Dna what is it
1 / 60

DNA what is it - PowerPoint PPT Presentation

  • Uploaded on

DNA what is it. Pentose sugar (deoxyribose) Phosphate molecule Four nitrogenous bases Pyrimidines: cytosine and thymine Purines: adenine and guanine. Proteins. One or more polypeptides Composed of amino acids

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
Download Presentation

PowerPoint Slideshow about ' DNA what is it' - imani-mcneil

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
Dna what is it
DNA what is it

  • Pentose sugar (deoxyribose)

  • Phosphate molecule

  • Four nitrogenous bases

    • Pyrimidines: cytosine and thymine

    • Purines: adenine and guanine


  • One or more polypeptides

  • Composed of amino acids

    • 20 amino acids of 64 total known are found in the structure of all plants and animals 6 we can not manufacture from scratch and are essential in diet

    • Directed by sequence of bases along DNA strans 3 consecutive bases = a codon

Dna replication
DNA Replication

  • Untwisting and unzipping of the DNA strand

    • Single strand acts as a template for replication and transcription to RNA

  • Complementary base pairing done by action of DNA polymerase

    • Adenine-thymine; cytosine-guanine Chargraf’s rules


  • Any inherited alteration of genetic material

    • Chromosome aberrations major changes in the entire DNA strand and entire piece missing or an extra chromosome or even an extra complete set examples include Cri – du – chat mising piece of #5 or trisomy 21 Downs syndrome extra 21 chromosome

    • Base pair substitution

      • One base pair is substituted for another

      • Silent substitution

        • Substitution that does not result in an amino acid change because genetic code is redundant

        • RNA codons GUU, GUC, GUA, GUG all code for the amino acid valine


  • Frameshift mutation

    • Insertion or deletion of one or more base pairs

    • Causes a change in the entire “reading frame”

    • Examples include sickle cell anemia


  • Spontaneous mutation

    • Mutation that occurs in absence of exposure to known mutagens

  • Mutational hotspots

    • Areas of the chromosomes that have high mutation rates

      • A cytosine base followed by a guanine are known to account for a disproportionately large percentage of disease-causing mutations


  • Agent known to increase the frequency of mutations

    • Radiation

    • Chemicals


  • RNA is synthesized from the DNA template

  • Results in the formation of messenger RNA (mRNA)

  • mRNA moves out of the nucleus and into the cytoplasm


  • Process by which RNA directs the synthesis of a polypeptide

  • Site of protein synthesis is the ribosome

  • tRNA contains a sequence of nucleotides (anticodon) complementary to the triad of nucleotides on the mRNA strand (codon)

  • The ribosome moves along the mRNA sequence to translate the amino acid sequence


  • Somatic cells

    • Contain 46 chromosomes (23 pairs)

    • Diploid cells

  • Gametes

    • Contain 23 chromosomes

    • Haploid cells

      • One member of each chromosome pair

  • Meiosis

    • Formation of haploid cells from diploid cells


  • Autosomes

    • The first 22 of the 23 pairs of chromosomes in males and females

    • The two members are virtually identical and thus said to be homologous

  • Sex chromosomes

    • Remaining pair of chromosomes

    • In females, it is a homologous pair (XX)

    • In males, it is a nonhomologous pair (XY)


  • Ordered display of chromosomes

Chromosome aberrations
Chromosome Aberrations

  • Euploid cells

    • Cells that have a multiple of the normal number of chromosomes

    • Haploid and diploid cells are euploid forms

  • When a euploid cell has more than the diploid number, it is called a polyploid cell

    • Triploidy: a zygote having three copies of each chromosome (69)

    • Tetraploidy: four copies of each (92 total)

  • Both triploid and tetraploid fetuses don’t survive

Chromosome aberrations1
Chromosome Aberrations

  • Aneuploidy

    • A somatic cell that does not contain a multiple of 23 chromosomes

    • A cell containing three copies of one chromosome is trisomic (trisomy)

    • Monosomy is the presence of only one copy of any chromosome

    • Monosomy is often lethal, but infants can survive with trisomy of certain chromosomes

      • “It is better to have extra than less”

Chromosome aberrations2
Chromosome Aberrations

  • Disjunction

    • Normal separation of chromosomes during cell division

  • Nondisjunction

    • Usually the cause of aneuploidy

    • Failure of homologous chromosomes or sister chromatids to separate normally during meiosis or mitosis

Autosomal aneuploidy
Autosomal Aneuploidy

  • Partial trisomy

    • Only an extra portion of a chromosome is present in each cell

  • Chromosome mosaics

    • Trisomies occurring only in some cells of the body

Autosomal aneuploidy1
Autosomal Aneuploidy

  • Down syndrome

    • Best-known example of aneuploidy

      • Trisomy 21

    • 1:800 live births

    • Mentally retarded, low nasal bridge, epicanthal folds, protruding tongue, poor muscle tone

    • Risk increases with maternal age

Sex chromosome aneuploidy
Sex Chromosome Aneuploidy

  • One of the most common is trisomy X. This is a female that has three X chromosomes.

  • Termed “metafemales”

    • Symptoms are variable: sterility, menstrual irregularity, and/or mental retardation

    • Symptoms worsen with each additional X

Sex chromosome aneuploidy1
Sex Chromosome Aneuploidy

  • Turner syndrome

    • Females with only one X chromosome

    • Characteristics

      • Absence of ovaries (sterile)

      • Short stature (~ 4'7")

      • Webbing of the neck

      • Edema

      • Underdeveloped breasts; wide nipples

      • High number of aborted fetuses

      • X is usually inherited from mother

Sex chromosome aneuploidy2
Sex Chromosome Aneuploidy

  • Klinefelter syndrome

    • Individuals with at least two Xs and one Y chromosome

    • Characteristics

      • Male appearance

      • Develop female-like breasts

      • Small testes

      • Sparse body hair

      • Long limbs

    • Some individuals can be XXXY and XXXXY. The abnormalities will increase with each X.

Alterations in chromosome structure
Alterations in Chromosome Structure

  • Chromosome breakage

    • If a chromosome break does occur, physiological mechanisms will usually repair the break, but the breaks often heal in a way that alters the structure of the chromosome

    • Agents of chromosome breakage

      • Ionizing radiation, chemicals, and viruses

Alterations in chromosome structure1
Alterations in Chromosome Structure

  • Breakage or loss of DNA

  • Cri du chat syndrome

    • “Cry of the cat”

    • Deletion of short arm of chromosome 5

    • Low birth weight, metal retardation, and microcephaly

Alterations in chromosome structure3
Alterations in Chromosome Structure

  • Duplication

    • Presence of a repeated gene or gene sequence

    • Rare occurrence

    • Less serious consequences because better to have more genetic material than less (deletion)

    • Duplication in the same region as cri du chat causes mental retardation but no physical abnormalities

Alterations in chromosome structure4
Alterations in Chromosome Structure

  • Inversions

    • Two breaks on a chromosome

    • Reversal of the gene order

    • Usually occurs from a breakage that gets reversed during reattachment

      • ABCDEFG may become ABEDCFG

Alterations in chromosome structure5
Alterations in Chromosome Structure

  • Translocations

    • The interchanging of material between nonhomologous chromosomes

    • Translocation occurs when two chromosomes break and the segments are rejoined in an abnormal arrangement

Alterations in chromosome structure8
Alterations in Chromosome Structure

  • Fragile sites

    • Fragile sites are areas on chromosomes that develop distinctive breaks or gaps when cells are cultured

    • No apparent relationship to disease

Alterations in chromosome structure9
Alterations in Chromosome Structure

  • Fragile X syndrome

    • Site on the long arm of the X chromosome

    • Associated with mental retardation; second in occurrence to Down syndrome

    • Higher incidence in males because they have only one X chromosome


  • Gregor Mendel

    • Austrian monk

    • Garden pea experiments

    • Mendelian traits


  • Locus

    • Position of a gene along a chromosome

  • Allele

    • A different form of a particular gene at a given locus

    • Example: Hgb A vs. Hgb S

    • Polymorphism

      • Locus that has two or more alleles that occur with appreciable frequency


  • Homozygous

    • Loci on a pair of chromosomes have identical genes

    • Example

      • O blood type (OO)

  • Heterozygous

    • Loci on a pair of chromosomes have different genes

    • Example

      • AB blood type (A and B genes on pair of loci)


  • Genotype (“what they have”)

    • The genetic makeup of an organism

  • Phenotype (“what they demonstrate”)

    • The observable, detectable, or outward appearance of the genetics of an organism

  • Example

    • A person with the A blood type could be AA or AO. A is the phenotype; AA or AO would be the genotype.


  • If two alleles are found together, the allele that is observable is dominant, and the one whose effects are hidden is recessive

  • In genetics, the dominant allele is represented by a capital letter, and the recessive by a lowercase letter

  • Alleles can be co-dominant


  • Carrier

    • A carrier is one that has a disease gene but is phenotypically normal

    • For a person to demonstrate a recessive disease, the pair of recessive genes must be inherited

    • Example

      • Ss = sickle cell anemia carrier

      • ss = demonstrates sickle cell disease


  • Used to study specific genetic disorders within families

  • Begins with the proband

Single gene disorders
Single-Gene Disorders

  • Recurrence risk

    • The probability that parents of a child with a genetic disease will have yet another child with the same disease

    • Recurrence risk of an autosomal dominant trait

      • When one parent is affected by an autosomal dominant disease and the other is normal, the occurrence and recurrence risks for each child are one half

Single gene disorders1
Single-Gene Disorders

  • Autosomal dominant disorder

    • Abnormal allele is dominant, normal allele is recessive, and the genes exist on a pair of autosomes

Single gene disorders2
Single-Gene Disorders

  • Autosomal dominant traits

Single gene disorders3
Single-Gene Disorders

  • Autosomal dominant trait pedigree


  • The percentage of individuals with a specific genotype who also express the expected phenotype

    • Incomplete penetrance

      • Individual who has the gene for a disease but does not express the disease

      • Retinoblastoma (eye tumor in children) demonstrates incomplete penetrance (90%)


  • Expressivity is the variation in a phenotype associated with a particular genotype

  • This can be caused by modifier genes

  • Examples:

    • von Recklinghausen disease

    • Autosomal dominant

    • Long arm of chromosome #17

    • Disease varies from dark spots on the skin to malignant neurofibromas, scoliosis, gliomas, neuromas, etc.

Single gene disorders4
Single-Gene Disorders

  • Autosomal recessive disorder

    • Abnormal allele is recessive and a person must be homozygous for the abnormal trait to express the disease

    • The trait usually appears in the children, not the parents, and it affects the genders equally because it is present on a pair of autosomes

Single gene disorders5
Single-Gene Disorders

  • Autosomal recessive disorder recurrence risk

    • Recurrence risk of an autosomal dominant trait

      • When two parents are carriers of an autosomal recessive disease, the occurrence and recurrence risks for each child are 25%


  • Mating of two related individuals

  • Dramatically increases the recurrence risk of recessive disorders

Sex linked disorders
Sex-Linked Disorders

  • The Y chromosome contains only a few dozen genes, so most sex-linked traits are located on the X chromosome and are said to be X-linked

Sex linked disorders1
Sex-Linked Disorders

  • Sex-linked (X-linked) disorders are usually expressed by males because females have another X chromosome to mask the abnormal gene

  • X-linked recessive

    • Most X-linked disorders are recessive

    • Affected males cannot transmit the genes to sons, but they can to all daughters

    • Sons of female carriers have a 50% risk of being affected