DNA Structure, Replication, and Chromosomes

DNA Structure

  • DNA has a ladder-like spiral structure called a double helix.

  • During the cell cycle:

    • Interphase: DNA is in chromatin form.

    • Mitosis: DNA is in chromosome form.

  • A DNA molecule comprises millions of subunits called nucleotides.

Nucleotides

  • Each nucleotide consists of:

    • Phosphate group

    • Pentose sugar

    • Nitrogenous base

Structure of a Nucleotide

  • Pentose Sugar

    • A 5-carbon sugar.

    • In DNA, the sugar is deoxyribose.

    • In RNA, the sugar is ribose.

Pentose Sugar Details

  • Carbons are numbered 1 through 5.

  • RNA (Ribose) vs. DNA (Deoxyribose):

    • The difference lies in the hydroxyl group on the #2 carbon.

Nucleotide: Phosphate Group

  • Phosphate Group

    • Links two sugars together.

Primary Structure of DNA

  • The sugar-phosphate backbone of a DNA strand.

  • Phosphodiester bonds link deoxyribonucleotides.

  • Nitrogen-containing bases project from the backbone.

  • Strand has a 5' end and a 3' end.

Phosphodiester Bonds

  • Join two sugars via the phosphate group.

Formation of Phosphodiester Bond

  • Chemical bonds form, releasing H2O.

Structure of a Nucleotide: Nitrogenous Base

  • Nitrogenous Base

    • In DNA, the four bases are: Thymine, Adenine, Cytosine, Guanine.

    • In RNA, the four bases are: Uracil, Adenine, Cytosine, Guanine.

DNA Strand

  • A strand of nucleotides is joined by covalent bonds (very strong).

  • The sequence of bases makes up the genetic code.

  • Bases are read in triplets.

    • These triplets send instructions to switch genes on and off, to make proteins and enzymes.

The 4 Nucleotides

  • Purines: Adenine (A) and Guanine (G)

  • Pyrimidines: Cytosine (C), Thymine (T), and Uracil (U; in RNA)

The 4 Nucleotides (Simplified)

  • Adenine (A)

  • Thymine (T)

  • Cytosine (C)

  • Guanine (G)

Nitrogenous Base Pairing

  • Each base bonds with one other specific base, called complementary base pairs.

    • Adenine (A) pairs with Thymine (T) - 2 hydrogen bonds.

    • Cytosine (C) pairs with Guanine (G) - 3 hydrogen bonds.

    • In RNA, Uracil (U) binds to Adenine (A).

Nitrogenous Base Pairing (Hydrogen Bonding)

  • Due to complementary base pairing, the order of bases in one strand determines the order of bases in the other strand.

Mutations

  • Mutation (genetic mutation) is a change in the nucleotide sequence of DNA.

  • Example of normal vs. mutated DNA sequence.

Nature of Genetic Material

  • Property 1: Contains, in a stable form, information encoding the organism’s structure, function, development, and reproduction.

  • Property 2: Replicates accurately so progeny cells have the same genetic makeup.

  • Property 3: Capable of some variation (mutation) to permit evolution.

DNA Replication

  • Two DNA molecules are made from one DNA molecule.

  • Each new DNA molecule (double helix) has one strand from the original molecule and one new strand.

DNA Replication Outcome

  • Parent DNA molecule and two daughter molecules are identical.

DNA Replication - Base Pairing

  • Adenine binds with Thymine.

  • Cytosine binds with Guanine.

  • Knowing the order of one strand allows you to know the order of the other.

DNA Replication Process

  • Step 1: Enzyme (helicase) breaks the hydrogen bonds between nitrogenous bases, causing the DNA helix to unzip or unwind.

  • Step 2: As DNA unzips, DNA polymerase moves along the single strands and helps each free nucleotide bind to a new corresponding base.

  • Step 3: A new strand is formed by pairing complementary bases with the old strand (A-T, C-G).

  • The process continues until the original molecule has been entirely unzipped and replicated, producing two new DNA molecules.

  • One strand acts as a template to make a new strand.

  • The process is semiconservative because each daughter strand is paired with a parent (original) strand (one old and one new).

Inheritance

  • Passing on genetic information from parents to offspring.

Chromosomes

  • Each chromosome contains many genes.

Gene Definition

  • A segment of a chromosome that contains the code for a single protein (enzyme).

  • The enzyme causes a chemical reaction that allows a trait to be expressed (hair/eye color, etc.).

Alleles

  • Alleles are alternate/different forms of a gene.

  • Homozygous individuals have two identical alleles for a trait.

  • Heterozygous individuals have two different alleles for a trait.

Chromosome Count

  • The number of chromosomes varies from species to species.

  • The number of chromosomes does not correlate with the complexity of organisms.

  • Human somatic cells have 46 chromosomes (organized into 23 pairs).

  • Each pair consists of one chromosome from the father and one from the mother.

Sex Chromosomes & Autosomes

  • Sex Chromosomes: 1 pair out of 23 (X and Y), determine individual’s sex (Female XX, male XY).

  • Autosomes/Somatic Chromosomes: 22 pairs out of 23; all chromosomes but sex chromosomes, paired based on similar characteristics.

Sex Chromosomes & Autosomes (Visual)

  • Illustration of autosomes and sex chromosomes.

Homologous Chromosomes

  • Pairs of chromosomes that are similar but NOT identical based on:

    • Length

    • Centromere location

    • Banding patterns (when dyed)

  • Carry genes for the same trait at the same location.

Homologous Chromosomes (cont.)

  • They are not identical because they carry different forms of the same gene (different DNA pattern/alleles).

  • Genes are sections of DNA that contain genetic information for specific traits (e.g., hair/eye color).

Karyotype

  • A photograph of a particular set of chromosomes for an individual.

  • Prepared by taking a sample cell in metaphase, staining the chromosomes to reveal banding patterns, then sorting and pairing them.