Unit 2 Lesson 6: DNA Structure and
Function
7th Grade Cells and Heredity (Mod A)
DNA•Deoxyribonucleic Acid
•Genetic material of a cell
Many scientists from all over the world contributed to our understanding of DNA.Write these names & dates in your notebook. Leave 3 lines between each one.
• 1857 – Gregor Mendel –
• 1869 – Johann Fredrich Miechner -
• 1951 - Rosalind Franklin & Maurice Wilkins–
• 1953 – James Watson & Francis Crick -
• 1857 – Gregor Mendel – did experiments with pea plants; observed offspring had same traits as parents. Hypothsized that parents pass down traits to offspring.
• 1869 – Johann Fredrich Miechner – isolated “nuclein” from white blood cells - DNA
• 1951 - Rosalind Franklin & Maurice Wilkins–made images of DNA with x-rays
• 1953 – James Watson & Francis Crick - used Franklin & Wilkins images to make 3D model of DNA
•<repeated info – skip if you did the previous slide>
•Many scientists from all over the world contributed to our understanding of DNA.
• Some scientists discovered the chemicals that make up DNA, others learned how these chemicals fit together.
• Still others determined the three-dimensional structure of the DNA molecule.• 1951 - Rosalind Franklin and Maurice Wilkins made images of DNA with x-rays
• 1953 - James Watson and Francis Crick credited with building first model of DNA
• Shape is double helix
• Sides (a.k.a. backbone) made
of sugars and phosphate groups
• “Rungs” made of pairs of bases• Adenine
• Thymine
• Cytosine
• Guanine
DNA Structure
4 nucleotides
• Base + sugar + phosphate = nucleotide (“building block” of DNA)
• Bases always pair in specific ways – complementary bases• adenine (A) pairs with thymine (T)
• cytosine (C) pairs with guanine (G)
• How can you remember this?
• The ORDER of the nucleotides matters – it is the code that tells cells what proteins to build
• Segments of DNA that code for a certain trait are called genes, which determine your traits
• Each gene codes for a specific protein
DNA Replication: making copies• 1. The double helix unwinds (“unzips” and the two strands
separate• Each strand is used as a pattern for the new strand
• 2. bases on each side are exposed, and complementary nucleotides are added• For example: an nucleotide containing
thymine attaches to an exposed adenine
• 3. Now you have two identical DNA molecules, each containing one old strand and one new strand!
Replication happens right before cell division
*It only takes a few hours!Replication happens at many places along the strand at once
DNA does not always copy correctly!
Mutations: changes in the number, type or order of bases on a piece of DNA
• In a deletion mutation, a base is left out.
• In an insertion mutation, an extra base is added.
• The most common mutation, substitution, happens when one base replaces another.
• Which type of mutation is shown in each row? (The first row is the original sequence.)
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Unit 2 Lesson 6 DNA Structure and Function
Mutations can be positive or negative, but most are neutral.
How do mutations happen?• Random error
• Damage to the DNA molecule by mutagens
• Ex. UV light and chemicals in cigarette smoke
•Cells make proteins that can fix errors in DNA, but sometimes the mistake is not corrected & mistake becomes part of the genetic code.
• A genetic disorder results from mutations that harm the normal function of the cell.
• Some genetic disorders are inherited, or passed on from parent to offspring.
• Other disorders result from mutations during a person’s lifetime. Most cancers fall in this category.
Copyright © Houghton Mifflin Harcourt Publishing Company
• Q: What cell organelle makes proteins?
• A: Ribosomes
• Q: Where are ribosomes found?
• A: In the cytoplasm and on rough ER
• Q: Where is the code for making the proteins?
• A: On the DNA
• Q: Where is the DNA?
• A: In the nucleus
• Q: How does the info from the DNA inside the nucleus get outside the nucleus to the ribosomes?
•A: RNA!
RNA = ribonucleic acid• Like DNA, RNA has a sugar-phosphate backbone
and the bases adenine (A), guanine (G), and cytosine (C)
• Instead of thymine (T), RNA contains uracil (U).
• Unlike DNA, it is only one strand, not two
• Three types of RNA have special roles in making proteins.
• mRNA – messenger RNA
• tRNA – transfer RNA
• rRNA – ribosomal RNA
Transcription: copying DNA to an mRNA strand• (mRNA = messenger RNA)
• 1. DNA strand unwinds (just like in replication)
• 2. mRNA fills in the complementary nucleotides (just like in replication)• Only one gene at a time is transcribed, not the whole strand
• 3. When transcription is complete, DNA strand winds up again
Translation: proteins are made from the mRNA code
• 1. mRNA travels outside the nucleus to a ribosome made of rRNA (ribosomal RNA)
• 2. As mRNA passes through the ribosome tRNA (transfer RNA) molecules deliver amino acids to ribosome
• Each group of three bases on the mRNA strand code for one amino acid
• The order of bases tells what amino acids to move into the ribosome
• 3. Amino acids join together to make proteins
Together, transcription and translation are often called “protein synthesis”