CONTINUITY OF LIFE

DNA Structure
DNA = deoxyribonucleic acid,
· a large molecule found within the cell nucleus,
· stores information for directing protein synthesis
· made of nucleotides

Nucleotides consist of: phosphate, deoxyribose (sugar), nitrogen-base. There are 4 possible bases in DNA nucleotides: adenine (A), thymine (T), guanine (G), and cytosine (C)
Nucleotides are linked sugar to phosphate by covalent bonds to form long chains. à 2 chains are twisted around each other like a spiral staircase (double helix). The two strands are held together by weak hydrogen bonds between nitrogen-bases. Only certain base pairings are possible to form the steps of the staircase.
A pairs with T, G pairs with C
The two strands are said to complement each other.
/ DNA Replication = the process of copying DNA
1)  DNA strands “unzip” and separate from one another
2)  Each strand serves as a template (pattern) for a new strand to form.
3)  The base-pair rule applies.


Each new DNA molecule consists of one original strand and one newly formed strand.

Goal 3Page 9

Protein Synthesis

· Proteins are made in the cytoplasm, but DNA never leaves the nucleus.

· RNA is used to carry DNA instructions to the ribosomes for proteins synthesis

RNA is similar to DNA, but has several differences

DNA / RNA
Sugar / Deoxyribose / Ribose
Bases / A, G, C, T / A, G, C, U(racil)
Strands / One / two

There are three main types of RNA:

RNA type / Function
Messenger RNA (mRNA) / Carries “recipe” for protein to ribosomes
Transfer RNA (tRNA) / Temporarily binds to specific amino acids and carries them to ribosomes for protein synthesis
Ribosomal RNA (rRNA) / Important component of ribosomes

Protein synthesis occurs in two phases: Transcription & Translation

Transcription = a segment of DNA (gene) is copied into a complementary strand of RNA.

  1. The two strands of DNA separate.
  2. Complementary RNA nucleotides are paired with exposed DNA bases
  3. A strand of RNA is made.

The instructions in mRNA are arranged as sequences of three nucleotides called codons. Each codon codes for either a specific amino acid or marks the start and end of a protein.

There are 64 different codons. (See chart at right)

To use the chart, find each letter of the codon along the designated side. For example, consider the codon CAG. Find the “C” block under “First Base” along the left side of the chart. That narrows your choices to the second row. Next, find the “A” under “Second Base” at the top of the chart. This further limits your possible choices to the second row, third block. Finally, identify where the “G” falls along “Third Base” on the right side of the chart. This identifies the line within the block. The codon CAG codes for the amino acid glutamine.

Once transcribed, mRNA moves into the cytoplasm.

Translation = information on mRNA is decoded and used to arrange amino acids.

tRNA (IV) brings amino acids (V) to the mRNA (II) at the ribosome. Amino acids are linked by peptide bonds to form polypeptides (III) which are folded into proteins.

Each tRNA has a three base sequence at one end that is complementary to one of the mRNA codons. This sequence is called the anticodon (I).

Gene regulation—All of an organism’s cells have the same DNA but differ based on the expression of genes.

· In multicellular organisms, cell differentiate (become specialized) during development.

· Cells respond to their environment by producing different types and amounts of protein.


Asexual and Sexual Reproduction

All reproduction depends on cell reproduction.

Asexual reproduction = production of offspring by a single parent. Such offspring are genetically identical to the parent.

· Prokaryotes reproduce by a process of binary fission (the cell splits into two parts).

· Eukaryotic cell cycle has 5 main stages.

  1. The cell grows and carries out routine functions.
  2. The cell’s chromosomes are copied.
  3. The cell prepares for the nucleus to divide.
  4. The nucleus divides into two in a process called mitosis. Mitosis ensures that each new cell receives a copy of each chromosome
  5. Division of the cytoplasm between the daughter nuclei.

In eukaryotic organisms mitosis is necessary for: asexual reproduction, growth, and repair.

Sexual Reproduction = production of offspring by the joining of special reproductive cells (gametes) contributed by two parents. Requires a reduction in chromosome number from diploid [2n] (two sets of chromosomes) to haploid [n] (one set of chromosomes).

Gametes are produced by meiosis a form of cell division that cuts chromosome number in half. During meiosis, chromosomes replicate once, but the cell divides twice.

When sex cells combine during fertilization the resulting cell (zygote) has the original diploid number of chromosomes—half from each parent. The offspring of sexual reproduction have a mix of genetic material from both parents. Offspring are not identical to either parent.

Meiosis introduces genetic diversity in a variety of ways: crossing-over, random assortment of chromosomes, nondisjunction and random fertilization. Gene mutation is another source of variation.

Advantage / Disadvantage
Asexual / · Rapid population growth
· Small energy expense / · Offspring lack genetic diversity
Sexual / · Offspring genetically diverse / · Slower population growth
· Large energy expense

Patterns of Inheritance

Key Terms/Concepts

· Homologous chromosomes = In diploid organisms, chromosomes occur in pairs (one from each parent) that are similar in size, shape and genes.

· Allele = alternative forms of a gene (dominant A, recessive a)

· Dominant = the allele that is expressed when an organism contains two different alleles.

· Recessive = the allele that is not expressed when two different alleles are present in the genotype.

Trait / Dominant / Recessive
Hair type / Curly hair / Straight hair
Hair color / Dark / Light
Earlobe / Free / Attached
Ability to taste PTC (phenylthiocarbamide) / Tasting / Nontasting

· Genotype = The combination of alleles for a character (AA, Aa, aa)

o  Homozygous = both alleles of a genotype are the same (AA, aa)

o  Heterozygous = genotype has one dominant and one recessive allele (Aa)

· Phenotype = the expression of the genotype in the “appearance” of the organism. (tall vs. short) that may be influenced by environment.

Gregor Mendel’s experiments and laws

I. Law of Segregation of Alleles

  1. Developed by Mendel using monohybrid (single-trait) crosses.

1.  Two true-breeding parents (P) but different traits were crossed

2.  The offspring (F1) from this cross all showed only the dominant trait. The parental traits did not blend.

3.  The F1 individuals were crossed with each other to produce F2 generation.

4.  ¾ of the F2 expressed the dominant trait. ¼ expressed the recessive trait.

B.  Mendel’s crosses for monohybrid cross are summarized in the figure at the right.

MENDEL UPDATED

·  Genes are found on chromosomes, and thus multiple traits assort independently as long as they are located on different chromosomes. Mendel studied traits in peas that were each on separate chromosomes. Genes on the same chromosome are linked and thus will not normally assort independently.

P
generation / Grey female (GG)
/ White male (gg)

Gametes produced by P generation / /
F1 generation / All Gg
Dominant G masks recessive g

All Gg / Gametes produced by F1 generation / / / F2 generation
/ GG
/ Gg

/ Gg
/ gg

II. Law of Independent Assortment – when gametes are made, the genes for traits found on different chromosomes assort (separate) independently.

TESTCROSS = a deliberate genetic cross that is used to determine the genotype of an individual that expresses the dominant phenotype, which could be homozygous or heterozygous. The individual of unknown genotype is crosses with one having the recessive phenotype. If all the offspring exhibit the dominant phenotype the unknown parent is likely homozygous. If any of the offspring exhibit the recessive phenotype the unknown parent has to be heterozygous.

Intermediate Traits

Description / Example
Incomplete Dominance / One allele is not completely dominant over another. / Mirabilis plant: A red-flowered plant crossed with a white-flowered plant, produces heterozygous pink plants.
Codominance / Both alleles contribute to the phenotype. / One variety of chicken: A black-feathered chicken is crossed with a white-feathered chicken, producing heterozygous checkered chickens
Multiple Alleles / A gene has more than one allele / Human blood type: The alleles are A and B (which are co-dominant) and O.
Polygenic traits / Traits controlled by more than one gene. / Human skin color: More than four different genes control this trait, leading to a wide range of colors.

Chromosomes And Sex Determination

·  In many animals special chromosomes determine sex, the remaining chromosomes are autosomes.

·  In humans there are 44 autosomes (22 pair) and two sex chromosomes; X and Y in males, X and X in females.

Sex-Linked Traits

·  In humans, the Y chromosome contains the genes that determine maleness, the X contains many genes. If a male gets a recessive (or dominant) allele on the X chromosomes from his mother, he will express the trait. Therefore, males are frequently afflicted with X-linked disorders.

Punnett Squares

= a diagram used to identify possible genotypes of offspring resulting from a genetic cross.

To create a Punnett square, divide a square into four sections. Write the letters that represent the alleles of one parent across the top of the square, one letter per column. Write the alleles of the other parent down the side of the square, one letter per row. In each box, combine the alleles from one parent with those from the other. See below.

Pedigrees

·  A graphic devise showing the inheritance of a specific trait over several generations within a family.

·  Uses standardized set of symbols:

o  £ = normal male, = normal female

o  Shaded symbols = individual affected by trait

o  Horizontal line between symbols = mating

o  Vertical line = offspring

o  Offspring arranged left to right according to birth order

o  Generations identified by Roman numerals (I, II, etc.)

o  Individuals of a generation identified by Arabic numerals (1, 2, etc.)

·  What a pedigree tell us:

o  Sex-linked or not – if sex linked, males affected far more that females

o  Dominant/recessive –

§  If dominant, every affected individual WILL have at least one affected parent

§  If recessive, affected individual may or may not have an affected parent

o  Homozygous/heterozygous for dominant phenotype –

§  Determined using Punnett square

BIOTECHNOLOGY

Human Genome Project—project, begun in 1990, to identify all the genes and to determine the sequences of the 3 billion base pairs in human DNA. Project completed in 2003.

Genetic Engineering = the process of manipulating genes for practical purposes.

·  Involves production and use of recombinant DNA (DNA made from two or more different organisms)

·  Produces transgenic organisms (those that contain genes from another species) which can useful in:

o  Agriculture—pest and drought resistant crops, more nutrition,

o  Pharmaceutical industry—to make proteins such as human insulin, growth hormone (HGH), or blood-clotting factors

Gel-electrophoresis –a technique that uses an electric current to separate molecules according to size. DNA treated with a restriction enzyme is cut into fragments of different sizes. The fragments can be separated using gel-electrophoresis. When visualized the fragments produce an image of light and dark bands that is unique to the DNA sample. This unique pattern is known as a DNA fingerprint.

DNA fingerprints are used to:

·  Isolated DNA fragments having a gene of interest

·  Identify the source of specific DNA sample, as in crime scene investigation (forensic science)

EVOLUTION

Biogenesis vs. Abiogenesis—Two, competing ideas about the origin of life.

·  Abiogenesis = living things arise from non-living material, also known as spontaneous generation

·  Biogenesis = at the present time, under current conditions on Earth, all living things arise from preexisting living things.

Experiments:

Francesco Redi

. . . demonstrated that living things, such as maggots do not arise from decaying meat, but are the larval offspring of flies.

Lazzaro Spallanzani

. . . demonstrated that microorganisms do not arise via abiogenesis. Criticized for destruction of “vital force” necessary for spontaneous generation.


Louis Pasteur

. . . demonstrated, once and for always, that living things (even microorganisms) arise from other living things not from non-living matter.

ORIGIN OF LIFE

Oparin’s hypothesis—Conditions of early Earth were very different than today. Earth was hot, volcanic, with little oxygen in atmosphere. Under these conditions, simple inorganic molecules in atmosphere and ocean could combine chemically to combine and form simple organic molecules, the building blocks of life. Called “Primordial Soup Model.”

Miller-Urey Experiment –

conducted experiment to test Oparin’s hypothesis. Placed compounds Oparin suggested (methane, ammonia, water vapor, carbon dioxide, nitrogen, hydrogen) in apparatus shown. Electric sparks simulated lightning. After several days, simple organic molecules were formed.

Bubble Model—in light of current understanding that less methane and ammonia were in the atmosphere, scientists suggest that these chemical reactions occurred in bubbles below the surface of the ocean.

Charles Darwin and the Theory of Evolution

Along with Alfred Wallace developed a theory of evolution by means of natural selection.

Theory has 4 major points:

  1. Overproduction of offspring
  2. Variation among individuals
  3. Struggle for survival
  4. Differential survival of variants

Sources of genetic variation:

·  Mutations