CELL BIOLOGY - CLUTCH CH MEIOSIS AND SEXUAL REPRODUCTION.

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2 CONCEPT: BASICS OF MEIOTIC GENETICS Sexual reproduction involves mixing DNA from individuals to produce genetically distinct offspring Beneficial because it allows for genetic diversity - Reshuffling of genes provides competitive advantages to offspring - Selects out mutated genes Asexual reproduction produces offspring to the patents and siblings - Offspring are not as adaptive EXAMPLE: Asexual vs. Sexual Reproduction Asexual Reproduction Sexual Reproduction Sexual reproduction involves dividing and segregating out genetic information from parents Germ cells (sex cells) each need to only contain on set of chromosomes - Egg or Sperm - Somatic cells (cells for rest of body) need both sets of chromosomes Haploid cells are cells with ½ the genetic information (one copy of every gene) - Two haploid cells can form a diploid cell - Human fertilization: Haploid egg and sperm form a diploid zygote Page 2

3 Diploid cells are cells with the full amount of genetic information (two copies of every gene) - Homologous chromosomes are chromosome pairs with the same genes One exception are the sex chromosomes (X and Y) which differ in genetic makeup EXAMPLE: Haploid and Diploid Cells Haploid (n) Fertilization Meiosis Diploid (2n) Page 3

4 PRACTICE: 1. True or False: Germ cells are diploid. a. True b. False 2. Fill in the blanks. Haploid cells have copy of genetic material, while diploid cells have copies of genetic material. a. 1,2 b. 2,1 c. 1,3 d. 3,1 Page 4

5 CONCEPT: MENDEL AND THE PRINCIPLES OF INHERITANCE Alleles Alleles are of a human gene Alleles can be classified as homozygous or heterozygous - Homozygous alleles are two identical variants - Heterozygous alleles are two different variants Alleles can be classified as dominant or recessive - Dominant alleles will always be phenotypically (physical appearance) expressed if they re present - Recessive alleles will only be phenotypically expressed if a dominant allele is absent EXAMPLE: Dominant Heterozygous Recessive Homozygous Rr rr Mendel studied allele and gene inheritance by mating plants and observing their offspring P1 Generation is the parental plants F1 Generation is the offspring created by the parental plants F2 Generation is the offspring created from the F1 generation - Backcrossing is when the F1 offspring are crossed (mated) with the P1 generation (their parents) Page 5

6 EXAMPLE: Page 6

7 Mendel s Laws Mendel studies of pea plants discovered two important laws regarding Law of segregation states that the two alleles for a single gene separate during gamete formation - Upon fertilization two alleles unite at random (so you get one random allele from each parent) Law of independent assortment states that alleles of different genes are passed independently of each other - Ex: Two pea genes (color/shape), aren t inherited together - Monohybrid cross looks at inheritance of one trait - Dihybrid cross looks at inheritance of two or more unrelated traits EXAMPLE: Dihybrid cross looking at cat color and cat tail length Page 7

8 Law of independent assortment is a bit more Genes on the same chromosome can also segregate independently - Crossing over can result in genes on the same chromosome being passed independently of each other Crossing over can only independently pass genes that are far away - Genes that are close to each other on a chromosome are inherited together Genetic linkage measures how frequently genes are co-inherited to determine their distance on a chromosome - Linkage group is a group of genes on the same chromosome that are inherited together EXAMPLE: Linkage Group Less likely to be co-inherited More likely to be co-inherited Page 8

9 PRACTICE: 1. Which of the following of Mendel s laws states that two alleles for a single gene separate during gamete formation? a. Law of independent assortment b. Law of segregation 2. Which of the following generations is created from mating the parental generation? a. P b. F1 c. F2 d. F3 Page 9

10 3. True or False: Genes closer together on a chromosome are more likely to be inherited together? a. True b. False Page 10

11 CONCEPT: MEIOSIS Overview Meiosis is the process of creating germ cells (sex cells) that are used to Meiosis I involves replicating the DNA and creating two cells containing a single set of chromosomes Meiosis II involves dividing the two cells into four cells with a single set of sister chromatids EXAMPLE: Overview of meiosis Meiotic DNA duplication results in a lot of different chromosome Sister chromatids are two copies (one pair) of each duplicated chromosome - Separate during Meiosis II Homologous chromosomes are two copies (maternal and paternal) of both sister chromatids - Bivalent is when the four sister chromatids stick together - Separate during Meiosis I Haploid(n) cells have only ½ the chromosomes of a diploid(2n) cell which have two copies of a chromosome Page 11

12 EXAMPLE: Homologous Chromosomes Replication Sister Chromatids Meiosis 1 Diploid Meiosis 2 Haploid Page 12

13 Genetic Variation In meiosis, DNA is reorganized and to produce genetically distinct offspring Homologous recombination exchanges similar DNA sequences between chromosomes - DNA-repair uses identical DNA sequence from sister chromatid to repair DNA - Meiosis uses non-identical DNA sequences from non-sister chromatids in the bivalent Crossing over is when maternal and paternal homologs physically swap chromosomal segments - Chiasma/Chaismata hold bivalents together where cross over events occur - Chiasmata are extremely important in keeping the bivalents together during meiosis Reassortment is the random division of chromosomes into the cells - As chromosomes line up in metaphase, they randomly assort to one side or the other and go into that cell - Nondisjunction occurs when homologs to separate - Causes aneuoploidy which are eggs with wrong number of chromosomes (down syndrome) EXAMPLE: Chiasmata during crossing over Page 13

14 Meiosis I Steps Meiosis I uses 4 steps to go through the first round of division 1. Prophase I is a long, complex phase with four stages that occurs after DNA replication a. Leptotene Condensation of chromatin fibers b. Zygotene More condensation and homologous chromosomes are paired (form bivalents) c. Pachytene Crossing over occurs d. Diplotene Pausing phase where chromosomes decondense a bit, but are still attached by chaisamata e. Diakinesis Chromosomes recondense, nuclear envelop break down, spindle forms Page 14

15 2. Metaphase I is when bivalents align at spindle equator - Chaismata keep homologous chromosomes together 3. Anaphase I is when homologous chromosomes move to opposite poles 4. Telophase I and Cytokinesis is when two cells are produced - These cells contain a single set of randomly sorted chromosomes (two sister chromatids) - These cells are haploid (haploid set of chromosomes) Page 15

16 Meiosis II uses 4 steps to go through the second round of division 1. Prophase II is a short, almost nonexistent phase occurs immediately after cytokinesis (NO DNA replication) 2. Metaphase II is when the chromosomes line up at equator 3. Anaphase II is when each sister chromatid is pulled towards opposite poles 4. Telophase II and Cytokinesis is when two cells are produced - These cells contain a set of sister chromatids - These cells are referred to as haploid (n) Page 16

17 Mitosis and Animal Life Stages Animals are classified into three based on when in their life cycle they perform meiosis 1. Gametic/terminal meiosis are organisms that use meiosis to produce haploid gametes (sex cells) - Most living organisms (including humans) - Meiosis is completed after fertilization (which forms a diploid cell) 2. Zygotic/Initial meiosis are organisms that use meiosis after fertilization - Uses meiosis to produce haploid spores - Diploid cells are only used as gametes 3. Sporic/Intermediate meiosis are organisms where meiosis occurrence has nothing to do with gamete formation - Sporophyte are diploid zygotes that undergo mitosis - Sporogenesis occurs through meiosis which produces gametophytes - Gamteophyes are used to produce gametes through mitosis EXAMPLE: Example of sporic/intermediate meiosis Page 17

18 PRACTICE: 1. In which of the following phases does crossing over occur? a. Metaphase I b. Prophase I c. Anaphase II d. Prophase II 2. In which of the following steps of prophase I does crossing over occur? a. Leptoene b. Zygotene c. Pachytene d. Diplotene e. Diakinesis Page 18

19 3. True or False: The cells produced from meiosis I and meiosis II are both haploid? a. True b. False 4. Fill in the blanks. In anaphase I the separate, while in anaphase II the separate. a. Sister chromatids, homologous chromosomes b. Homologous chromosomes, sister chromatids c. Homologous chromosomes, Homologous chromosomes d. sister chromatids, sister chromatids Page 19

20 5. Which of the following life cycles uses meiosis after fertilization? a. Gametic meiosis b. Zygotic meiosis c. Sporic meiosis Page 20