1. Unit 2 Lesson 6: DNA Structure and Function
7th Grade Cells and Heredity (Mod A)
Unit 2 Lesson 6: DNA Structure and Function

2. DNA
Deoxyribonucleic Acid
Genetic material of a cell

3. 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.
Rosalind Franklin and Maurice Wilkins made images of DNA with x-rays
James Watson and Francis Crick credited with building first model of DNA

4. DNA Structure 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

5. 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?
4 nucleotides

6. 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

7. DNA Replication: making copies
1. The double helix unwinds 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!

8. Replication happens right before cell division
Replication happens right before cell division *It only takes a few hours! Replication happens at many places along the strand at once

9. 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.

10. Unit 2 Lesson 6 DNA Structure and Function
Which type of mutation is shown in each row? (The first row is the original sequence.)
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11. Mutations can be positive or negative, but most are neutral.

12. 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.

13. 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.
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14. A: RNA! 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!

15. 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

16. 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
Start at 0:45 or so -

17. 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

18. Together, transcription and translation are often called “protein synthesis”
PBS DNA workshop -