Genetics

Genetics

7/21/2008

Basics• The molecular basis for

genes is deoxyribonucleic acid (DNA)

• DNA is composed of a chain of nucleotides, of which there are four types: adenine (A), cytosine (C), guanine (G), and thymine (T)

• DNA normally exists as a double-stranded molecule, coiled into the shape of a double-helix http://upload.wikimedia.org/wikipedia/commons/thumb/e/e4/

DNA_chemical_structure.svg/350px-DNA_chemical_structure.svg.png

Basics

• Genes are arranged linearly along long chains of DNA sequence, called chromosomes

• Haploid organisms have only one copy of each chromosome, most animals and many plants are diploid, containing two of each chromosome and thus two copies of every gene. The two alleles for a gene are located on identical loci of homologous chromosomes, each allele inherited from a different parent.

Basics

• Sex chromosomes - specialized chromosomes many animals have evolved that play a role in determining the sex of an organism.

• Females have two copies of the X chromosome, but males have one Y and only one X chromosome—this difference in X chromosome copy numbers leads to the unusual inheritance patterns of sex-linked disorders. (humans)

Mitosis• Mitosis is the process by

which a cell separates the chromosomes in its cell nucleus, into two identical sets in two daughter nuclei.

• It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two daughter cells containing roughly equal shares of these cellular components.

http://upload.wikimedia.org/wikipedia/commons/3/38/300px-Anaphase-flourescent.jpg

Mitosis• During S phase, the period

that precedes the mitotic phase in the cell cycle where preparation for mitosis occurs(DNA replication occurs)

• Each new chromosome now contains two identical copies of itself, called sister chromatids, attached

together in a specialized region of the chromosome known as the centromere

http://upload.wikimedia.org/wikipedia/commons/e/e0/Major_events_in_mitosis.svg

Mitosis

• Microtubules, essentially miniature strings, splay out from opposite ends of the cell and shorten, pulling apart the sister chromatids of each chromosome.

• As the cell elongates, corresponding sister chromosomes are pulled toward opposite ends. A new nuclear envelope forms around the separated sister chromosomes.

Cell cycle

• A series of events that take place in eukaryotic cell leading to its replication

• A vital process for a single fertilized eggs to develop into a mature organism

• Also important for process by which hair, skin and blood cells are renewed

Cell cycle• The cell cycle is divided

into four phases: G1, S, G2(collectively known as interphase) and M phase

• M phase consists of mitosis followed by cytokinesis

• Cells that have temporarily stopped dividing are said to have entered G0 phase

http://upload.wikimedia.org/wikipedia/en/2/22/Cell_Cycle_2.png

Cell cycle

• G1 phase starts from the end of the previous M phase till the beginning of DNA synthesis(S phase). Synthesis of enzymes required for S phase

• S phase is when DNA replicates. All of the chromomsome have replicated by the end of it. Two sister chromatids in each chromosome

• G2 phase results in production of microtubules

Meiosis• meiosis is a process of reduction division in which the

number of chromosomes per cell is cut in half. In animals, meiosis always results in the formation of gametes.

• the genome of a diploid germ cell, undergoes DNA replication followed by two rounds of division, resulting in four haploid cells.

• Each of these cells contain one complete set of chromosomes, or half of the genetic content of the original cell.

• If meiosis produces gametes, these cells must fuse during fertilization to create a new diploid cell, or zygote before any new growth can occur.

Meiosis• Meiosis I consists of segregation

of the homologous chromosomes from each other, then dividing the diploid cell into two haploid cells each containing one of the segregates.

• Meiosis II consists of decoupling each chromosome's sister strands (chromatids), segregating the DNA into two sets of strands (each set containing one of each homologue), and dividing both haploid, duplicated cells to produce four haploid, unduplicated cells.

http://www.carisgamba.com/CTmeiosis/meiosis-big.gif

Nondisjunction

• The normal separation of chromosomes in Meiosis I or sister chromatids in meiosis II is termed disjunction.

• Not normal – nondisjunction, which can occur in either meiosis I or meiosis II

• Down syndrome – trisomy of chromosome 21

Genetic diversity

• Independent assortment• Genetic recombination(genetic crossover)- process by which two chromosomes pair up

and exchange sections of their DNA. This often occurs during meiosis in a process called synapsis

http://upload.wikimedia.org/wikipedia/commons/0/0e/Morgan_crossover_1.jpg

Transcription

• only a small fraction of the total sequence of the genome encodes protein.

• 15% of the human genome consists of protein-coding sequence

• Transcription is the synthesis of RNA under the direction of DNA

• DNA sequence is enzymatically copied by RNA polymerase to produce a complementary nucleotide RNA strand, called messenger RNA (mRNA)

Transcription• One significant difference

between RNA and DNA sequence is the presence of U, or uracil in RNA instead of the T, or thymine of DNA.

• These two strands run in opposite directions to each other and are therefore anti-parallel.

• The asymmetric ends of DNA strands are referred to as the 5 (′ five prime) and 3 (′ three prime) ends, with the 5' end being that with a terminal phosphate group and the 3' end that with a terminal hydroxyl group.

http://upload.wikimedia.org/wikipedia/commons/thumb/e/e4/DNA_chemical_structure.svg/350px-DNA_chemical_structure.svg.png

Transcription• RNA is synthesized in

the 5' → 3' direction (from the point of view of the growing RNA transcript).

• Only one of the two DNA strands is transcribed -template strand

• The other strand is called the coding strand, because its sequence is the same as the newly created RNA transcript

http://fig.cox.miami.edu/~cmallery/150/gene/c7.17.7b.transcription.jpg

Translation• Amino acids are the

monomers which are polymerized to produce proteins, The synthesis of proteins is known as translation

• Translation occurs in the cytoplasm where the ribosomes are located. Ribosomes are made of a small and large subunit which surrounds the mRNA. In translation, messenger RNA (mRNA) is decoded to produce a specific polypeptide according to the rules specified by the genetic code.

http://library.thinkquest.org/04apr/00217/images/content/ribosome.jpg

Translation

• The ribonucleotides are "read" by translational machinery in a sequence of nucleotide triplets called codons. Each of those triplets codes for a specific amino acid.

• Primary structure of a protein is determined

http://en.wikipedia.org/wiki/Genetic_code http://library.thinkquest.org/C004535/media/translation.gif

Translation

• the reading of codons in mRNA (5' -> 3') requires that the anticodons bind in the opposite direction

• Anticodon: 3' CGA 5' Codon: 5' GCU 3‘• Most of the amino acids are encoded by

synonymous codons that differ in the third position of the codon(wobble amino acid).

Translation• Protein synthesis begins at a start codon

near the 5' end of the RNA. • The small ribosomal subunit, typically

bound to a tRNA containing the amino acid methionine, binds to an AUG codon on the mRNA and recruits the large ribosomal subunit.

• The large ribosomal subunit contains three tRNA binding sites, designated A, P, and E. The A site binds an aminoacyl-tRNA (a tRNA bound to an amino acid); the P site binds a peptidyl-tRNA (a tRNA bound to the peptide being synthesized); and the E site binds a free tRNA before it exits the ribosome.

http://library.thinkquest.org/04apr/00217/images/content/ribosome.jpg

Mutation

• mutations are changes to the nucleotide sequence of the genetic material of an organism

• caused by copying errors in the genetic material during cell division, by exposure to ultraviolet or ionizing radiation, chemical mutagens, or viruses

• In multicellular organisms, mutations can be subdivided into germ line mutations, which can be passed on to descendants, and somatic mutations, which are not transmitted to descendants in animals.

Reference• http://en.wikipedia.org/wiki/Mitosis• http://en.wikipedia.org/wiki/Meiosis• http://en.wikipedia.org/wiki/Genetics• http://en.wikipedia.org/wiki/Cell_cycle• http://en.wikipedia.org/wiki/Translation_%28biology%29• http://en.wikipedia.org/wiki/Eukaryotic_translation• http://en.wikipedia.org/wiki/Mutation#Causes_of_mutation• http://en.wikipedia.org/wiki/Ribosome