Gr12 Life Sciences

• Deoxyribonucleic acid (DNA) is a molecule that contains genetic information or instructions that controls the production of specific proteins that are essential for life.
• Location of DNA

  • DNA is mainly found in the nucleus.
  • On the chromosome, the DNA is divided into specific parts that encode certain characteristics. We call these parts genes.
  • Each gene has its own locus (position) on the DNA strands.
  • Extranuclear DNA is found outside the nucleus, in the mitochondria (mtDNA) and in the chloroplasts.
• The Structure of DNA:

  • Double-stranded
  • Twisted and folded to form a double helix.
  • Made of repeating units called nucleotides (monomers).
  • Nucleotides are the foundation for genetic code.
• Each nucleotide consists of:

  • One phosphate group
  • Deoxyribose sugar
  • One nitrogen containing base
• There are four nitrogenous bases:

  • Adenine (A) and Thymine (T)
  • Guanine (G) and Cytosine (C)
• Complimentary base pairs:

  • A purine (A or G) always bind to a pyrimidine (C or T)
  • A and T are complimentary base pairs. They bind together.
  • G and C are complimentary base pairs. They bind together.
  • We therefore always have the same amounts of A and T in a DNA molecule.
  • We therefore always have the same amounts of G and C in a DNA molecule.
• • Less than 2% of DNA contains codes for proteins.
  • The rest is known as non-coding DNA.
  • Form functional RNA molecules.
• • Mitochondrial DNA is the small circular chromosome found inside the mitochondria.
  • These organelles found in cells have often been called the powerhouse of the cell.
  • The mitochondria, and thus mitochondrial DNA, are passed almost exclusively from mother to offspring through the egg cell.
• DNA Replication:

  • Process of making a NEW, IDENTICAL DNA molecule  ​
  • ​Takes place in the nucleus during Interphase of the cell cycle  ​
  • Necessary to ensure that genetic code is passed on 
• DNA Replication steps (enzyme controlled process)

  1. DNA molecule unwinds.
  2. DNA molecule unzips – hydrogen bonds between nitrogenous bases are broken by an enzyme.
  3. Free floating nucleotides pair up with their complementary bases on the exposed DNA strands.
  4. DNA polymerase joins the nucleotides to each other (along the backbone) and to the complementary bases (creating the hydrogen bond – “rezipping”)
  5. The two new DNA molecules rewind to form helixes.

  This is semi-conservative DNA replication – one strand is from the previous molecule and one strand in newly built.
• DNA Replication is important because:

  1. Identical copies of DNA are made so that the chromosome number can be kept constant from one generation to the next.

  ​2. It allows the daughter cells after mitosis to be identical to each other and to the cell from which they were formed.​

  ​3. Mistakes caused during replication (mutations) can cause variation in the appearance and functioning of the offspring​. Variation is important for evolution. 
• • Ribonucleic acid (RNA) is a single stranded molecule.
  • It is made up of monomers that we call nucleotides.
  • In RNA, the nucleotides consist of 3 parts: a phosphate group, a ribose sugar molecule and a nitrogen base.
  • The nitrogen base Uracil (U) now replaces Thymine (T).
• Messenger RNA (mRNA)

  Nucleus

  Single strand Long

  Codon = every 3 bases

  Copy the genetic information from the DNA molecule to the ribosomes in the cytoplasm
• Transfer RNA (tRNA)

  Cytoplasm

  Fold over itself

  Anticodon on one side Amino acid folds over

  Carries specific amino acids to the ribosome. Attaches amino acid to developing peptide chain.
• Ribosomal RNA (rRNA)

  Ribosome 

  Single strand

  Largest part of the ribosome

  No definitive form

  Place for protein synthesis
• • Proteins are very large, organic molecules.
  • A protein consists of monomers that we call amino acids.
  • There are 20 different amino acids.
  • Amino acids are bound by peptide bonds to form polypeptide chains.
  • Proteins are produced according to the genetic code of DNA.
  • Protein synthesis takes place in 2 stages: Transcription and Translation
• Transcription:

  • Takes place in nucleus​
  • A section of the DNA molecule unwinds​ (the section for that specific protein)
  • Weak hydrogen bonds between the nucleotides break under enzyme action.​
  • One DNA strand acts as a template​
  • Free floating nucleotides pair with the complimentary bases on the DNA template (with T being replaced with U​)
  • To form a (single stranded) mRNA molecule​
  • mRNA strand leaves the nucleus through the nuclear pore into the cytoplasm.​
  • The DNA strands then re-join/rezip and rewind to form a helix​
• Translation:

  • Takes place in cytoplasm
  • mRNA attaches to a ribosome in the cytoplasm
  • According to the codons (3 nucleotide bases) of mRNA​
  • complementary anticodon of the tRNA molecule pairs with the mRNA codon
  • Each specific tRNA brings a specific amino acid for the codon
  • The mRNA strand moves through the ribosome to read all codons.
  • The Amino acids link together by peptide bonds .
  • To form the required protein. (less than 51 amino acids makes it a polypeptide chain)
  • A number of polypeptide chains can be folded together to form a protein.
• DNA can be extracted from a person's body to obtain your “barcode” pattern. 

  • The bars correspond to the base pairs you inherit from your parents.
  • This is known as a DNA profile.
  • Each person has a unique DNA profile, except identical twins.
  • At crime scenes, DNA samples can be collected and used in court as forensic witnesses / evidence.
  • Skin, blood, saliva, semen and hair are DNA samples that can be collected for forensic evidence.
• Uses of DNA Profiles:

  • To identify crime suspects. 
  • Do paternity tests.
  • To determine if a person has any genetic disorders.
  • To see if tissue types match when an organ transplant needs to be done.
  • To identify missing persons. (NOT LOCATE)
  • To identify unrecognizable bodies.

   
• Chromosomes are found in the nucleus of all living cells
• A chromosome consists of the nucleic acid DNA
• Double helix DNA is coiled around proteins called histones
• During interphase, when the cell is not dividing, the chromosome appears as a chromatin network
• When the cell is dividing, the DNA shortens and thickens, becoming visible as a chromosome
• Before cell division, genetic material must be replicated through DNA replication
• After DNA replication, the chromosome consists of two identical units called sister chromatids
• Identical sister chromatids are connected by a centromere
• Chromosome number

  Chromosome number of different organisms differs.But every specific type of organism always has the same number of chromosomes.

  For example:

  For example:Maize = 20 chromosomesHuman = 46 chromosomesStrawberry = 56 chromosomes

  NOTE: It is therefore NOT the case that a larger or more complex or advanced organism has more chromosomes. 

  The 46 human chromosomes are organized into 23 pairs, of which one pair is derived from the father and the other pair is derived from the mother. These pairs are known as homologous chromosome pairs.
• Homologous chromosomes

  • The number of chromosomes of organisms that reproduce sexually is always an even number because it consists of two sets of chromosomes.
  • This double set of chromosomes is known as the diploid chromosome number (2n).
  • In humans, the 2n = 46 chromosomes.
  • The chromosome in each set is usually similar in size, shape, and genetic composition.
  • One chromosome in a set is derived from the father and the other pair is derived from the mother.
  • These pairs of chromosomes are known as Homologous chromosomes.
  • It ensures that the organism possesses the characteristics of both parents.

   
• What is Mitosis?

  Mitosis is a type of cell division in which one cell (the mother cell) divides to produce two new cells (the daughter cells) that are genetically identical to the mother cell.
• Functions of Mitosis:

  • Growth of an organism
  • Replacement of worn or damaged tissue
  • Reproduction of unicellular organism
• Phases of Mitosis:

  1.   Interphase

  2.   Prophase

  3.   Metaphase

  4.   Anaphase

  5.   Telophase
• Interphase

  Interphase is the phase just before cell division processes such as mitosis begin.

  Interphase is the period during which the cell's normal metabolic response takes place and when the cell does the work for which it was made.

  During interphase, the chromatin network will always be visible. 
• Prophase

   

  • The Chromatin network shortens and thickens.
  • Chromosomes become visible.
  • Chromosomes replicated during interphase so that there are now 2 sets of identical DNA molecules.
  • The nuclear membrane begins to disappear and the nucleolus disintegrates.
  • Nuclear plasma changes to a gel state.
  • Cell organelles move away from the nucleus.
  • Centrioles double and begin to move to opposite poles.
  • Spindles form between centrioles.
• Metaphase

  Chromosomes are arranged on the equatorial plane of the cell so that the centromere connecting two chromatids lies on a spindle.
• Anaphase

  • The spindles shorten, the centromeres divide in two, and the chromatids separate.
  • The chromatids, now called daughter chromosomes, move to opposite poles.
• Telophase

  • Daughter chromosomes uncoil to form a chromatin network again.
  • The DNA / chromosomes of the two cells are identical to each other.
  • The nuclear membrane and nucleolus reappear.
  • Division of the cytoplasm occurs so that two cells are formed that are identical to each other. The process is known as cytokinesis.
• Process of Meiosis

  Before meiosis occurs, the cell is in Interphase during which DNA replication takes place. After replication, the chromosomes have a dual nature, which is necessary for meiosis to occur.
• Meiosis is divided into two distinct stages:

  - Meiosis I (first meiotic division)

  - Meiosis II (second meiotic division)