chromosomes of these two cells are not copied before Meiosis II – Both these cells divide during Meiosis II 4 daughter cells 5. Prophase II: the two copies of each n chromosome are attached together 6. Metaphase II: The chromosomes line up along each cell’s center 7. Anaphase II: The two attached copies of each chromosome separate and are pulled to opposite poles in each cell 8. Telophase II and Cytokinesis: A new cell membrane forms in the center of each cell, as each cell divides into two 1n daughter cells 4: 1n cells http://highered.mcgrawhill.com/sites/0072495855/student_view0 /chapter28/animation__how_meiosis_wo rks.html Meiosis • One cell in an organism’s reproductive system divides twice to form four 1n cells • In males, these gametes become sperm • In females, at least one of these cells becomes an egg – In some species (humans) only one of four daughter cells becomes and egg • The rest dissolve back into the organism or are never produced Meiosis vs Mitosis • Only cells that are to become gametes go through meiosis • A cell that divides by meiosis undergoes two divisions, but the chromosomes are not copied before the 2nd division…in mitosis, chromosomes are always copied before dividing • Daughter cells produced by meiosis (haploid/1n) contain only ½ the genetic material of the parent cell (one of each chromosome) • Daughter cells produced by mitosis (diploid/2n) contain exactly the same genetic material as the parent (pairs of chromosomes) http://highered.mcgrawhill.com/sites/0072495855/stude nt_view0/chapter28/animation_ _unique_features_of_meiosis.ht ml • try it • Meiosis How Cells Divide