Download Homologous chromosomes

Welcome Back!
Wednesday, November 30th.
Bellwork: You have two types of cells in your body: somatic cells, and germ cells.
Somatic cells­ are your body cells. They undergo mitosis when they need to divide.
Germ cells­ are your reproductive cells also referred to as gametes.
­male gametes are called sperm cells
­female gametes are called egg cells.
Sexual reproduction­ involves the fusion of two gametes that results in offspring that are a genetic mixture of both parents.
1
Gametogenesis ­ the production of gametes.
Sperm cells (male gamete)
­smaller than female gamete
­contribution to the embryo is DNA
­must be able to swim, modifications are made to the cell for this to happen.
Egg cells (female gamete)
­gives DNA, contributes organelles, molecular building blocks, and other materials for a successful embryo to develop
Fertilization­ the actual fusion of two gametes. Occurs when the sperm fertilizes the egg.
­A zygote is formed that will have the number of chromosomes in a diploid cell.
­Develops by mitosis into a multicellular organism.
Diploid­ a cell with 2 copies of each chromosome, (2n) number of chromosomes, body cells are diploid.
­one from female parent
­one from male parent
Haploid­ gametes that contain only
one of each kind of chromosome (n) number of chromosomes.
2
Each organism has a specific amount of chromosomes in their cells. The number of chromosomes is not related to the complexity of an organism.
Examples: Adder's Tongue Fern 1260(2n) 630(n)
Fruit Fly 8(2n) 4(n)
Humans 46(2n) 23(n)
Autosomes­ chromosomes that contain genes for characteristics not directly related to the sex of an organism.
­Chromosome pairs 1­22 are autosomes
Sex Chromosomes­ ones that control the development of sexual characteristics. ­Very different from one another. ­X and Y are standard
­XX indicates female, XY indicates male
­23 chromosomes All other human cells have 46 chromosomes.
­If cells only reproduced through mitosis then when an egg cell and a sperm cell fuse, the new cell would have 92 chromosomes!
Meiosis is the type of cell division that produces gametes with half the number of chromosomes as a parent's body cells.
­Two separate divisions occur (Meiosis I and II)
­Begins with 1 diploid (2n) cell, and results in 4 haploid (n) cells.
3
Homologous chromosomes ­ two chromosomes (one from mom and one from dad) that have the same length and same general appearance.
­contain the same genes in the same order.
Can you identify Homologous Chromosomes?
The diagram below shows chromosomes 1 with four different genes present. These genes are represented by the letters F, g, h, and J. Possible homologous chromosomes of chromosome 1 are labeled 2­
5. Examine the five chromosomes and the genes they contain to determine which of chromosomes 2­5 are homologous with chromosome 1.
4
1. Could chromosome 2 be homologous with chromosome 1? Explain.
yes, because the genes are in the
same location and position
2. Could chromosomes 4 be homologous with chromosome 1? Explain.
no, because the last gene is not the
same
3. Could chromsome 4 be homologous with chromosome 1? Explain.
no, because they have completely
different genes
4. Could chromosome 5 be homologous with chromosome 1? Explain
yes, same as #1
Review Mitosis:
5
Prophase I
­DNA coils into chromosomes
­Spindle fibers form
­Nuclear membrane breaks down
­Centrioles move to opposite sides of the cell.
­Crossing over occurs here.
­Homologous chromosomes line up gene to gene in a four­part structure called a tetrad. ­Tetrad is made up of 2 homologous chromosomes, with 2 sister chromatids.
­Sex chromosomes also pair up with one another
6
Crossing Over­ The exchange of chromosome segments between homologous chromosomes during prophase I of meiosis.
­Exchange of genetic material creates more genetic diversity.
­Results in a new combination of alleles, called genetic recombination.
­Can occur at any location on a chromosome and in several locations at the same time.
Draw a detailed picture using colored pencils to show crossing over occuring. pg 190
In humans approximately 2­3 crossovers for each pair of homologous chromosomes.
Linked Genes or (genetic linkage)
­genes that are close together on the same chromosome are more likely to be inherited together (linked together)
­the farther apart two genes are located, the more likely they are to be separated when crossing over.
7
Metaphase I
­Centromere attaches to spindle fibers
­Homologous chromosomes are lined up side by side in the middle as tetrads. **Remember: In mitosis they were a single file line**
8
Anaphase I:
­Homologous chromosomes separate and move to opposite ends of the cell.
­Centromere does NOT split, sister chromatids stay together.
­Ensures that each new cell will only have 1 chromosome from each homologous pair.
Telophase I:
­Nuclear membrane forms again.
­Spindle fibers break down
­Cytoplasm divides
­We end up with 2 cells with a unique combination of 23 duplicated chromosomes coming from both parents.
Note: Still contains doubled chromosomes. Therefore, a second cell division is necessary to separate the doubled chromsomes
9
Meiosis II: Exactly like mitosis. It divides sister chromatids resulting in undoubled chromosomes. The process described below relates to both cells.
Note: DNA is NOT copied again between the two stages.
Directions: Use pages 174­175 to describe what occurs during each phase and complete a detailed picture of each stage.
Prophase II
­ The nuclear membrane breaks down
­ Centrioles move to opposite sides of the cell.
­Spindle fibers form
10
Metaphase II
­Spindle fibers align the 23 chromosomes at the cell equator (in the middle).
Anaphase II
­Sister chromatids are pulled apart from each other and move to opposite sides of the cell.
Telophase II:
­Nuclear membranes form around each set of chromosomes at opposite ends of the cell. ­Spindle fibers break apart.
­cell undergoes cytokinesis
END RESULT OF MEIOSIS
­Four haploid cells with a combination of chromosomes from mom and dad.
­Haploid cells become gametes transferring the genes they contain to the offspring.
11
12
13