Cell Growth, Specialization, and Death: Nucleic Acids – DNA & RNA

Nucleic Acids – DNA & RNA

Specific Learning Objectives

• Describe the structure and functions of proteins.
• Describe the structure of nucleic acids, differentiating between DNA and RNA.
• Define the components of a nucleotide.
• Differentiate between the nucleotide bases of DNA and RNA.
• Explain what the genetic code is and what it is coding for.
• Describe the two-step process (transcription & translation) that results in gene expression.
• Explain the role of DNA, rRNA, tRNA and mRNA in the production of a protein.
• Describe the cell cycle.
• Give the details of DNA replication.
• Explain what occurs during mitosis and cytokinesis.
• Describe how mitosis differs from meiosis.

Textbook Readings: Nucleic acids

• First discovered in nuclei of cells, found in all cells
• Organic macromolecules (C, H, O, N, P), main information carrying molecules
• Polynucleotides – a chain of repeating monomers called nucleotides

Nucleotide

• A pentose sugar – deoxyribose, ribose
• A phosphate group
• A nitrogenous base – adenine, guanine, cytosine, thymine, uracil

Nucleoside

• Pentose sugar + nitrogenous base

Nucleotide

• Nucleoside + phosphate group
• Sequence of nitrogenous bases carries the information
• Two major classes – Deoxyribonucleic Acid (DNA), Ribonucleic Acid (RNA)

DNA: Deoxyribonucleic Acid

• Mainly in the nucleus, but also in mitochondria.
• Constitutes the blueprint that codes for protein synthesis.
• Approximately 20,000 – 25,000 genes in the human genome.
• Only 1.5% of DNA is due to genes.
• 98.5% of DNA is non-coding – e.g., regulatory sequences, introns, and noncoding DNA – e.g., repeat elements.

DNA - Structure

• Watson and Crick (1953) – double helix model
• Double-stranded polymer - two polynucleotide chains, antiparallel
• Alternating sugar-phosphate backbone
• Pentose sugar - deoxyribose
• Complementary nitrogenous bases form rungs of the ladder
  • adenine - thymine
  • guanine - cytosine
• Nitrogenous bases held together by hydrogen bonds
• Base pairing rule: A=T, C=G

Organisation of DNA

• Double strands of DNA – twisted ladder
• DNA wrapped around proteins called histones.
• Histones & DNA bundled together is called chromatin
• Chromatin twists and condense to form chromosomes
• Each chromosome contains hundreds to thousands of genes

Quantity of DNA

• Each somatic human cell nucleus has two copies of each chromosome – one inherited from Mum and the other from Dad.
• Somatic cells with 46 chromosomes (23 pairs) are said to have the full amount of DNA - diploid
• The maternal and paternal chromosomes of a pair are called homologous chromosomes (make a “homologous pair)
• Gametes (sperm and egg) only have 1 chromosome of each homologous pair (have 23 chromosomes), have half the normal amount of DNA - haploid
• Humans have:
  • 22 pairs of autosomal chromosomes and 1 pair of sex chromosomes
    • Women have 2 X chromosomes (22 + XX)
    • Men have an X and a Y chromosome (22 +XY)
• When cells are dividing, the chromatin condenses to form chromosomes, is easier to see and can be arranged next to their pair – this kind of map is a karyotype

Karyotype

• A map of chromosomes in a dividing cell
• Male karyotype (22 autosomes + XY)

RNA: Ribonucleic Acid

• Single stranded polymer, self complementary sequences forms folds, bulges and helices
• Supports DNA during protein synthesis
• Found both in the nucleus and cytoplasm
• Alternating sugar-phosphate backbone
• Pentose sugar – ribose
• Nitrogenous bases
  • adenine - uracil
  • guanine - cytosine
• Three types – mRNA, tRNA, rRNA
• Different relative sizes, shapes and roles to play in protein synthesis

Terminology

• Genetics - study of heredity
• Gene – segment of DNA that codes for a protein
• Allele – alternative form of a gene
• Genotype – the actual gene (AA, Aa, aa)
• Phenotype - person’s appearance (blue eyes, brown hair)
• Dominant and recessive alleles
• Sex-linked traits: traits affected by genes on sex chromosomes

Compendium questions

1. Why do we need proteins in the body?
2. What is a nucleic acid?
3. What are the similarities and differences between DNA and RNA?
4. What is a gene?
5. What is the connection between DNA, RNA and protein synthesis?
6. Describe transcription and translation.
7. What is meant by the terms phenotype and genotype?
8. What is the difference between a chromosome and a chromatid?
9. What are the stages of mitosis?
10. What are the stages of meiosis?
11. Which cell types undergo each type of cell division?.