DNA Organization, Genes, and the Genome

• Genetic material is located in the nucleus
• Genetic information is stored in deoxyribonucleic acid (DNA)
• DNA contains all the information needed to build an individual

DNA Organization

• Chromosome size is about 1400 nm during metaphase
• Condensed scaffold associated form size is about 700 nm
• Extended scaffold-associated form has fibers connected to a chromosome scaffold at 300 nm
• Chromatin fibers, at the level of packed nucleosomes are around 30 nm
• Chromatin in the "beads-on-a-string" form is about 11 nm
• A short section of the DNA double helix measures about 2 nm

Gene

• It is a sequence of DNA nucleotides containing the information that specifies the amino acid sequence of a single polypeptide chain
• A gene is a unit of hereditary information
• A single molecule of DNA contains many genes

Genome

• The total genetic information coded in the DNA of a typical cell within an organism
• The human genome contains 50,000 - 100,000 genes
• The information in the genome is required for producing 50,000 - 100,000 different proteins
• The largest chromosome has 250 million pairs of nucleotides
• The smallest chromosome has 47 million pairs of nucleotides
• The size of the gene in the chromosome is about 50 thousand pairs of nucleotides
• The nucleus of the human cell contains 23 pairs of chromosomes

Human Genome Project (HGP)

• An international research effort to determine the sequence of the human genome
• HGP identified the genes that the human genome contains
• The HGP was a 13-year effort co-ordinated by the Department of Energy (DOE) and the National Institutes of Health (NIH)

DNA

• Genetic information is used for gene expression
• Information of a gene is transferred from DNA and converted to protein
• RNA molecules act as messengers
• Proteins are the biological workers
• The synthesis of an RNA molecule is called transcription
• RNA directs protein synthesis in translation
• Protein's 3D structure determines its function
• Information can only transfer in one direction
• A DNA sequence is divided into series of units of three bases which make a codon
• One codon is specific to one amino acid, which is a structural component of a protein
• The four bases can form 64 codons
• 20 amino acids are found in nature

Genetic Language

• Similar to a written language
• It consists of a set of symbols such as A, B, C, D, that form an alphabet
• The letters are arranged in specific sequences forming words
• The words are arranged in linear sequences to form sentences
• The genetic language contains 4 letters, corresponding to the bases A, G, C, and T
• The genetic words are 3-base sequences which specify particular a.a that is, each word in the genetic language is only 3 letters long
• A codon is a triplet code
• The sequence of 3-letter code words (triplets) along a gene in a single strand of DNA specifies the sequence of a.a in a polypeptide chain
• A gene is equivalent to a sentence, then the genetic information in the human genome is equivalent to a book containing 50,000 to 100,000 sentences

Genetics Vocabulary

• Gene: a section of DNA that encodes a particular protein
• Allele: different version of a gene - there are two alleles for each gene, one from each parent
• Dominant: An allele that always shows up if you have it
• Recessive: An allele that only shows up if there is no dominant allele
• Genotype: The genetic make-up (combination of alleles)
• Phenotype: The characteristics/traits that show up

DNA

• It is a polymer of nucleotide monomers
• Contains 2'-deoxyribose sugar
• Have four bases: adenine, guanine, thymine, and cytosine
• Together a sugar and a base are called a nucleoside
• Adenine and guanine are purine bases (two carbon rings)
• Thymine and cytosine are pyrimidine bases (single carbon ring)
• Nucleosides are joined together with a phospsodiesteri bond in DNA chains
• The sequence of bases is the genetic information
• DNA molecules are composed of two polynucleotide chains
• The chains form a double helix twisted in a right-handed way
• There is a twist a full circle in every 10 bases
• Bases are the steps
• Sugars and phosphates are supporting pilars
• Two nucleotide chains run in opposite directions in a chemical direction
• Purines with two carbon rings interact only with single ring pyrimidines
• Space between the chains is limited
• Complementary pairing is vital for retaining the genetic information
• Interaction is stabilized by hydrogen bonds in the pattern A-T (two hydrogen bonds), and G-C (three hydrogen bonds)

The Genetic Code

• Describes how base sequences are converted to protein sequence
• The DNA sequence is divided into series of units of three bases - a codon
• One codon is specific to one amino acid
• 4 bases can form 64 codons
• There are about 20 aminoacids
• Codons hava also alternative functions needed to regulate protein synthesis
• There is a right reading frame is obligatory
• Mutations are alterations in DNA sequence

Types of Mutation

• Mutations are alterations in DNA sequence, caused by chemical and physiological agents and errors in DNA replication
• Cells can repair some mistakes
• Once introduced and not repaired, changes in DNA sequence are made permanent during DNA replication
• Single Nucleotide Polymorphisms cause alteration of a single base
• This can cause an alteration in the amino acid which the codon codes, not cause the amino acid alteration, or cause the alterations of a new codon

Frameshift mutation

• Insertion or deletion of bases, meaning reading frame is altered
• Human genome has about 3 billion base pairs
• Human genome has 30000 genes
• Human genes: 23 chromosome pares, 46 chromosomes Approximately 25% of the DNA is gene related; Only 5% encodes proteins; Genes include exons and introns; Beside coding areas also additional secuences are found
• Phenotype: The outlook of an organism
• Genotype: The genetic information written in the DNA
• Information from DNA is passed on in sexual reproduction and is needed for characteristics to develop
• Humans have 46 chromosomes (44 autosomes, 2 sex chromosomes)
• Females are XX
• Males are XY

Genetics principles

• Each individual has two copies of each gene, one from the mother, and one from the father which becomes the Genotype
• A dominant character needs only one allele to cause the phenotype (heterozygous)
• A recessive character: both alleles needed to cause the phenotype (homozygous)
• Inheritance helps observe patterns of inheritance
• Autosomal recessive: the disease appears in male and female children of unaffected parents
• Autosomal dominant: affected males and females appear in each generation of the pedigree; affected mothers and fathers transmit the phenotype to both sons and daughters
• X-linked recessive: Many more males than females show the disorder, all the daughters of an affected male are "carriers", none of the sons of an affected male show the disorder or are carriers
• X-linked dominant: Affected males pass the disorder to all daughters but to none of their sons; affected heterozygous females married to unaffected males pass the condition to half their sons and daughters
• Codominant inheritance: two different versions (alleles) of a gene can be expressed, and each version makes a slightly different protein; both alleles influence he genetic trait or determine the characteristics of the genetic condition
• An example of a condition that is codominant is an ABO locus
• Mitochondrial inheritance: This type of inheritance applies to genes in mitochondrial DNA; mitochondrial disorders can appear in every generation of a family and can affect both males and females, but fathers do not pass mitochondrial traits to their childrenLeber’s hereditary optic neuropathy is an example

Medical notes

• Cystic Fibrosis is a disease affecting the mucus lining of the lungs, then leading to breathing problems
• Huntington disease, Huntington's chorea, is an inherited disorder characterized by abnormal body movements called chorea, and loss of memory; incidence is 5 to 8 per 100,000
• Hemophilia is an illness that impairs the body's ability to control bleeding
• Fragile X syndrome is a genetic condition that causes developmental problems including learning disabilities and mental retardation; affected males tend to be restless, fidgety, and inattentive