DNA Deoxyribonucleic Acid

Scientists Discovering DNA Erwin Chargaff: discovered base pairing –Percentage of adenine = thymine –Percentage of cytosine = guanine Rosalind Franklin: ( 1952) Used X-ray diffraction to: –show that DNA is a spiral shape (helix) –suggest that DNA is made of 2 strands –bases are near the center of DNA

Scientists Discovering DNA James Watson and Francis Crick: (1953) –Created the first accurate model of DNA using clues from Frankiln’s X-ray diffraction –Showed that DNA was a double helix –Won the Nobel Prize for their discovery Hydrogen-bonded bases Sugar phosphate backbone

Structure of DNA DNA is a double helix Long molecule made up of nucleotides Nucleotide: has 3 basic components –5 carbon sugar (deoxyribose) –Phosphate group –Nitrogenous base nucleotide

Nitrogenous Bases DNA contains four nitrogenous bases: –adenine (A) –cytosine (C) –guanine (G) –thymine (T) Purines (2 ring structure) –adenine and guanine Pyrimidines (1 ring structure) –cytosine and thymine

DNA Strand PurinesPyrimidines AdenineGuanineCytosineThymine Phosphate group Deoxyribose

Chargaff’s Rule of base pairing Base Pairing: bonds in DNA can only form between adenine and thymine and between cytosine and guanine  A=T and C=G  Base pairs are held together by hydrogen bonds A == T C == G T == A G == C A == T T == A Sugar- phosphate backbone

8 DNA P P P O O O P P P O O O G C TA copyright cmassengale

9 Antiparallel Strands One strand of DNA goes from 5’ to 3’ (sugars) The other strand is opposite in direction going 3’ to 5’ (sugars) copyright cmassengale

DNA double helix Hydrogen bonds Nucleotide Sugar-phosphate backbone Key Adenine (A) Thymine (T) Cytosine(C) Guanine (G)

Chromosome Structure DNA molecules are very long A human cell contains more than 1 meter of DNA DNA is tightly packed to fit inside the nucleus Chromosomes are composed of DNA that is tightly coiled around proteins Chromatin: consists of DNA tightly coiled around proteins Histones: protein that DNA coils around

Chromosome Structure Chromosome Supercoils Coils Nucleosome Histones DNA double helix

13 DNA Replication copyright cmassengale

14 Synthesis Phase (S phase) S phase during interphase of the cell cycle Nucleus of eukaryotes Mitosis -prophase -metaphase -anaphase -telophase G1G1 G2G2 S phase interphase DNA replication takes place in the S phase. copyright cmassengale

DNA Replication DNA replication: process in which DNA is duplicated by producing 2 new complementary strands following the rules of base pairing (A=T C=G) How DNA Replication Occurs: 1.Part of the double helix separates (unzips) by breaking the hydrogen bonds 2.Old strand of DNA is used as a template to form a new strand of DNA by attaching complementary bases EX: Old strand: ATCGTTC; New strand TAGCAAG DNA polymerase: main enzyme involved in DNA replication –Adds complementary bases during replication –Proofreads DNA to minimize errors during replication

DNA Replication Growth Replication fork DNA polymerase New strand Original strand DNA polymerase Replication fork New strand Original strand Nitrogenous bases

17 Replication of Strands Replication Fork Point of Origin copyright cmassengale

18 Semiconservative Model of Replication Idea presented by Watson & CrickIdea presented by Watson & Crick TheThe two strands of the parental molecule separate, and each acts as a template for a new complementary strand New DNA consists of 1 PARENTAL (original) and 1 NEW strand of DNA Parental DNA DNA Template New DNA copyright cmassengale