Biology

Genetics and Heredity

Concepts/Ideas/Facts:

  1. The second meiotic division is essentially the same as ordinary mitosis but the results differ.
  2. in meiosis nuclei chromosomes different from the chromosomes of the original nucleus are produced
  3. in mitosis chromosome compliments are identical to those of the original nucleus
  4. Genes do not necessarily occur in a fixed position on a chromosome but may move around and attach at places where they share a common base sequence.
  5. in DNA, there are 10 base pairs per complete turn of the double helix
  6. the ration of purines to pyrimidines in DNA is always 1:1
  7. in diploid organisms every gene is present twice.
  8. there are many tRNA molecules that take part in transcription

Definitions:

  1. Meiosis – the process of nuclear division in which the chromosome number is reduced from the diploid (2n) to the haploid (n) number.  During meiosis, the nucleus of a diploid cell undergoes two division, one of which is a reduction division resulting in the production of four daughter nuclei, each containing one-half the number of chromosomes of the original nucleus.  The end result of meiosis is that the genetic material present in the diploid nucleus has divided twice.
  2. Fertilization (syngamy) – process by which two haploid cells (gametes) fuse to form a diploid zygote.  Fertilization reestablishes the diploid chromosome number.
  3. Spore – a cell that, without fusing with another cell, develops into a mature haploid organism.
  4. Homologs (homologous chromosomes) – pairs of chromosomes formed from the two sets of diploid chromosomes in diploid organisms
  5. Histones – positively charged, DNA-binding proteins, that play a role in packing the DNA into the nucleus in an orderly manner.  They contain high concentrations of the amino acids, arginine and lysine.
  6. Chromatin – DNA and proteins which together make up chromosomes.
  7. DNA – negatively charged structure that holds genes
  8. Nucleosomes – repeating structures of histones that bind tightly around DNA.  They look like beads-on-a-string
  9. Heterochromatin – chromosomes packaged in a highly condensed state which prevents mRNA transcription
  10. Euchromatin – not condensed chromosomes
  11. Chromomeres – tightly coiled regions of DNA
  12. Gamete – a cell that unites with another gamete to produce a diploid zygote.
  13. Spore – a cell that can develop into an organism without uniting with another cell
  14. Zygote – the diploid (2n) cell resulting from the fusion of male and female gametes
  15. Synapsis – the pairing of homologous chromosomes
  16. Bivalent – the associated pairs of homologous chromosomes
  17. Chiasma (chiasmata) – X-like configuration of chromosomes due to genetic crossing-over.
  18. Crossing-over – the breaking and rejoining chromosomes or the exchange of corresponding segments of genetic material between chromatids of homologous chromosomes occurring in prophase I of meiosis.  The greater the distance between two genes on a chromosome, the greater the chance of crossover.
  19. Synaptonemal Complex – occurs when homologous chromosomes are close together during prophase I.
  20. Continuous Variation – the variation shown by a bell-shaped curve on a graph which shows variation as continuous and indicates that the variation cannot be divided into a series of sharply contrasting forms.
  21. Genetic Maps – maps can be constructed based on the amount of crossing-over that occurs between genes providing an idea of gene position on chromosomes.
  22. Degenerate – the fact that certain amino acids are specific to more than one amino acid.
  23. Translation – process of assembling protein molecules from information encoded in mRNA.  It involves the transfer of information from one language (nucleotides) to another (amino acids).
  24. Polypeptide (protein) Targeting or Sorting – a process that begins in the ground substance whereby polypeptides encoded by nuclear genes are synthesized and are eventually directed to the right cellular compartments.
  25. Signal Sequence (signal peptide) – a signal sequence of hydrophobic amino acids that causes the mRNA-ribosme complex to bind to the endoplasmic reticulum.  Eventually the ribosomal units separate from the mRNA and the polypeptide ends up sequestered in the lumen of the endoplasmic reticulum.
  26. Inducible Enzymes – enzymes that are lacking in an organism but are produced only when a certain substance is present.
  27. Repressibe Enzymes – enzymes that are not produced when a particular amino acid is present

Meiosis: consists of two successive nuclear divisions producing four nuclei (pg126)

First Meiotic Division

  1. Prophase I – chromosomes present in the diploid number first become visible as long, slender threads.  Appearing to be single rather than double at this early stage meiosis, chromosomes at this time have already duplicated and consist of two identical chromatids attached at the centromere.  Homologous chromosomes, one from each parent, pair together forming homologs of two identical chromatids.  Thus, a homologous pair consists of four chromatids.  The pairing of homologous chromosomes is called synapsis and the associated pairs of homologous chromosomes are called bivalents.  During this phase the chromosomes thicken and shorten and it is at this time that crossing-over occurs.  At the end of prophase I the nuclear envelope breaks down and the nucleolus disappears.
  2. Metaphase I – spindles appear and become attached to the centromeres of the chromosomes of each bivalent (paired homologus chromosomes) then move randomly to the center of the cell with the centromeres lined up on opposite sides of the equatorial plane but not directly on the equatorial plane.
  3. Anaphase I – begins when homologous chromosomes separate and move towards the cell poles.  The centromeres do not separate and the sister chromatids remain together, it is the homologs that separate.
  4. Telophase I – chromosomes relax and uncoil and become elongated and indistinct, a new nuclear envelop develops, and the spindles disappear.  Four new nuclei, each with the haploid number of chromosomes, are formed.

Second Meiotic Division – essentially the same as ordinary mitosis

  1. Prophase II – chromosomes reappear, each consisting of two chromatids but due to crossing-over, they no longer are identical
  2. Metaphase II – chromosomes line up at the equatorial plane with the centromeres on the plane.
  3. Anaphase II – centromeres of each chromosome divide and move towards opposite poles
  4. Telophase II – chromosomes completely separated and new cell walls are forming

Genetics:

  1. Medelian Genetics – branch of genetics that deals with relatively clear-cut traits and their inheritance.
  2. Monohybrid Crosses – crosses between individuals that differ in one single trait
  3. Dihybrid Crosses – crosses between individuals that involve two traits
  4. Allele – one of two or more alternative forms of the same gene.  Alleles occupy the same site (locus) on homologous chromosomes.  Each diploid cell has two alleles for each gene, one on each of the homologous chromosomes.
  5. Locus –gene site on a chromosome
  6. Dominant – gene or allele that covers or is dominant over recessive alleles.  It is expressed in the phenotype.
  7. Recessive – gene whose phenotype is hidden or masked by the dominant allele.  It is nsot expressed in the phenotype.
  8. Genotype – genetic make-up
  9. Incomplete Dominance – occurs when the phenotype is intermediate between the phenotype of the parent homozygotes, since the action of one allele does not completely mask the action of the other.
  10. Phenotype – appearance or apparent characteristic of a genotype (what you see, or measure)
  11. Homozygous – having two identical alleles at a particular locus on a homologous chromosome
  12. Heterozygous – having different alleles at a particular locus on a homologous chromosome
  13. Testcross – crossing of an individual showing a dominant characteristic with a second individual that is homozygous recessive for that trait.
  14. Principle of Segregation (Mendel’s First Law) – hereditary traits are determined by discreet factors (genes) that appear in pairs, one of each pair being inherited from each parent.  Thus, each pair of factors is separated or segregated.
  15. Principle of Independent Assortment – the inheritance of a pairs of factors for one trait is independent of the simultaneous inheritance of factors for other traits; in other words factors assort independently, as though no other factors were present.  This is due to the random manner in which the centromeres of the bivalents line up on either side of the equatorial pane at metaphase I.
  16. Linkage – genes closely located near each other generally will not segregate independently and are held together as if they were “linked” to each other.
  17. Polygenic Inheritance – the interaction of various genes in combination with each other that produce a continuous pattern of variations or produce complex characteristics in organisms.
  18. Transporons – small, mobile portions of genes that migrate from one chromosomal position to another at random.
  19. Pleiotrophy – phenomenon whereby a single gene controls whole complexes of traits
  20. Epistasis – phenomenon where most traits are controlled by the combined effect of several or many genes.  One gene modifies the phenotypic expression of another, non-allelic gene.

Mutations:

  1. Mutation – hereditary change in one of the alleles of a gene or any change in the genetic message of an organism.  This may involve alterations in the coding sequence itself, or changes in the way in which the genetic message is organized.  Mutation allows organisms in a species to vary and to adapt to changing conditions and provide the basis fro evolutionary change.  Mutations in eukaryotes occur spontaneously at the rate of about 5 x 10-6 per locus per cell division (1 mutant gene per locus per 200,00 cell divisions)
    1. Point Mutations – involve only one or a few nucleotides and arise from chemical or physical damage to the DNA
    2. Deletion – occurs when small sections of the chromosome is deleted.  Here, certain nucleotides are deleted, which affects the coding of proteins that use this DNA sequence. If for example, a gene coded for alanine, with a genetic sequence of C-G-G, and the cytosine nucleotide was deleted, then the alanine amino acid would not be able to be created, and any other amino acids that are supposed to be coded from this DNA sequence will also be unable to be produced because each successive nucleotide after the deleted nucleotide will be out of place.
    3. Insertion – Similar to the effects of deletion, where a nucleotide is inserted into a genetic sequence and therefore alters the chain thereafter. This alteration of a nucleotide sequence is known as frameshift.
    4. Substitution – certain nucleotide is replaced with another, which will affect any amino acid to be synthesized from this sequence due to this change. If the gene is essential, i.e. for the coding of haemoglobin then the effects are serious, and organisms in this instance suffer from a condition called sickle cell anaemia.
    5. Position Effect – the random movement of small, mobile portions of a chromosome from one chromosomal position to another which may disrupt the action of their neighbors, or vice versa, and lead to effects that we recognize as a mutation.
    6. Inversion – occurs when a sequence of genes breaks off from a chromosome and reorients itself in the direct opposite position of its original position on the chromosome.
    7. Translocation – occurs when a sequence of genes breaks off from one chromosome and reattaches itself on another chromosome.  Often translocations are reciprocal.  Usually this sort of mutation is lethal.
    8. Changes in chromosome number – whole chromosomes may be added or subtracted from the basic set
    9. Polyploidy – the adding of whole sets of chromosomes to the basic set (duplicating of chromosomes)
  2. Mutant – organism carrying a mutation
  3. Mutagens – substances that cause mutations

Nature of DNA

  1. it must carry a great deal of genetic information from cell to cell and from generation to generation
  2. it must be able to copy itself with great precision
  3. genes must be able to sometimes mutate
  4. it must have some mechanism for “reading out” the stored information and translating it into action in living individuals.

DNA Structure:

  1. DNA has a double helix structure
  2. two sides are made up of alternating deoxyribose sugar molecules and phosphate groups
  3. the rungs are formed by the nitrogen bases – adenine (A) pairing with thymine (T) and cytosine (C) pairing with guanine (G)
  4. there is one base for each sugar, with two bases forming each rung.
  5. paired bases are joined by hydrogen bonds and are always purine-pyrimidine combinations,  Adenine forms two hydrogen bonds with thymine and guanine forms three hydrogen bonds with cytosine.
  6. sugar of each nucleotide is linked by a phosphate group (P)
  7. strands of DNA on either side of the helix run antiparallel, that is they run opposite and upside down.
  8. DNA is synthesized only in the 5-carbon end to 3-carbon end direction

DNA:

  1. DNA Replication – process of how DNA copies itself
  2. Origin of Replication – specific nucleotide where DNA replication begins
  3. Helicases – special initiator proteins and enzymes which break the hydrogen bonds linking complimentary bases at the origin of replication, opening up the helix so replication can occur
  4. DNA Polymerases – enzymes that synthesize new strands of DNA
  5. DNA Replication Forks – localized areas of DNA replication that appear as Y-shaped structures
  6. Leading Strand – first strand to form after DNA  “unzips” and is sequentially continuous.
  7. Lagging Strand – second strand to form at a slower rate after DNA “unzips” and is discontinuous.
  8. Okazaki Fragments – the short DNA fragments that synthesize on the lagging strand of DNA
  9. DNA Ligase – enzyme that join Okazaki fragments to the growing DNA strand
  10. Topoisomerases – enzymes that prevent DNA tangling during replication.
  11. RNA primase – enzyme that synthesizes RNA primer which initiates the beginning of a new DNA strand
  12. Codon – sequence of three nucleotides which specifies a single amino acid
  13. Genetic Code – code by which the sequence of nucleotides in mRNA is translated into amino acids
  14. Nulceotide – monomer of DNA and RNA consisting of a nitrogen base (cytosine, guanine, thymine, adenine, and uracil in RNA), a sugar and a phosphate group

RNA:

  1. RNA Transcription (tRNA) – synthesis of RNA along an “unzipped” DNA strand
  2. RNA Polymerases – enzymes that transcribe RNA synthesis
  3. Uracil –  the amino acid  which replaces thymine in RNA transcription
  4. Promoters – specific nucleotide sequences of DNA act as start signals for RNA transcription
  5. Terminators – specific nucleotide sequences of DNA that stop RNA transcription
  6. Messenger RNA (mRNA) – working copies of the genetic information that dictate the amino acid sequence in proteins.  In prokaryotic cells, most proteins are encoded by a continuous segment of DNA from which a functional mRNA molecule is copied.  In eukaryotic cells there is a gene coding sequence, called exons, interrupted by non-coding sequences, called introns
  7. Introns – segments of DNA of a eukaryote gene, which are transcribed into mRNA and then are excised from the RNA leaving behind exons to be translated into proteins.
  8. Exons – segments of DNA of a eukaryote gene, which are transcribed into mRNA and then into proteins
  9. Primary Transcription – the entire copied gene in RNA transcription with both intron and exon sequences
  10. RNA Splicing – the removal of introns by special RNA processing enzymes and the splicing of the remaining exons together before the molecule leaves the nucleus.
  11. Transfer RNA (tRNA) – at least one for each of the 20 amino acids, tRNA molecules are small, consisting of about 80 nucleotides that form a single strand that fold back on itself.  tRNA translates mRNA into the language of proteins. N The tRNA chain always terminates in a CCA sequence (codon).
  12. Anticodons- bind the codon of a mRNA to the anticodon of tRNA.  Anticodons serve to “plug in” the tRNA molecule to an mRNA codon.  It is basically the opposite (complimentary) amino acids.
  13. Aminoacyl-tRNA synthetases – enzyme that attaches tRNA molecules to a particular amino acid.
  14. Ribosomal RNA (rRNA) – the RNA that manipulates protein synthesis and is associated with ribosomes
  15. Ribosomes – large protein-synthesizing machines on which tRNA molecules position themselves in precise relationship to mRNA molecules so as to read accurately the genetic message encoded in the mRNA.   Two sites involved in protein synthesis:
    1. A site (aminoacyl site) – the binding site for incoming tRNA, with its amino acid
    2. P site (peptidyl site) – site to which the growing polypeptide chain is attached.  The   enzyme peptidyl transferase forges the peptide bond between the first amino acid and the second amino acid.

   16.  Polyribosome or Polysome – a group of ribosomes translating the same mRNA molecule

Stages in Translation:

  1. Initiation – begins when smaller ribosomal units attach to a strand of mRNA exposing its first codon, the initiation codon.  The first tRNA then pairs with the initiation codon of the mRNA in an anti-parallel fashion.
  2. Initiation Codon – where translation begins
  3. Initiation Complex – combination of the small ribosomal subunit, mRNA, and the initiator tRNA.
  4. Elongation – period of the growth of the polypeptide chain as amino acids are added to the chain
  5. Chain Termination – the encountering of a stop codon on mRNA which terminates the translation of a polypeptide.  Once a stop codon is encountered, no tRNA recognizes these codons, so translation ceases.  At this point a release factor from the cytoplasm binds to the stop codon and translation is complete.  

Enzyme Biosynthesis Regulation:

  1. Operon – consists of a promoter, one or more structural genes, and an operator
  2. Structural Genes – genes that code for proteins, often enzymes that work sequentially in a particular reaction pathway.
  3. Operator – sequence of nucleotides located between the promoter and the structural genes.  It may overlap the promoter, and /or the structural gene(s).  It is the site at which a repressor protein can bind.
  4. Regulator – located anywhere on the bacterial chromosome, it codes for a protein called the repressor, which binds to the operator, obstructing the promoter.  No mRNA transcription can occur if this occurs.
  5. Effector – depending on the operon, an effector can either activate or inactivate the repressor for that particular operon.
  6. Repressor – protein that precedes the operator, that binds to the operator and obstructs (inhibits) the promoter.  It is the binding site for RNA polymerase.
  7. Promoter - specific nucleotide sequences of DNA act as start signals for RNA transcription

“Central Dogma” of Genetics:

Gene ΰ mRNA ΰproteins

                    or

Gene ΰ primary transcription ΰ mRNA ΰ protein

Websites:

http://www.biology-online.org/i