Human Genome and DNA Structure

Learning Objectives

  • Define a genome and describe its arrangement in chromosomes (autosomes and sex chromosomes).

  • Describe DNA structure.

  • Explain arrangement of eukaryotic and prokaryotic DNA in cells.

Genome Overview

  • Complete DNA complement in a cell = genome.

  • Prokaryotic genome: single, double-stranded DNA (loop/circle); located in nucleoid.

  • Plasmids: smaller loops of DNA (not essential for growth).

  • Eukaryotic genome: multiple double-stranded, linear DNA molecules; forms chromosomes.

  • Human somatic cells: 46 chromosomes (diploid, 2n).

  • Gametes (sex cells): 23 chromosomes (haploid, n).

DNA Structure

  • 1950s: Watson and Crick determined DNA structure using Rosalind Franklin's X-ray crystallography data.

  • Chargaff's rules: A = T and C = G (nucleotide pairing).

  • 1962: Nobel Prize awarded to Watson, Crick, Wilkins for DNA structure discovery.

  • DNA building blocks: nucleotides (deoxyribose, phosphate group, nitrogenous base).

  • Four nitrogenous bases: Adenine (A), Thymine (T), Guanine (G), Cytosine (C).

  • Polymer of nucleotides forms a sugar-phosphate backbone connected by covalent bonds.

  • DNA structure: two strands held together by hydrogen bonds between nitrogenous bases.

Learning Objectives

  • What is a genome? (It's like a recipe book for living things!)

  • How is this recipe book organized in chromosomes? (Like chapters in the book, some for body stuff, some for boy/girl stuff.)

  • What does DNA look like? (Like a twisted ladder!)

  • Where does DNA live in different kinds of cells? (In a special spot or floating around?)

Genome Overview

  • The genome is ALL the instructions for a living thing, like a giant recipe book for your whole body.

  • Germs & Tiny Bugs (Prokaryotes): Have one big circle of DNA, like a rubber band, floating in the middle of their cell.

  • Extra Powers (Plasmids): Some germs have tiny extra circles of DNA called plasmids. These are like secret power-up cards, giving them special abilities, like being tough against medicine.

  • You & Other Animals (Eukaryotes): We have lots of long pieces of DNA. Imagine super long threads that are carefully rolled up into X-shapes called chromosomes. These chromosomes live inside a special control room in our cells (the nucleus).

    • Your Body Cells: Each of your body cells has 46 of these X-shaped chapters. They come in pairs, so you get half from your mom and half from your dad.

    • Most of these chapters (44 of them) tell your body how to grow everything else (like your eyes, heart, or hair). We call these autosomes.

    • Two special chapters (the last pair) tell if you'll be a boy or a girl. These are sex chromosomes.

    • Baby-Making Cells (Gametes): The special cells (sperm and egg) that help make new babies only have half the recipe book – 23 chapters each. That way, when they join, they make a complete new set of 46!

DNA Structure

  • Imagine DNA is a super special twisted ladder.

  • Way back in the 1950s, smart scientists named James Watson, Francis Crick, and Maurice Wilkins, using a cool picture taken by Rosalind Franklin, figured out what this ladder looked like!

  • Another scientist named Erwin Chargaff found a secret rule for the ladder's rungs:

    • The rungs are made of four special letters: A, T, C, and G.

    • Chargaff learned that A always wants to hug T, and C always wants to hug G! They always pair up this way. It's like specific LEGO bricks only fitting with certain other LEGO bricks.

  • Guess what? Watson, Crick, and Wilkins won a huge prize (the Nobel Prize) for their amazing discovery!

  • What's DNA made of? Each step of our twisted ladder is made of three parts:

    • A tiny sugar piece (like a sweet treat).

    • A phosphate piece (like a little bouncy ball).

    • One of the four special letters: A, T, C, or G.

  • The Letters:

    • A stands for Adenine

    • T stands for Thymine

    • G stands for Guanine

    • C stands for Cytosine

  • Building the Ladder: These little sugar-phosphate-letter pieces link up in a long chain to make the sides of our twisted ladder.

  • The Twisted Ladder: The DNA ladder is actually made of two of these chains, twisted around each other. The A-T and C-G letters from each side connect in the middle, making the steps of the ladder. These connections are like tiny magnets (hydrogen bonds) holding the steps together. This special twisted shape is called a "double helix."