DNA Structure and Protein Synthesis

DNA Structure

• DNA is a polymer called a nucleic acid, with nucleotides as its monomers.
• Nucleotides consist of a 5-carbon sugar (deoxyribose in DNA, ribose in RNA), a phosphate group, and a nitrogenous base.
• DNA contains the nitrogenous bases adenine (A), thymine (T), cytosine (C), and guanine (G). RNA contains uracil (U) instead of thymine (T).
• Chargaff's rule states that A always pairs with T (or U in RNA), and G always pairs with C.
• The nitrogenous bases are connected by hydrogen bonds.
• DNA has an antiparallel structure where the 5' end of one strand faces the 3' end of the other strand.
• DNA forms a right-handed helix structure.

Protein Synthesis

• Messenger RNA (mRNA) carries genetic instructions from DNA to ribosomes for protein synthesis.
• Transfer RNA (tRNA) brings specific amino acids to the ribosomes during translation.
• Ribosomal RNA (rRNA) combines with proteins to form ribosomes.
• Transcription is the process where the DNA code is copied into mRNA.
• Translation is the process where mRNA is read by tRNA, and a polypeptide chain (protein) is built.
• Three nitrogenous bases form a codon, and each codon corresponds to a specific amino acid.

DNA Replication

• Helicase separates the two DNA strands by breaking hydrogen bonds.
• DNA polymerase brings and binds complementary nucleotides to the original strands.
• DNA polymerase builds continuously in the 5' to 3' direction on the leading strand.
• On the lagging strand, DNA polymerase builds discontinuously, forming Okazaki fragments.
• DNA ligase seals the Okazaki fragments together on the lagging strand to create a continuous molecule.
• DNA replication is semi-conservative, resulting in each new DNA molecule containing one original strand and one newly synthesized strand.
• In eukaryotes, replication can begin at multiple sites simultaneously due to the long chromosomes.

Mutations

• Mutations are changes in the DNA sequence that can occur due to errors during DNA replication.
• Most mutations have no effect.
• Mutations can be inherited or acquired.
• Point mutations involve a change to a single nucleotide in a strand.
• Substitution mutations occur when one nucleotide is replaced by another.
  • Missense mutations change one amino acid.
  • Nonsense mutations change a codon to a stop codon.
  • Silent mutations have no noticeable effect due to redundancy in the genetic code.
• Frameshift mutations shift the reading frame of the genetic code.
  • Insertion mutations add a nucleotide.
  • Deletion mutations remove a nucleotide.
• Frameshift mutations are generally more severe as they affect all codons after the mutation.

Cell Cycle and Mitosis

• The cell cycle is a cycle of cell growth and division.
• One cell (mother) produces an identical copy (daughter) through mitosis.
• Mitosis occurs only in somatic (body) cells.
• The three main phases of the cell cycle are interphase, mitosis, and cytokinesis.
• Interphase consists of three subphases:
  • G1: Cell grows, and cellular components (except chromosomes) are duplicated.
  • S: Each chromosome is duplicated.
  • G2: Cell checks for errors and prepares for mitosis.
• The stages of mitosis are:
  • Prophase: Chromatin condenses into chromosomes (sister chromatids linked by centromeres), and centrosomes form mitotic spindles.
  • Prometaphase: The nucleus breaks apart, and centrioles move to opposite poles of the cell.
  • Metaphase: Centrioles reach the poles, and chromosomes align along the metaphase plate. The M checkpoint ensures that sister chromatids are attached to opposite ends of the mitotic spindle.
  • Anaphase: Sister chromatids are pulled apart, creating two separate sets of the genome, and the cell elongates.
  • Telophase: Two nuclei are formed, and chromosomes unwind back into chromatin.
• Cytokinesis: The cleavage furrow is pulled apart until two separate, distinct cells are formed.

Meiosis

• Meiosis is the process by which gametes (sex cells) are created.
• Daughter cells have half the DNA of the parent cell (haploid).
• Meiosis involves one round of DNA replication and two rounds of cell division.
• Replicated chromatids (sister chromatids joined at a centromere) are called dyads. Overlapping dyads form a tetrad.
• Crossing over occurs when overlapping homologous chromosomes in tetrads exchange genes.
• Independent assortment is the random separation of homologous chromosomes in Meiosis I. Random fertilization also contributes to genetic variation.
• A diploid cell has a full set of chromosomes, while a haploid cell has half a set.
• A fertilized egg is called a zygote.

Meiosis I

• Prophase I: Homologous chromosomes pair up and exchange genetic material (crossing over), forming tetrads.
• Metaphase I: Homologous pairs line up at the cell's center (metaphase plate).
• Anaphase I: Homologous pairs separate, moving to opposite poles of the cell.
• Telophase I: The cell divides, forming two haploid daughter cells.

Meiosis II

• Prophase II: Chromosomes condense again if they became dispersed in telophase I.
• Metaphase II: Sister chromatids line up at the metaphase plate.
• Anaphase II: Sister chromatids separate and move to opposite poles.
• Telophase II: The cell divides, resulting in four haploid daughter cells.
• Cytokinesis: Four distinct gametes are formed.

Gene Expression and Regulation

• A karyotype is an organized profile of an individual's chromosomes.
• Two chromosomes specify sex: XX (female) or XY (male).
• The remaining chromosomes are arranged in pairs by homology, from largest to smallest.
• Genes are either expressed or repressed based on an individual cell's needs.

Prokaryotic Gene Regulation

• Operons consist of one or more genes that encode proteins necessary for a specific function.
• Components of an operon include:
• Promoter: DNA sequence where RNA polymerase binds and begins transcription.
• Repressor: Binds to the operator to turn off transcription.
• Operator: DNA sequence that controls RNA polymerase's access to structural genes.
• Genes: DNA fragments that transcribe into RNA.
• Inducer: Prevents the repressor from binding to the operator.
• Regulatory gene: The gene that codes for the repressor protein.
• Inducible operons are normally off but can be turned on.
• Repressible operons are normally on but can be turned off.

Cancerous Cells

• Benign tumors are not dangerous.
• Malignant tumors are dangerous and invasive.
• Size does not determine the danger of a tumor; malignant tumors are dangerous because they can metastasize, spreading and colonizing via the bloodstream.
• Blood vessels provide oxygen and nutrients to cancerous tumors.
• Cancer is rarely hereditary.
• Mutations in certain genes can lead to cancerous growth:
• Oncogenes stimulate growth, and mutations can cause them to work uncontrollably.
• Suppressor genes normally inhibit growth, and mutations can cause them to stop functioning.
• Repair genes normally limit mutations, and mutations can cause them to stop functioning.
• Cancerous cells lack contact inhibition, leading to uncontrolled replication and tumor formation in three dimensions.