DNA E RNA - ÁCIDOS NUCLEICOS - BIOQUÍMICA | Biologia com Samuel Cunha

Key Concepts

Nucleic acids (DNA and RNA), organic compounds, polymers, monomers, nucleotides, phosphate, pentose (deoxyribose and ribose), nitrogenous bases (adenine, thymine, cytosine, guanine, uracil), purines (adenine, guanine), pyrimidines (thymine, cytosine, uracil), double helix, chromosomes, genes, chromatin, histone, DNA replication (semi-conservative), DNA polymerase, RNA types (mRNA, rRNA, tRNA), transcription, translation, protein synthesis, hydrogen bonds, phosphodiester bonds, 5' and 3' ends, Chargaff's rule.

Nucleic Acids: DNA and RNA

Introduction

• Nucleic acids are named for their acidic character and initial discovery in the nucleus.
• They are organic compounds, a key category in biochemistry alongside vitamins, carbohydrates, and lipids.
• While named for their presence in the nucleus, nucleic acids are also found in the cytoplasm (tRNA), mitochondria, chloroplasts, and rough endoplasmic reticulum (rRNA).

Function of Nucleic Acids

• Storage of Genetic Information: DNA stores all genetic information.
• Transmission of Genetic Information: Gametes (sperm and oocytes) transmit genetic information to offspring.
• Protein Synthesis: Nucleic acids facilitate the transfer of genetic information from DNA to the cytoplasm for protein production, which determines physical traits and functions.
• Understanding DNA and RNA is fundamental to understanding protein synthesis and genetics (chromosomes are DNA molecules).

Nucleic Acids as Polymers

• DNA and RNA are polymers, large molecules composed of repeating smaller units called monomers.
• The monomers of nucleic acids are nucleotides.
• Proteins are also polymers, with amino acids as their monomers.

Nucleotide Structure

• A nucleotide consists of three components:
  • Phosphate group: Remains constant.
  • Pentose sugar: A five-carbon sugar. This differs between DNA and RNA.
  • Nitrogenous base: Varies and is represented by the letters A, T, C, G, and U.

Pentose Sugar Differences

• DNA: Contains deoxyribose as its pentose sugar. This is why it's called deoxyribonucleic acid.
• RNA: Contains ribose as its pentose sugar. This is why it's called ribonucleic acid.

Nitrogenous Base Differences

• DNA: Contains the nitrogenous bases adenine (A), thymine (T), cytosine (C), and guanine (G).
• RNA: Contains the nitrogenous bases adenine (A), uracil (U), cytosine (C), and guanine (G).
• RNA does not contain thymine (T), and DNA does not contain uracil (U).

Purines and Pyrimidines

• Nitrogenous bases are classified as either purines or pyrimidines.
• Purines: Adenine (A) and Guanine (G). They have a double-ring structure. Mnemonic: "Pure As Gold" (Pure = Purines, As = Adenine, Gold = Guanine).
• Pyrimidines: Thymine (T), Cytosine (C), and Uracil (U). They have a single-ring structure.
• Visual identification: Purines have two rings in their molecular structure, while pyrimidines have one.

DNA (Deoxyribonucleic Acid)

Discovery and Structure

• The structure of DNA as a double helix was determined in the 1950s by Watson and Crick, building on the work of Rosalind Franklin.
• DNA consists of two strands (double helix) connected by molecular forces, resembling a spiral ladder.

DNA Organization

• A DNA molecule is very long and condenses by wrapping around proteins called histones.
• Further condensation leads to the formation of a chromosome.
• Humans have 46 chromosomes (46 DNA molecules) in each cell (excluding cells like red blood cells).
• A gene is a specific sequence of nucleotides within the DNA molecule.
• A gene contains the information needed to produce a protein.

DNA Base Pairing

• The two DNA strands are connected through specific base pairing:
  • Adenine (A) always pairs with Thymine (T).
  • Cytosine (C) always pairs with Guanine (G).
• This pairing is crucial for DNA replication and transcription.
• Mnemonic devices: AT (Agnaldo Timóteo), GC (Gal Costa).

Nucleotide Structure in DNA

• Each nucleotide in DNA consists of a phosphate group, deoxyribose sugar, and a nitrogenous base (A, T, C, or G).
• The phosphate and sugar components remain constant; only the nitrogenous base varies.

Chemical Bonds in DNA

• Nitrogenous bases on opposite strands are connected by hydrogen bonds.
  • Guanine (G) and Cytosine (C) are connected by three hydrogen bonds (stronger bond).
  • Adenine (A) and Thymine (T) are connected by two hydrogen bonds.
• Nucleotides within a single strand are connected by phosphodiester bonds.

DNA Strand Orientation

• DNA strands have a directionality, referred to as 5' (five prime) and 3' (three prime) ends, based on the carbon numbering of the deoxyribose sugar.
• The two DNA strands are antiparallel, meaning they run in opposite directions (one strand runs 5' to 3', and the other runs 3' to 5').
• DNA is always assembled in the 5' to 3' direction.

DNA Replication

• DNA replication is semi-conservative.
• The double helix unwinds, and each original strand serves as a template for a new strand.
• The enzyme helicase breaks the hydrogen bonds, separating the DNA strands.
• DNA polymerase assembles the new DNA strands, ensuring correct base pairing (A with T, C with G).
• The result is two DNA molecules, each containing one original strand and one newly synthesized strand.

Chargaff's Rule

• In DNA, the amount of adenine (A) is equal to the amount of thymine (T), and the amount of guanine (G) is equal to the amount of cytosine (C).
• This rule can be used to calculate the percentage of each base in a DNA sample.
  • Example: If a DNA molecule has 20% adenine, it must also have 20% thymine. The remaining 60% must be divided equally between guanine and cytosine (30% each).

RNA (Ribonucleic Acid)

RNA Structure

• RNA is a single-stranded molecule, unlike the double-stranded DNA.
• RNA contains uracil (U) instead of thymine (T).

Types of RNA

• mRNA (messenger RNA): Carries genetic information from DNA to the ribosomes for protein synthesis.
• rRNA (ribosomal RNA): A component of ribosomes, the site of protein synthesis.
• tRNA (transfer RNA): Transports amino acids to the ribosomes for protein synthesis.

RNA Function

• RNA copies information from a gene and transports it to the cytoplasm for protein synthesis.
• mRNA carries the genetic code for a specific protein.
• rRNA combines with proteins to form ribosomes.
• tRNA carries specific amino acids to the ribosome, matching them to the codons on the mRNA.

Nucleotide Structure in RNA

• Each nucleotide in RNA consists of a phosphate group, ribose sugar, and a nitrogenous base (A, U, C, or G).
• The phosphate group remains constant, while the nitrogenous base varies.

Protein Synthesis Overview

• mRNA is formed from the information of a gene.
• rRNA associates with proteins to form ribosomes.
• tRNA carries amino acids to the ribosome.
• The ribosome reads the mRNA sequence and uses tRNA to assemble the corresponding amino acids into a protein.

Example Exercise

• Given a DNA sequence, determine the corresponding mRNA sequence.
  • DNA: A T C G G C A T
  • mRNA: U A G C C G U A (U replaces T)

Conclusion

Understanding the structure, function, and interactions of DNA and RNA is crucial for comprehending genetics, protein synthesis, and cellular processes. The key differences between DNA and RNA, including their sugar composition, nitrogenous bases, and structure, are essential for distinguishing their roles in the cell.