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DNA Structure, Chromosomes, and Replication

Learn about the components and structure of DNA, the role of genes, and the process of DNA replication. Understand the organization of DNA into chromosomes and the enzymes involved in replication.

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DNA Structure, Chromosomes, and Replication

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  1. Chapter 12 12.1 DNA 12.2 Chromosomes and DNA Replication 12.3 RNA and Protein Synthesis 12.4 Mutations 12.5 Gene Regulation

  2. 12.1 DNA • The Components and Structure of DNA

  3. The Components and Structure of DNA • What do genes do? • Carry information from one generation to the next. • Put that information to work by determining the heritable characteristics of organisms. • Had to be easily copied, because all of a cell’s genetic information is replicated every time a cell divides.

  4. Structure of DNA • A long molecule made up of units called nucleotides. • Nucleotide made up of: • A 5-carbon sugar called deoxyribose • A phosphate group • A nitrogenous base • 4 different nitrogenous bases • Purines: two rings • Adenine • Guanine • Pyrimidines: one ring • Thymine • Cytosine

  5. Structure, cont. • The backbone of a DNA chain is formed by sugar and phosphate groups of each nucleotide. • The nitrogenous bases stick out sideways from the chain. • The nucleotides can be joined together in any order, meaning that any sequence of bases is possible. • After research it was decided that DNA is in the structure of a double helix which looks like a twisted ladder. • Base pairing was recognized (base pairs made up the “rungs” of the later • A = T and C = G

  6. 12.2Chromosomes and DNA Replication • DNA and Chromosomes • DNA Replication

  7. DNA and Chromosomes • Prokaryotic: lack nuclei and some organelles • DNA located in the cytoplasm • Single circular DNA containing all genetic information • Eukaryotic: 1000 x the DNA in a prokaryote • Located in the cell’s nucleus in the form of a number of chromosomes

  8. DNA Size • DNA is surprisingly large • In the E. Coli bacteria, the DNA is approximately 1.6 mm. • It must be folded into a space one thousandth its length. • The DNA in a human nucleus is over a meter long!

  9. Chromosome Structure • Chromosomes contain both DNA and protein tightly packed together to form a substance called chromatin. • Chromatin consists of DNA that is tightly coiled around proteins called histones. • Together, the DNA and histone molecules form a beadlike structure called a nucleosome. • Nucleosomes pack with one another to form a thick fiber, which is shortened by a system of loops and coils

  10. DNA Replication • The structure of the double helix explains how it can be copied. • Each strand of the DNA double helix has all the information needed to reconstruct the other half. • Each strand can be used to make the other strand, these strands are complementary • If you could separate the two strands, the rules of base pairing would allow you to reconstruct the base sequence of the other strand.

  11. DNA Replication • Prokaryotes: DNA replication begins at a single point in the chromosome and proceeds, often in two directions, until the entire chromosome is completely copied.

  12. Duplicating DNA • Replication: the copying of DNA • Ensures that each resulting cell will have a complete set of DNA molecules. • During DNA replication, the DNA molecule separates into two strands, then produces two new complementary strands following the rules of base pairing. • Each strand of the double helix of DNA serves as a template, or model, for the new strand • Note that each DNA molecule resulting from replication has one original strand and one new strand, making semi-conservative

  13. How Replication Occurs • Carried out by a series of enzymes. • These enzymes unzip a molecule of DNA. • The unzipping occurs when the hydrogen bonds between the base pairs are broken and the two strands of the molecule unwind • Each strand acts as a template for the attachment of complementary bases • DNA polymerase: enzyme involved in DNA replication; joins individual nucleotides to produce a DNA molecule; proofreads new DNA strands.

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