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.

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• 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”