DNA Structure & Replication

DNA and its Structure DNA stands for deoxyribonucleic acid and carries the genetic information in all types of living organisms DNA consists of two strands of nucleotides wound together in a spiral called a double helix

DNA and its structure Each nucleotide is composed of: A phosphate group A sugar molecule One of four different nitrogenous bases: Adenine (A) Thymine (T) Guanine (G) Cytosine (C) The phosphate and sugar parts of the nucleotides form the sides of the ladder in the DNA double helix

DNA and its structure The nitrogenous bases of the nucleotides extend toward the center of the double helix, forming the rungs of the ladder Each base is matched with a complementary base in the other strand In accord with the base-pairing rules: Adenine (A) pairs with Thymine (T) Guanine (G) pairs with Cytosine (C) The two strands are held together by hydrogen bonds formed between the paired bases

DNA and its structure

dna & Chromosomes Chromosomes are thread-like structures located inside the nucleus of animal and plant cells Each chromosome contains a DNA molecule, which contains multiple genes Chromosomes are not always visible; They usually sit around uncoiled and as loose strands called chromatin When it is time for the cell to divide, they condense and wrap up very tightly The tightly-wound DNA is the chromosome

DNA & chromosomes Chromosomes look like an “X” and they are usually found in pairs Scientists count individual strands of chromosomes because not every organism has pairs Humans have 46 chromosomes (or 23 pairs) Peas only have 12 chromosomes, and dogs have 78 More chromosomes does NOT equal a more complex or intelligent creature

Dna & chromosomes

DNA Replication Replication is the process where DNA makes a copy of itself It produces two new DNA molecules that are identical to the original DNA molecule Each of the new DNA molecules carry the same genetic information as the original DNA molecule

DNA Replication Why does DNA need to copy? It’s simple: Cells divide for an organism to grow or reproduce, and every new cells needs a copy of the DNA or instructions to know how to be a cell DNA replicates right before a cell divides

DNA Replication DNA replication is semi-conservative That means that when it makes a copy, one half of the old strand is always kept in the new strand This helps reduce the number of copy errors (a.k.a. mutations)

DNA Replication During replication, enzymes called helicases separate DNA strands by moving along the strand and breaking the hydrogen bonds between base pairs The Y-shaped region that results when the two strands separate is called a replication fork Each old strand is used as a template to form a new DNA strand

DNA Replication Next, enzymes called DNA polymerase add complementary nucleotides (free floating in the nucleus) to each of the original strands The result is two separate and identical DNA molecules that are ready to move to new cells for in cell division In each new DNA double helix, one strand is from the original molecule, and one strand is new (hence the name, ‘semi-conservative’)

DNA replication

DNA Replication