NOTES FOR BIOLOGY 1201

Section 001

Spring 2005

DR. STEVEN POMARICO

Reproduction is an emergent property associated with life. The reproduction of an organism is a consequence of heredity.

---Heredity

-Results from the transmission of genes.

-When individuals share similar genes (e.g., offspring, siblings, etc.) they often resemble each other.

---Variation

-While offspring resemble their parents and siblings there are still some differences.

-Some of these similarities and differences are studied using genetics.

---Genetics

>>>>>>Offspring acquire genes from parents by inheriting chromosomes

---Genes

-specific sequences of nucleotides containing genetic information

-most genes code for proteins

-each chromosome contains hundreds or thousands of genes.

-each gene is in a specific region or locus

---Locus

Sexual Reproduction and the Role of Meiosis - 8.3.1

>>>>>>A Comparison of asexual and sexual reproduction

---Asexual reproduction

---Sexual reproduction

ASEXUAL REPRODUCTION	SEXUAL REPRODUCTION
Only one parent	Two parents give rise to offspring
Single parent passes on all its genes	Each parent passes on half its genes
Results in a genetically identical offspring or clone	Offspring have a unique combination of genes inherited from both parents.
Rare genetic differences in offspring are the result of DNA changes, or mutations	Greater genetic variation with offspring being genetically different from their parents and siblings

>>>>>Bacteria reproduce by binary fission

---Binary fission

Features:

-Chromosome replicates

-Each copy is attached to the plasma membrane

-Membrane elongates between the two copies until the cell is twice its normal size

-Plasma membrane pinches inward

-A cell wall forms between the two daughter cells.

---Clone

---Mutation

>>>>>Fertilization and meiosis alternate in sexual life cycles.

The human life cycle follows the basic pattern shared by all sexually reproducing organisms.

---Life cycle

-Each species has a characteristic number of chromosomes in the somatic cells

---Somatic cells

-somatic cells usually are diploid

---Diploid

Meiosis and fertilization result in alterations between the haploid and diploid condition.

---Meiosis

-Gametes (Reproductive cells) are haploid and contain half the number of chromosomes of somatic cells.

---Gametes

---Haploid

-Human somatic cells contain 46 chromosomes that are distinguished by size,

shape and banding pattern

-The full compliment of chromosomes can be photomicrographed and the

homologous chromosome pairs can be arranged to

produce a karyotype

---Karyotype

---Homologous chromosomes (homologues)

-the homologous autosomes carry the same genetic loci.

---Autosomes

---Sex chromosome

-Human females have a homologous pair of X chromosomes

-Human males have one X and one Y chromosome.

-Humans have 22 pairs of autosomes and 1 pair of sex chromosomes (the diploid number of 46)

-Human gametes have 22 autosomal chromosomes and one sex chromosome

(either an X or a Y)

-Sperm cells and ova are gametes and have the haploid number of chromosomes.

-When two haploid gametes unite during fertilization the diploid number is restored.

---Fertilization (syngamy)

---Zygote

-restoration of diploid condition

-contains the maternal and paternal haploid chromosomes

-beginning with the zygote, mitosis is the method of cell division until the production

of gametes

Homologous Chromosomes: Thanks, Mom and Dad! - 8.3.2

>>>>>>Meiosis reduces chromosome number from diploid to haploid

-Chromosomes replication preceded meiosis (just like mitosis)

-replication is followed by two consecutive cell divisions.

Meiosis I and Meiosis II

-divisions produce 4 daughter cells instead of 2

-these daughter cells have half the number of chromosomes.

STAGES OF MEIOSIS

Unique events occur during meiosis I

1 - Interphase I

-Chromosome replication

Meiosis: Prophase I - 8.3.3

2 - Meiosis I - First division

a) Prophase I (90% of meiosis)

-Chromosomes condense

-Homologous chromosomes undergo synapsis (pairing up) to form a tetrad

-Sister chromatids are linked at centromeres, nonsister chromatids are linked at chiasmata

-Genetic crossing over occurs

-centrioles move apart

-nucleoli disappear

-nuclear envelope disappears

-the spindle forms but its different than in mitosis

Meiosis kinetochores attached to same centrosome

Mitosis kinetochores attached to different centrosomes

---Synapsis

---Tetrad

---Chiasmata

Disjunction and Meiosis II - 8.3.4

b) Metaphase I

-tetrads align on metaphase plate

-kinetochores of sister chromatids face same pole

-centromeres of homologues face opposite poles.

c) Anaphase I

-spindle microtubules interact with kinetochore fibers

-homologous chromosomes move toward opposite poles

-sister chromatids remain attached (this is different than mitosis)

d) Telophase I and cytokinesis

-chromosomes arrive at poles

-each pole has haploid set, but each chromosome has sister chromatids

-nuclear envelope may or may not reform

-cell divides

-NO FURTHER REPLICATION OF CHROMOSOMES.

3 - Meiosis II - Second division (very similar to mitosis)

a) Prophase II

-if nuclear membrane reformed at telophase I, then the nuclear membrane

disperses

b) Metaphase II

-Chromosomes align at metaphase plate as in mitosis

c) Anaphase II

-Sister chromatids separate and move toward opposite poles

d) Telophase II and cytokinesis

-nuclei form at opposite poles

-cell division occurs

-four haploid daughter cells are the final result

Mitosis vs. Meiosis - 8.3.5

COMPARISON OF MEIOSIS I WITH MITOSIS

	MEIOSIS I	MITOSIS
Prophase	-Synapsis creates tetrads -Chiasmata form as a result of crossing over	-No synapsis therefore no tetrads and no chiasmata
Metaphase	-Tetrads align on the metaphase plate	-Individual chromosomes align on the metaphase plate
Anaphase	-separates the pairs -centromere does not divide -sister chromatids to the same pole	-separates the chromatids -centromere divides -sister chromatids to opposite poles

Meiosis II is virtually identical in mechanism to mitosis

Mitosis versus meiosis

-Mitosis conserves the number of chromosomes

-Meiosis reduces the chromosome number by 1/2

-Meiosis creates genetic variation

-Meiosis is 2 successive nuclear divisions

>>>>>>Sexual life cycles promote genetic variation among offspring

Three sources of genetic variation that result from sexual reproduction:

1. Independent assortment

2. Crossing over

3. Random fusion of gametes

Independent Assortment - 8.4.1

1. Independent assortment of chromosomes

---Independent assortment

-orientation of homologous pairs is random

=> 50% chance daughter cell will get a certain chromosome of the

homologous pair

-there are 2^C possible combinations, where C is the number of

chromosomes/gamete

=> in humans 2²³ = about 8.4 million possible combinations

2. Crossing over (a.k.a genetic recombination)

---Crossing over

-made possible by synapsis of homologous pairs

-a single crossover may influence many genes

-many crossovers per homologous pair may occur

In humans 2-3 per chromosome pair is average.

-results in a chromosome with genes from both parents

3. Random fertilization

-there are 2^C+C zygotes that can results from an egg and a sperm that each have 2^C possible chromosomal combinations

(remember that C is the number of chromosomes/gamete)

=> so for humans there are 2⁴⁶ possible zygotes from one fertilization.

About 70 trillion possible combinations

>>>>>>Evolutionary adaptation depends on a population’s genetic variation

There are two basic sources of genetic variation:

1. Sexual reproduction

-very common

2. Mutations

-very rare