GeneticsGeneticsStandard 4: The student should be Standard 4: The student should be

able to demonstrate an able to demonstrate an understanding of the molecular understanding of the molecular

basis of heredity.basis of heredity.

Essential questionEssential question

What is the structure of DNA? What is the structure of DNA?

KEY CONCEPT KEY CONCEPT DNA was identified as the genetic DNA was identified as the genetic material through a series of material through a series of

experimentsexperiments..

People to knowPeople to know• Gregor Mendel – 1st to suggest

that paired factors carry traits• Walter Flemming – 1st to see

chromosomes• Thomas Morgan – determined

that specific genes are on specific chromosomes

• Meischer – 1st to isolate DNA

•Wilhelm Johannsen – 1st to use the term gene

•Walter Sutton – developed the chromosome theory

•Rosalind Franklin – the first to see DNA structure

•Watson and Crick-first to describe the double helix of DNA

• Ervin Chargaff – determined how nucleotides pair in DNA

• Griffin – determined that there is a “transforming principle” that is passed on

• Avery- identified DNA as the“transforming principle”• Hershey and Chase – proved DNA is

the genetic material

Watson and Crick determined the Watson and Crick determined the three-dimensional structure of three-dimensional structure of

DNA by building models.DNA by building models. • They realized that DNA

is a double helix that is made up of a sugar-phosphate backbone on the outside with bases on the inside.

• Watson and Crick’s discovery built on the work of Rosalind Franklin and Erwin Chargaff.

– Franklin’s x-ray images suggested that DNA was a double helix of even width.

– Chargaff’s rules stated that A=T and C=G.

Griffith finds a ‘transforming Griffith finds a ‘transforming principle.’ DNA can changeprinciple.’ DNA can change

• Griffith experimented with the bacteria that cause pneumonia.• He used two forms: the S form (deadly) and the R form (not

deadly).• A transforming material passed from dead S bacteria to live R

bacteria, making them deadly.

Avery identified DNA as the Avery identified DNA as the transforming principle.transforming principle.

• Avery isolated and purified Griffith’s transforming principle.

• Avery performed three tests on the transforming principle.

– Qualitative tests showed DNA was present.– Chemical tests showed

the chemical makeupmatched that of DNA.

– Enzyme tests showedonly DNA-degradingenzymes stoppedtransformation.

Hershey and Chase confirm that Hershey and Chase confirm that DNA is the genetic material.DNA is the genetic material.

• Hershey and Chase studied viruses that infect bacteria, or bacteriophages.

• Tagged DNA was found inside the bacteria; tagged proteins were not.

– They tagged viral DNA with radioactive phosphorus.– They tagged viral proteins with radioactive sulfur.

KEY CONCEPT KEY CONCEPT DNA structure is the DNA structure is the

same in all same in all organisms. organisms.

DNADNA

1. Deoxyribonucleic acid2. Double strand helix3. Codes for proteins which

determine our traits

DNA is composed of four types DNA is composed of four types of nucleotides.of nucleotides.

•Each nucleotide has three parts.– a phosphate group– a deoxyribose sugar– a nitrogen-containing base

phosphate group nitrogen-containingbase

deoxyribose

• The nitrogen containing bases are the only difference in the four nucleotides.

Nucleotides always pair in the Nucleotides always pair in the same way. same way.

• The base-pairing rules show how nucleotides always pair up in DNA.

• This is also know as Chargaff’s rule

– A pairs with T

– C pairs with G

• The backbone is connected by covalent bonds.

hydrogen bond covalent bond

• The bases are connected by hydrogen bonds.

Summary Nitrogen Summary Nitrogen basesbases

• Pyrimidines -single– Thymine (T)– Cytosine (C)

• Purines - double– Adenine (A)– Guanine (G)

– Thymine pairs Adenine– Cytosine pairs Guanine

Essential Question: How does Essential Question: How does DNA Code for life?DNA Code for life?

KEY CONCEPT KEY CONCEPT DNA DNA replicationreplication copies the copies the

genetic information of a cell.genetic information of a cell.

DNA replication – makes a DNA replication – makes a copy of itselfcopy of itself

reviewreview• DNA is replicated during the

S (synthesis) stage of thecell cycle.

• Each body cell gets acomplete set ofidentical DNA.

• DNA serves only as a template. (pattern)• Enzymes and other proteins do the actual

work of replication.– Enzymes unzip the double helix.– Free-floating nucleotides form hydrogen bonds

with the template strand.

nucleotide

The DNA molecule unzips in both directions.

– Polymerase enzymes form covalent bonds between nucleotides in the new strand.

– DNA polymerase enzymes bond the nucleotides together to form the double helix.

DNA polymerase

new strand nucleotide

• DNA replication is semiconservative. original strand new strand

Two molecules of DNA

• Two new molecules of DNA are formed, each with an original strand and a newly formed strand.

DNA ReplicationDNA Replication

1. Strands separate between the nitrogen bases (unzips)

2. free nucleotides bond to the exposed bases

3. There are now two complete strands made of one new and one old strand

KEY CONCEPT KEY CONCEPT TranscriptionTranscription converts converts a gene into a single-a gene into a single-

stranded RNA stranded RNA molecule.molecule.

The transcription process is The transcription process is similar to replicationsimilar to replication. .

Replication

• Copies all the DNA

• Makes one copy• takes place in

the nucleus

transcription• Copies a gene

(part) of the DNA making RNA

• Can make many copies.

• moves to different locations

• RNA is a link between DNA and proteins.

• Proteins give organisms their traits

replication

transcription

translation

RNA carries DNA’s instructions.

• RNA differs from DNA in three major ways.

1. RNA has a ribose sugar.2. RNA is a single-stranded structure.3. RNA has uracil instead of thymine.

• Transcription

– DNA unzips at the start site to the termination site

start site

nucleotides

transcription complex

– RNA polymerase bonds the nucleotides together.

– The DNA helix winds again as the gene is transcribed.

– Nucleotides pair with one strand of the DNA.

DNA

RNA polymerase moves along the DNA

– The RNA strand detaches from the DNA once the gene is transcribed.

RNA

• Transcription makes three types of RNA.

1. Messenger RNA (mRNA) carries the message that will be translated to form a protein to the ribosome from the nucleus.

2. Transfer RNA (tRNA) brings the message from the nucleus to a ribosome in the cytoplasm.

3. Ribosomal RNA (rRNA) found at the ribosomes where proteins are made.

Summary of TranscriptionSummary of Transcription1. The DNA unzips only the gene2. The nucleolus produces mRNA off of

the open strand of DNA3. Uracil replaces thymine4. tRNA travels to the ribosome 5. The code is read in groups of 3

called codons as rRNA (ribosomal)

KEY CONCEPT KEY CONCEPT Translation converts Translation converts an mRNA message an mRNA message into a polypeptide, into a polypeptide,

or protein.or protein.

• A codon is a sequence of three nucleotides that codes for an amino acid.

codon formethionine (Met)

codon forleucine (Leu)

• A change in the order in which codons are read changes the resulting protein.

• Regardless of the organism, codons code for the same amino acid.

Amino acids are linked to Amino acids are linked to become a protein. become a protein.

• An anticodon is a set of three nucleotides that is complementary to an mRNA codon.

• An anticodon is carried by a tRNA.

• Ribosomes consist of two subunits.

– The large subunit has three binding sites for tRNA.

– The small subunit binds to mRNA.

• For translation to begin, tRNA binds to a start codon and signals the ribosome to assemble.

– A complementary tRNA molecule binds to the exposed codon, bringing its amino acid close to the first amino acid.

– The ribosome helps form a polypeptide bond between the amino acids.

– The ribosome pulls the mRNA strand the length of one codon.

– The now empty tRNA molecule exits the ribosome.

– A complementary tRNA molecule binds to the next exposed codon.

– Once the stop codon is reached, the ribosome releases the protein and disassembles.

CodonsCodons• Three nucleotide bases from the

rRNA move through the ribosome• Called tRNA (transfer)• Each code gives a new amino acid• 64 combinations• Build proteins

Summary of translationSummary of translation• rRNA moves through the ribosome• The sequence is read in blocks of 3

called codons to form amino acids• The amino acids are linked to form

the proteins• The proteins determine the traits of

the organism

• The genetic code matches each codon to its amino acid or function.

The genetic code matches each RNA codon with its amino acid or function.

Rules for reading codon Rules for reading codon chartscharts

1. Break into codons2. Read bases in order3. Base one gives the row4. Base two gives the column5. Base three gives the amino acid

Essential Question:Essential Question:

•How are traits passed How are traits passed from one generation to from one generation to another?another?

KEY CONCEPT KEY CONCEPT Gametes have half the Gametes have half the

number of number of chromosomes that chromosomes that body cells have.body cells have.

You have body cells and You have body cells and gametes. gametes.

• Body cells are also called somatic cells.• Germ cells develop into gametes.

– Germ cells are located in the ovaries and testes.– Gametes are sex cells: egg and sperm.– Gametes have DNA that can be passed to offspring.

body cells sex cells (sperm) sex cells (egg)

• Your body cells have 23 pairs of chromosomes.– Homologous pairs of

chromosomes have the same structure.

– For each homologous pair, one chromosome comes from each parent.

• Chromosome pairs 1-22 are autosomes (body cells).

• Sex chromosomes, X and Y, determine gender in mammals.

Your cells have autosomes Your cells have autosomes and sex chromosomes.and sex chromosomes.

Body cells are diploid; Body cells are diploid; gametes are haploidgametes are haploid. .

• Diploid (2n) cells have two copies of every chromosome.– Body cells are diploid.– Half the chromosomes come from each parent.

• Haploid (n) cells have one copy of every chromosome.

– Gametes are haploid.– Gametes have 22 autosomes and 1

sex chromosome.

ReviewReview• Meiosis reduces chromosome number and

creates genetic diversity.

• Meiosis I and meiosis II each have four

phases, similar to those in mitosis.

homologous chromosomes

sisterchromatids

sisterchromatids

– Pairs of homologous chromosomes separate inmeiosis I.

– Homologous chromosomes are similar but not identical.– Sister chromatids divide in meiosis II.– Sister chromatids are copies of the same chromosome.

• Meiosis I occurs after DNA has been replicated.

• Meiosis I divides homologous chromosomes in four phases.

• Meiosis II divides sister chromatids in four phases.

• DNA is not replicated between meiosis I andmeiosis II.

• Meiosis differs from mitosis in significant ways.

– Meiosis has two cell divisions while mitosis has one.

– In mitosis, homologous chromosomes never pair up.

– Meiosis results in haploid cells; mitosis results in diploid cells.

• Gametogenesis is the production of gametes and differs between females and males.

– Sperm become streamlined and motile.

– Sperm primarily contribute DNA to an embryo.

– Eggs contribute DNA, cytoplasm, and organelles to an embryo.

– During meiosis, the egg gets most of the contents; the other cells form polar bodies.

spermatogenesis

Oogenesis

KEY CONCEPT KEY CONCEPT Mendel’s researchMendel’s research

Mendel laid the groundwork Mendel laid the groundwork for geneticsfor genetics. .

1. Traits are distinguishing characteristics that are inherited.

2. Genetics is the study of biological inheritance patterns and variation.

3. Gregor Mendel showed that traits are inherited as discrete units.

Mendel’s data revealed Mendel’s data revealed patterns of inheritance. patterns of inheritance.

• Mendel made three key decisions in his experiments.– use of purebred plants– control over breeding– observation of seven

“either-or” traits

• Mendel used pollen to fertilize selected pea plants.

Mendel controlled thefertilization of his pea plantsby removing the male parts,or stamens.

He then fertilized the femalepart, or pistil, with pollen froma different pea plant.

– P generation crossed to produce F1 generation

– interrupted the self-pollination process by removing male flower parts

• Mendel allowed the resulting plants to self-pollinate.

– Among the F1 generation, all plants had purple flowers

– F1 plants are all heterozygous

– Among the F2 generation, some plants had purple flowers and some had white

• Mendel observed patterns in the first and second generations of his crosses.

• Mendel drew three important conclusions.

1. Traits are inherited as discrete units.2. Organisms inherit two copies of each

gene, one from each parent.3. The two copies segregate

during gamete formation.

purple white

Terms of HeredityTerms of Heredity• Heredity – the passing on of

characteristics from parent to offspring

• Genetics – the study of heredity• Traits – characteristics of inherited• Genes – carry the traits of

inheritance• Alleles – a single member of a pair of

genes

– Each parent donates one allele for every gene.

– Homozygous describes two alleles that are the same at a specific locus.

– Heterozygous describes two alleles that are different at a specific locus.

Laws of heredityLaws of heredity• Chromosome theory

– Chromosomes carry genes that separate

• Law of segregation– Genes for a trait separate

• Law of dominance– One gene can hide another

• Law of Independent Assortment– Genes separate independently of one an

other

Law of DominanceLaw of DominanceGenes can be dominant

– Observed (stronger) trait– Can be seen with only one– Symbolized by a capital letter– Tt or TT

Genes can be recessive– Hidden trait– Only shows when both alleles are recessive– Symbolized by a lower case letter– tt

Expression of genesExpression of genes• Phenotype

– Physical expression of the trait– Tall or short

• Genotype– The genes being expressed

• Homozygous– Same – TT or tt

• Heterozygous– Different– Tt

Law of segregationLaw of segregation• Alleles separate when gametes (sex

cells) form• The alleles that combine are at

random

Law of independent Law of independent assortmentassortment

• Alleles separate independently of one an other

• This is seen better in dihybrid or trihybrid crosses

Predicting outcomesPredicting outcomes• Monohybrid crosses

– One pair of alleles – TT

• Dihybrid crosses– Two pairs of alleles– TtGg

• Trihybrid crosses– Three pairs of alleles– TtGgWw

Punnett SquarePunnett Square• Developed in 1905 by Reginald

Punnett• Allows for accurate predictions of

outcomes of crosses and calculations of probablity

• Mendel’s rules of inheritance apply to autosomal genetic disorders.– A heterozygote for a recessive disorder is a carrier. – Disorders caused by dominant alleles are uncommon.

(dominant)

Exceptions to the rulesExceptions to the rules• Incomplete dominance

– Blending – Red flower crossed with a white flower and

you get pink flowers

• Codominance– Both trait show – Black chicken crossed with a white chicken

and the offspring have both black and white feathers

Phenotype can depend on Phenotype can depend on interactions of alleles.interactions of alleles.

• In incomplete dominance, neither allele is completely dominant nor completely recessive.– Heterozygous phenotype is intermediate between

the two homozygous phenotypes

– Homozygous parental phenotypes not seen in F1 offspring

• Codominant alleles will both be completely expressed.– Codominant

alleles are neither dominant nor recessive.

– The ABO blood types result from codominant alleles.

• Many genes have more than two alleles.

• Sex Determination– Two different chromosomes work together– XX gives you a female– XY gives you a male

• Sex-linked traits– Traits on the sex chromosomes– Color blindness is a trait only found on the X

chromosomes for humans

• Polygentic inheritance– Controlled by two or more genes– Skin color is a combination of 3 to 4 genes

sex-linked traitssex-linked traits. .

– Y chromosome genes in mammals are responsible for male characteristics.

– X chromosome genes in mammals affect many traits.

. .

• Males (XY) express all of their sex linked genes.• Females (XX) can carry sex-linked genetic disorders• Expression of the disorder depends on which parent carries the allele

and the sex of the child.

XY

• Male mammals have an XY genotype.– All of a male’s

sex-linked genes are expressed.

– Males have no second copies of sex-linked genes.

• Female mammals have an XX genotype.– Expression of sex-linked genes is similar to

autosomal genes in females.– X chromosome inactivation randomly

“turns off” one X chromosome.

Many genes may interact to Many genes may interact to produce one trait. produce one trait.

• Polygenic traits are produced by two or more genes.

Order of dominance: brown > green > blue.

Epistasis Epistasis • Multiple genes giving

multiple factors• Hair color in mice is a

5 gene phenotype with genes for color shading and spots

• Albinism in hedgehogs is caused by epistatic genes blocking color

Environmental Environmental

• The sex of sea turtles and many reptiles depends on both genes and the temperature of the environment

• Phenotype can be a combination of genotype and environment.

• Crossing over is the exchange of chromosome segments between homologous chromosomes.

Human genetics follows the Human genetics follows the patterns seen in other patterns seen in other

organismsorganisms. . • The basic principles of genetics are the same

in all sexually reproducing organisms.– Inheritance of many human

traits is complex.– Single-gene traits are

important in understandinghuman genetics.

Patterns of heredity Patterns of heredity • Tracing genetics through a family is

done using a pedigree chart1. Seperated by generations 2. Represents the males and females 3. Represents those that have the trait

or carry the trait

• Phenotypes are used to infer genotypes on a pedigree.• Autosomal genes show different patterns on a pedigree

than sex-linked genes.

• If the phenotype is more common in males, the gene is likely sex-linked.

Several methods help map Several methods help map human chromosomes. human chromosomes.

• A karyotype is a picture of all chromosomes in a cell.

X Y

• Karyotypes can show changes in chromosomes. – deletion of part of a chromosome or loss

of a chromosome– large changes in chromosomes– extra chromosomes or duplication of part

of a chromosome