Genetics Chapters 1-2

Transmission Genetics

• The basic principles of heredity 

• Focus is on the individual organism

Molecular Genetics 

• The chemical nature of the gene 

• How genetic information is encoded, replicated, and expressed 

• Focus is the gene and its structure, organization, and function

Population genetics 

• The genetic composition of populations (group of organisms of the same species) 

• How genetic composition changes geographically and with the passage of time 

• Focus is the group of genes found in a population 

Model Genetic Organisms

 Organisms with characteristics that make them useful for genetic analysis

Characteristics of Model Organisms

• Short generation time 

• Production of numerous progeny  

• Ability to carry out controlled genetic crosses 

• Ability to be reared in a laboratory environment 

• Availability of numerous genetic variants 

• Accumulated body of knowledge about their genetic systems 

Why can’t humans be model organisms?

• Humans take 20-30 years for one full generation, making experiments take decades to get results.

• Humans have very few offspring per reproductive cycle (usually one at a time), so sample sizes would be too small for a reliable genetic study.

• Controlled genetic crosses: We cannot ethically choose who breeds with whom. Controlled mating is impossible in humans.

• Reared in a laboratory environment: Humans cannot be raised or kept in lab conditions for experimentation—this is unethical and impractical.

• Availability of numerous genetic variants: While humans do have genetic diversity, we cannot create or selectively breed variants for study the way we can with model organisms.

• Accumulated body of genetic knowledge: Human genetics is well-studied, but the knowledge is partly based on model organisms. We needed simpler organisms first to understand fundamental genetics.

Don’t follow the common characteristics

Name the 6 Common Model Organisms

• Drosophila melanogaster (fruit fly) 

• Escherichia coli (Bacterium) 

• Caenorhabditis elegans (Nematode) 

• Arabidopsis thaliana (Thale-cress plant) 

• Mus musculus (House Mouse) 

• Saccharomyces cerevisiae (Baker’s yeast) 

Pangenesis

Theory of Heredity

• Each part of the body contains genetic information for that particular part 

• Specific particles (gemmules) carry information from parts of the body to the reproductive organs 

• Then passed to the embryo at the moment of conception 

Inheritance of acquired characteristics

Theory of Heredity

• Traits acquired in a person’s lifetime 

• Become incorporated into that person’s hereditary information 

• Are passed on to offspring

Preformationism

Theory of Heredity/Embryology

• Inside the egg or sperm exists a full formed miniature adult (a homunculus) 

• Simply enlarges during development 

Blending Inheritance

Theory of Heredity

Traits of offspring are a blend, or mixture, of parental traits 

What are the defining characteristics of Prokaryotes?

• Unicellular

• No membrane-bound organelles

• Made up of eubacteria and archaea

• DNA isn’t highly ordered or in a packed arrangement like Eukaryotes 

What are the defining characteristics of Eukaryotes?

• Both unicellular and multicellular

• Membrane-bound organelles 

• Genetic material is surrounded in a nuclear envelope to form a nucleus 

• DNA is closely associated with histones to form tightly packed chromosomes 

Parts of a Prokaryote

Cell Wall, Plasma membrane, cytoplasm, ribosomes, nucleioid, flagella

Parts of a Eukaryote

Nucleus, plasma membrane, cytoplasm, ribosomes, mitochondria, endoplasmic reticulum, Golgi apparatus, chloroplast (plant), cell wall (plant), vacuole (plant)

Nucleus - Prokaryote vs Eukaryote

• Absent in Prokaryotes (Nucleioid region instead)

• Present in Eukaryotes (Genetic material is surrounded in a nuclear envelope to form a nucleus)

Cell Diameter Range - Prokaryotes vs Eukaryotes

• For prokaryotes, relatively small, from 1-10 μm

• For eukaryotes, relatively large, from 10-100 μm

Genome structure in Prokaryotes vs Eukaryotes

• Prokaryotes, one circular DNA molecule

• Eukaryotes, multiple linear DNA molecules

DNA Structure, Arrangement, and Amount in Prokaryotes vs Eukaryotes

• Prokaryotic, isn’t highly ordered or packed, relatively small amount of DNA

• Eukaryotic, closely associated with histones to form tightly packed chromosomes, relatively large amount of DNA

Viruses

• Neither prokaryotic nor Eukaryotic

• Outer protein coat surrounding nucleic acid 

Prokaryotic Cell Reproduction

Occurs through binary fission, involving the simple division and separation of the replicated circular chromosome, high rate of replication

Structural Maintenance of Chromosomes

Proteins that encircle DNA and compact and organize into chromosomes 

Eukaryotic Cell Reproduction

By mitosis for growth and repair, or by meiosis for sexual reproduction

Do all eukaryotic organisms have the same amount of chromosomes?

No. And there is no relation between complexity of organism and amount of chromosomes they have.

Homologous Chromosomes

pairs of chromosomes that have the same length, centromere position, and genes in the same order, with one chromosome inherited from each parent

Diploid and Haploid

• Diploid: two sets of genetic information/chromosomes

• Haploid: one set of genetic information/chromosomes

Centromere

middle point of chromosome, where sister chromatids are attached, contains a kinetochore (Attachment point for spindle microtubules)

What would be the result if a chromosome didn’t have a kinetochore or something was wrong with it?

Nondisjunction (failure of correct separation of homologous chromosomes)

• Spindle microtubules wouldn’t attach to the chromosome

• The chromosome wouldn’t be drawn into a newly formed nucleus

• The resulting daughter cells would be missing a chromosome (have the incorrect amount)

Telomere

Tips of a linear chromosome (only in eukaryotes)

Origin of Replication

A specific DNA sequence where replication or synthesis starts. One in prokaryotes, a lot in eukaryotes

Chromosome Structure

Types of Chromosome Structures

• Metacentric (Middle

• Submetacentric (Almost middle)

• Telocentric (end)

• Afrocentric (almost end)

Cell Cycle

The life cycle of the cell. Consists of:

Interphase (cell growth, DNA synthesis before division): G1, S, G2

M Phase (cell division): Prophase, Prometaphase, Metaphase, Anaphase, and Telophase

G0

A stable, nondividing period of variable length that some cells enter temporarily and others enter permanently during G1

Interphase

An extended period between cell divisions, including cell growth, DNA synthesis. It consists of G1, S, and G2 phases

G1

First phase of growth, where cell grows/increases in size, proteins necessary for DNA synthesis and cell division are synthesized; G1/S Checkpoint

G1/S Checkpoint

A regulated decision point in the cell cycle. Access whether cell is ready to undergo DNA synthesis

S Phase

DNA synthesis, the chromosome is replicated, resulting in two identical sister chromatids joined together

G2

Preparation for division, cell growths, and protein synthesis necessary for cell division; G2/M Checkpoint

G2/M Checkpoint

A regulated decision point in the cell cycle. only passed if DNA is completely replicated and undamaged

M Phase

Mitotic phase: includes mitosis and cytokinesis

Includes Prophase, Prometaphase, Metaphase, Anaphase, and Telophase

Prophase

Chromosomes condense and mitotic spindle forms

Prometaphase

Nuclear envelope disintegrates, and spindle microtubules anchor to kinetochores

Metaphase

Chromosomes align on the metaphase plate; spindle assembly checkpoint

Spindle-Assembly Checkpoint

Ensures that each chromosome is aligned on the metaphase plate and attached to spindle microtubules from opposite poles before anaphase

Anaphase

Early anaphase: sister chromatids separate, becoming individual chromosomes that migrate toward the poles.

Late anaphase: contractile ring forms (animal cells); cell plate begins to form (plant cells)

Telophase 

Chromosomes arrive at spindle poles, the nuclear envelope reforms, and condensed chromosomes relax. Cleavage furrow forms and deepens (animal cells); cell plate becomes obvious and grows (plant cells)

Cytokinesis

Cytoplasm divides; animal cells split completely; cell plate/wall completes separating cells

Mitosis and Consequences of Cell Cycle

Process of cell division for growth and repair; separation of sister chromatids

Consequences:

• Two genetically identical cells

• Each cell with approximately have the cytoplasm as the parent

Cytokinesis

Separation of cytoplasm

Meiosis and Consequences

Haploid gamete production; consists of meiosis I and meiosis II

Consequences:

• Four haploid cells produced 

• Newly formed cells from meiosis are genetically different from one another and from the parental cell 

What state precedes meiosis I but not meiosis II?

Interphase (same as before mitosis)

Fertilization

Fusion of Haploid Gametes

Meiosis I

separation of homologous chromosome pairs, and reduction of the chromosome number by half; reduction division (chromosomes number halves)

Prophase I, Metaphase I, Anaphase I, Telophase I, Cytokinesis, Interkinesis

Prophase I

Chromsomes condense, homologous chromosomes synapse, crossing over takes place, the nuclear envelope breaks down, and the mitotic spindle forms

Synapsis

The close pairing of homologous chromosomes during Prophase I

Tetrad

The closely associated structure composed of four-sister chromatids of two homologous chromosomes during Prophase I

Crossing Over

The exchange of genetic information/chromosome segments between the sister chromatid of one chromosome and the sister chromatid of the other synapsed chromosome. This is the first mechanism of generating genetic variation in newly formed gametes. 

Metaphase I

Random alignment of homologous pairs of chromosomes along the metaphase plate 

Anaphase I

Separation of homologous chromosome pairs and the random distribution of chromosomes into two newly divided cells. This is the second mechanism of generating genetic variation in the newly formed gametes.

Telophase I

chromosomes arrive at the spindle poles 

Cytokinesis (Meiosis I)

cytoplasm is divided to produce two cells, each having half the original number of chromosomes 

Interkinesis

In some type of cells, the spindle breaks down, chromosomes relax, and a nuclear envelope reforms, but no DNA synthesis takes place

Meiosis II

separation of sister chromatids, also known as equational division (chromosomes number remains equal)

Prophase II

Chromosomes condense, the spindle forms, and the nuclear envelope disintegrates 

• Only in cells in which the spindle has broken down after interkinesis, chromosomes have relaxed, and the nuclear envelope has reformed in telophase I. Other types of cells proceed directly to metaphase II after cytokinesis 

Metaphase II

Individual chromosomes line up on the metaphase plate

Anaphase II

sister chromatids separate and move as individual chromosomes toward the spindle poles 

Telophase II

Chromosomes arrive at the spindle poles; the spindle breaks down and nuclear envelope re-envelopes 

Cohesin and Shugohisin

• Cohesin: protein that holds the chromatids together and is key to the behavior of chromosomes in mitosis and meiosis

• Shugohisin: protein that protects cohesin 

How does shugohisin affect sister chromatids in meiosis I and meiosis II?

In meiosis, cohesin is protected at the centromeres by shugohisin during anaphase I, so homologous chromosomes, but not sister chromatids, separate in meiosis I. The breakdown of shugohisin and thus centromeric cohesin allows sister chromatids to separate in anaphase II of meiosis.

Compare Mitosis, meiosis I, and meiosis I. 

Cell division, reduction in chromosome #, genetic variation produced, crossing over, random distribution of maternal and paternal chromosomes, metaphase, anaphase

Meiosis in animals

Spermatogenesis: male gamete production

Oogenesis: female gamete production

Spermatogenesis

The process of male gamete production

Spermatogonium

A diploid cell located in the testis that is capable of undergoing meiosis to produce sperm

Primary spermatocyte

A spermatogonium that has entered Prophase I, which is still diploid

Secondary spermatocyte

The haploid product of Meiosis I in male animals.

Spermatids

The immediate, haploid product of Meiosis II from secondary spermatocytes; these mature to become sperm.

Oogenesis

The process of female gamete production

Oogonium

A diploid cell in the ovary capable of undergoing meiosis to produce an egg cell

Primary oocyte

An oogonium that has entered Prophase I

Secondary Oocyte

One of the products of Meiosis I in oogenesis; it is haploid and receives most of the cytoplasm; what fuses with sperm during fertilization

First Polar Body

One of the products of Meiosis I in oogenesis; it is the smaller cell, which is haploid and contains half the chromosomes but only a small part of the cytoplasm

Ovum

The mature female gamete; it is the larger cell that acquires most of the cytoplasm during oogenesis

Second Polar Body

One of the products of Meiosis II in oogenesis; it contains a set of chromosomes but little of the cytoplasm.

Meiosis in plants

one of the alternating life stages of plants

Microsporocyte

Diploid reproductive cell in the stamen of a plant; undergoes meiosis to produce four haploid microspores.

Microspore

Male haploid product of meiosis in plants.

Megasporocyte

Diploid reproductive cell in the ovary of a plant that undergoes meiosis to produce haploid megaspores.

Megaspores

Female haploid product of meiosis in plants.