Chapter 3

Human Genetics

RNA – Ribonucleic Acid

Differs from DNA in that:

1. Contains the sugar Ribose

2. Is mostly single stranded

3. Contains the base Uracil

instead of Thymine

Transcription

The synthesis of RNA on a DNA

template

(copying process that makes RNA)

Process of Transcription:1: RNA polymerase binds with part of the DNA

strand

2: RNA polymerase unwinds a portion of the double helix (separating the strands). RNA

nucleotides pair with complementary bases one at a time

3: A continuous RNA strand is formed

4: The RNA molecules separate, polymerase comes off, and the DNA strands rejoin

The product of transcription is RNA

Messenger RNA (mRNA)

Transfer RNA (tRNA)

Ribosomal RNA (rRNA)

Information flows from DNA RNA Protein

Translation(protein synthesis)

Why is mRNA made?

• DNA canNOT leave the nucleus

What does mRNA do?• Carries DNA information to the ribosomes• Is used to make protein

How does mRNA make proteins?

• At the ribosome, mRNA assembles amino acids into chains– Codons – group of 3 nitrogenous bases

• tRNA helps assemble the AA

• AA chains combine to form proteins

tRNA

• Carries anticodon and an amino acid

Figure 3.4

Figure 3.5 (1)

Figure 3.5 (2)

Figure 3.5 (3)

Translation uses all 3 forms of RNA:

• mRNA as the template

• tRNA to match a codon to an amino acid

• rRNA to form the platform where the process takes place

Figure 3.2a

MutationsHeritable changes in DNA sequence

• Can result from mistakes during DNA replication• Are fixed by mechanisms in your body• In somatic cells can affect individuals but not necessarily

the next generation• In gametes may be passed on to the next generation

Point Mutations

• Single-base mutation

Eg. Substitution of 1 nitrogenous base for another

Frameshift Mutation

• 1 or 2 extra nitrogenous bases are inserted/deleted into a DNA sequence

Chromosomal aberrations (larger changes in genes)

Chromosomal fragment may:

1. Be duplicated

2. Become attached at new location

3. Be lost completely

4. Be inverted and reinserted (*most frequent)

5. Change in chromosome #

Karyotype – an arrangement of homologous

chromosome pairs Autosomal chromosomes – same in both sexes

(22 of 23 pairs) Sex chromosomes – 23rd pair; have a role in

determining the sex of an individual

(most of the genes on the X chromo. are not on the Y chromo.)

Genes that are carried on sex chromosomes determine sex and

sex-linked traits

Sex determination

Females

XX

Males

XY

female male

Exceptions to the rule:Not all females are XX and not all males are XY

A cross-over during meiosis can occur between an X & Y chromosome and is followed by an exchange of DNA pieces ---translocation

1/20,000 normal males is XX (chromosomally)1 of X chromo. contains small translocated piece of Y chromo.

1/20,000 normal females is XY (chromosomally)missing same small piece of the Y chromo.

Lead to the assumption that:

• An XY female has 99.8% of the Y chromosomal DNA; therefore, the male-determining factor must be located in the .2% portion of the chromo. she did not have

• The .2% of the chromo. has the sry gene – which is thought to induce male development

www.biointeractive.org

Sex-linked traits• Most genes are located on the X chromo.

• Genes that are on the X chromo. and not on the Y chromo. are sex-linked genes

Females

• Can be either homozygous or heterozygous for sex-linked traits (b/c they have 2 X chromo. and 2 alleles of every sex-linked gene)

Males• Because males only have

1 X chromo., they have 1 allele for each sex-linked trait (which determines his phenotype for the trait)

• Hemizygous

e.g., red-green colorblindness

Trait: red-green colorblindness

XC Y

Xc

Xc

Male gametes

Female

gametes

Genotype: (XCY x XcXc)

Phenotype: male is normal, female is colorblind

Trait: red-green colorblindness

XC Y

XC

Xc

Male gametes

Female

gametes

Genotype: (XCY x XCXc)

Phenotype: male is normal, female is carrier (normal)

Chromosomal variation

• Normal human –

22 pairs of autosomal chromo.

1 pair of sex chromo.

• Any difference in chromosomal # has consequences called syndromes

Variations in sex chromosomes:Klinefelter’s syndrome• XXY• Male phenotype• Sterile• Some symptoms treated

with hormones

Turner’s syndrome• XO• Female phenotype• Infertile (ovaries do not

produce female hormones, therefore puberty isn’t reached and gametes do not develop)

• Some symptoms treated with hormones

Klinefelter’s

Figure 2.15

• Klinefelter’s and Turner’s are thought to result from nondisjunction

• Nondisjunction –

abnormal cell division of sex chromo.

Variations in autosomal chromosomes:

Down’s syndrome Trisomy 21 Distinct facial characteristics Heart abnormalities Variable amounts of mental retardation

Frequency of Down’s Syndrome

Variations in autosomal chromosomes:

Patau’s syndrome or Trisomy 13 Small head, extra fingers and toes Cleft lip, large triangular nose, wide-spaced eyes Severe mental retardation Death usually by 1 year

Trisomy 13 karyotype

Variations in autosomal chromosomes:Cri du chat syndrome (Cry of the Cat) Deletion in chromosome 5 Small head, mental retardation, catlike cry Short lifespan 1 in 50,000 births

Cri du chat karyotype

Other diseases and abnormalities that are inherited in genes:

(not chromosomal aberrations)

• Albinism– Single mutation that prevents the formation of

the pigment melanin (which blocks ultraviolet light) in eyes, skin, hair and internal organs

– Affects all races of humans and other species– In U.S. 1 in 17,000 people have some type of

albinism

Pedigrees

• Diagrams giving the pattern of mating and descent of certain traits– most basic methods for presenting genetic data– are most useful when they span many generations

Pedigrees help determine if traits are:

1. Inherited

2. Dominant or recessive

3. Have a more complex genetic basis

Hereditary diseases associated with known genes:1. Alkaptonuria (error in a series of closely related metabloic pathways)2. Phenylketonuria (PKU)3. Duchenne’s Muscular Dystrophy (X chromo.)4. Huntington’s Disease (chromo. 4)5. Cystic fibrosis (chromo. 7)

And have found linkage areas for:6. Alzheimer’s Disease (chromo. 21)

7. 1 form of colon cancer (chromo. 2)

8. 2 forms of manic depression (chromo. 11 & X chromo.)

Human Genome Project

Proposed in 1986

Completed in 2003

Purpose: 1. Make a genetic map of the human genome

a. Including chromosomal location of all human genes

2. Complete DNA sequence of the human genome

3. DNA sequence of each human is unique

a. People share same genes, but sequences vary (different alleles)

Genome - Complete set of inherited genetic information encoded in our DNA

• 3.1 billion nucleotide base pairs in each cell• Only about 2% of the human genome contains

genes, which are the instructions for making proteins

• Approx. 28,000 – 35,000 genes in the human genome

• The human genome is nearly the same (99.9%) in all people

Human Genome Project:Legal Issues:

1. Who owns the human genome?

2. Who owns the sequence of a particular gene?

3. Can genes be patented?

4. Who should have access to the information?a. Physicians?

b. Insurance companies?

Ethical Issues:1. Genetic discrimination?2. Misuse of genetic info. due to ignorance. 3. Who should be tested?

Those at higher risk, for ex.:a. African Americans – sickle-cell anemiab. Mediterranean or southeast Asian –thalassemiac. Ashkenazi Jews –Tay-Sachs diseased. European Americans – cystic fibrosis

4. Eugenics - Altering a gene poola. Positive eugenics – encouraging certain genotypes to breed in

greater numbersb. Negative eugenics – preventing certain genotypes from breeding

• What is a favorable genotype?

• Hitler??? & the “Master Race”

Gene Therapy• Introduction of genetically engineered (functional

genes) into cells altering cell’s genotype – For curing a disease or genetic defect

Cloning

Asexually produced organism (or cells)

Procedure:1. Nucleus from a fertilized egg (zygote) is removed

2. and replaced with the nucleus from a cell of a fully developed individual

3. implanted in the womb

4. and brought to term

Now to the FUN stuff!!!!

Many genes are highly complex and involve the interaction of many genes

(such as hair color, eye color, skin color, height, etc.)

Several traits are determined by only one gene, and if you have the gene then you have the trait.

such as the following: • Come to Class for Examples!! *see, it’s not all right

here

Review Mitosis & Meiosis

46 46

46 46 4646

23 23 23 23

Mitosis

Meiosis

Review Cell Structure

Cytoplasm with organelles (ribosomes)

nucleus

chromosomes

Review Cell Processes

• Replication

• Transcription

• Translation