Human Cells 2 Structure and replication of DNA Higher Human Biology Human Cells 2 Structure and replication of DNA

Learning Intentions Revise DNA structure Learn more detail on structure of DNA Learn some of the detail on DNA replication

Getting started What is DNA? What is DNA made from? Why is DNA a unique molecule? Draw a section of DNA?

DNA – DeoxyriboNucleic Acid double helix It is the molecule of inheritance It can direct its own replication It is the code to ensure proteins are produced It is unique in everyone

Chromosomes and DNA Chromosomes are made of DNA tightly coiled along with some associated proteins called histones

DNA sub units Sub units of DNA are called nucleotides 1 = P it is an phosphate molecule 2 = deoxyribose sugar 3 = a nitrogenous base molecule (4 possible ones – guanine, cytosine, adenine and thymine)

Joining Nucleotides Nucleotides are joined together between the phosphate group and the deoxyribose sugar (sugar – phosphate bonds)

DNA the whole structure Double stranded helix Strands run antiparallel Complimentary base pairing across the strands (G-C and A-T) Strands joined by hydrogen bonds Each strand exposes a 3’ and a 5’ carbon end

How does DNA code for life? There are approximately 3 billion base pairs in our DNA The DNA is organised into genes Genes contain a sequence of bases that code for specific proteins The sequence of bases are unique Humans have 20,000 to 25,000 essential genes This only makes up a small part (1 - 2%) of all the DNA Scientists are still working on what the other DNA does

Consolidate Read pages 16 to 23 in the text book Complete the questions on page 23 Draw a labelled diagram of DNA showing the structure

DNA replication Replication is unique to DNA. It requires: An original piece of DNA as a template Primers (short lengths of nucleotides) A supply of the 4 different nucleotides ATP Enzymes (e.g. DNA polymerase and DNA ligase)

DNA and replication basic

DNA replication more detail DNA uncoils Hydrogen bonds are broken (enzyme ). Molecule unzips exposing the nitrogenous bases A short primer molecule (a few nucleotides joined together) joins to the 3’ end. New bases can only be added at the 3’ end Bases pair in a complimentary way (A-T, G-C) from the primer on the one strand. These join with sugar – phosphate bonds using DNA polymerase Bases then join to other bases with hydrogen bonds

DNA and replication

DNA replication more detail On the antiparallel strand new nucleotides align with the exposed bases and short fragments of DNA are formed. This is called the lagging strand These short fragments are joined together by DNA ligase This process for each strand continues until both sides have been copied. Each DNA molecule will then coil into the double helix This process is semi conservative because each new DNA molecule has a strand from the original plus a new strand making the double helix

DNA and replication

When is DNA replicated? DNA is replicated just before cells divide by mitosis and meiosis. Mitosis produces 2 diploid somatic cells that all contain the same number of chromosomes as the original parent cell. Meiosis produces 4 haploid gametes that all contain 50% of the original parent cell chromosome number.

Consolidation Read pages 24 to 31 in the text bock Answer the questions on page 31/32 Go over the section in scholar Complete Chapter 2 in the matching and multiple choice book Extended response - Homework Give an account of the structure and replication of DNA