MEIOSIS & MENDELIAN GENETICS– CHAPTER

Freshman Biology; Semester Two

Chromosomes Def – the genetic information passed

down from parent to offspring Each/every human body cell has 46

chromosomes 44 = non-sex chromosomes (22 pairs) 2 = sex chromosomes X or Y (1 pair)

All body cells (except sex cells) go through mitosis

Mitosis produces cells that are: Clones/genetically identical to parent

BEFORE chromoso

me replication

AFTER chromoso

me replication

Cell Cycle Review

Asexual reproduction that occurs in body (somatic) cells; not sex cells

Interphase G1 S G2

Mitosis Prophase Metaphase Anaphase Telophase

Cytokinesis

End product is 2 cells that are genetically identical to the parent (original) cell

Characteristics of Meiosis

Meiosis occurs in gametes (sex cells) ONLY

TWO divisions with 4 phases each (8 phases total) creating 4 unique cells

Cells start out diploid and end haploid

Initial ComparisonMitosis Meiosis

# of cells produced

2 4

Daughter cells vs.

parent cells

Identical Not identical (Why? crossing over)

# of chromosomes

Same (46 46 in humans)

Cut in ½ (46 23 in humans)

Purpose To produce new cells (growth,

repair old/damaged

cells)

To produce gametes -egg and sperm

(for sexually reproducing organisms)

Diploid vs. Haploid

Diploid (2n) cells have two sets of chromosomes One inherited from mom;

one from dad All somatic (body) cells

are diploid (all cells except sex cells)

Humans’ diploid number is 46, but other species have other numbers.

The chromosomes that are alike from each set are called homologous chromosomes.

Haploid (1n) cells have one set of chromosomes

Gametes (sex cells) are haploid

Humans’ haploid number is 23, but other species have different numbers.

When fertilization occurs, the organism will again be diploid. 23 chromosomes from

male parent + 23 chromosomes from female parent = 46 total (diploid)

Meiosis I: Prepping for Meiosis Interphase I

Cells replicate DNA ONCE, forming duplicate chromosomes

There will only be ONE interphase during the whole process of meiosis.

Meiosis I has four stages: Prophase I Metaphase I Anaphase I Telophase I

Cytokinesis Page 273, Figure 5

Meiosis I: Stage One

Prophase I Each chromosome (2

sister chromatids) pairs with its corresponding homologous chromosome to form a tetrad

Crossing-over occurs Result: the exchange of

alleles between homologous chromosomes and produces new combos of alleles

Meiosis I: Stage Two

Metaphase I Spindle fibers

attach to the chromosomes

Still attached at the centromere

Forms tetrad (2 homologous c’somes lined up at equator)

Meiosis I: Stage Three

Anaphase I Spindle fibers pull

apart homologous chromosomes toward opposite ends of the cell

Sister Chromatids are still connected at the centromere!

Meiosis I: Stage Four & Cytokinesis Telophase I

Nuclear membranes form

Sister Chromatids may not be identical due to crossing over

Cytokinesis The cytoplasm

separates (just like in mitosis)

Cell splits into two haploid (n) cells

Meiosis II

Meiosis II is very similar to mitosis; however there is NO chromosome replication that takes place before it begins (no interphase II)

Both haploid (n) cells created in meiosis I divide Ends with four new haploid (n) cells

Sperm or egg cells Four stages:

Prophase II Metaphase II Anaphase II Telophase II

CytokinesisMEIOSIS II

Meiosis II: Stage One

Prophase II The two haploid (n)

daughter cells that were produced at the end of meiosis I have half the number of chromosomes as the original cell

NO REPLICATION OF CHROMOSOMES happens during meiosis II

Meiosis II: Stage Two

Metaphase II Chromosomes

line up in the center of each cell

Spindle fibers are attached at centromeres of sister chromatids

Like metaphase in mitosis. Why?

Meiosis II: Stage Three

Anaphase II Spindle fibers

shorten Sister chromatids

separate and move apart toward opposite ends of each cell

Meiosis II: Stage Four & Cytokinesis Telophase II

Nuclear envelopes reform in both cells

Cytokinesis The cytoplasm in both cells

splits to form 4 haploid (n) daughter cells with HALF the number of chromosomes as the original cell

So if parent cell has 46 chromosomes, each cell at the end of meiosis II would have 23 chromosomes.

Result: Sexual Reproduction allows for genetic variation

Crossing over makes 4 possibilites!

Spermatogenesis & Oogenesis Spermatogenesis

Formation of sperm

Starts at puberty Forms 4 sperm

during each meiosis

Men will make 5 to 200 million sperm per day!!

Oogenesis Formation of the egg Meiosis starts inside the

womb and continues in some during every cycle after puberty

1 egg and 3 polar bodies are created after every meiosis

The egg must contain a lot of cytoplasm to support the developing embryo after fertilization

Mitosis/Meiosis Video

Genetics

Genetics is the study of traits and how they are passed from one generation to the next.

BrainPop Greatest Discoveries

Gregor Mendel

Austrian monk Performed genetic experiments in the

1850’s and 1860’s Considered the “Father of Genetics” His work was performed with no

knowledge of DNA, cells, or meiosis!

Mendel’s Experiments

Worked with pea plants in the monastery gardens

Followed the inheritance patterns of seven different traits (Ex.: Green seed Vs. Yellow seed) in the plants

Creating the F1 Generation

For each trait: Mendel used a true-breeding plant for each

form of the trait for the parent (P) generation Ex- True-breeding purple flower x true-breeding

white flower Cross-pollinated the plants to produce

offspring Created F1 generation which only displayed

one form of the trait (hybrids or heterozygous) Ex- all F1 plants were purple flowered

Conclusions

Pea plants were passing a chemical message from one generation to the next that was controlling the trait (Ex- flower color) This is a gene (Ex- gene for flower color)

Genes are sections of DNA on chromosomes that code for a trait

Different forms of a trait are called alleles There is a purple and a white allele for flower

color

More Conclusions

Principle of Dominance One allele is dominant over the other Dominant will always be displayed when

present Recessive is only seen when it is the only

allele present

Creating the F2 Generation

For each trait Mendel self-pollinated plants from the F1

generation Ex- F1 purple flower is crossed with itself

Created the F2 generation which displayed both traits in a 3:1 ratio For every 4 flowers, 3 were purple flowered and

one was white flowered

Conclusions

Each pea plant has two copies of every gene Each copy is found on one of the

homologous chromosomes Each individual has three possible types of

combinations Two dominant alleles- homozygous dominant Two recessive alleles- homozygous recessive One of each- heterozygous

More Conclusions

Principle of Segregation The two copies of a gene that an

individual has separate (segregate) from each other during gamete formation (Meiosis)

The copy to be put in the gamete is chosen at random

This happens during Anaphase I when the tetrads separate

Tetrad Separation (Segregation)

Predicting Inheritance Outcomes Probability- rules that predict the

likelihood of an event occurring Punnett squares- tool used in

genetics to figure out the probability of a genetic cross Monohybrid cross- Punnett square

showing the outcome of the inheritance of one trait

Dihybrid cross- Punnett square showing the outcome of the inheritance of two traits

Information About Traits

Physical form of the trait seen is the phenotype (show either dominant or recessive)

Genotype is the alleles that an individual has for a trait (2 alleles/trait) Represented by letters (capital for dominant, lower-

case for recessive) Letter is chosen based on dominant allele Possibilities (using flower color as example)

Homozygous dominant PP Heterozygous Pp Homozygous recessive pp

Heredity

Setting Up a Punnett Square

One parent’s possible gametes go on the top

Other parent’s possible gametes go on the side

Squares are filled in with the column and row header Dominant letter is

written first

Mendel’s Dihybrid Experiment Mendel crossed two plants that were true-

breeding for two traits Ex- True-breeding round and yellow peas

(RRYY) x True-breeding wrinkled and green peas (rryy)

F1 generation phenotype: all round and yellow

F1 generation was self-pollinated to create F2

F2 generation showed all 4 possible phenotype combinations in a 9:3:3:1 ratio

Conclusions

Law of Independent Assortment Each gene segregates on its own The inheritance of one trait does not

influence the inheritance of another; each trait is chosen randomly and independent from each other For example, a pea plant that inherited the

dominant yellow pea color did not automatically inherit the round (dominant) pea shape.

Setting Up a Dihybrid Punnett Square

All possible allele combinations from one parent are placed along the top (4 total) For example- an F1 round and yellow pea plant

(RrYy) could produce RY, Ry, rY, and ry gametes

All possible allele combinations from the other parent are placed along the side (4 total)

Square are filled with the column and row headers (16 squares) Letters from one trait go first, then the other Capital letter for that trait are put in front

Dihybrid Punnett Square

Uses for Punnett Squares

Give all possible outcomes for a cross between two different parents

Predicts expected (not actual) ratios among the offspring