Download Unit 2 Lesson 6: DNA Structure and Function

7th Grade
Cells and Heredity (Mod A)
Unit 2 Lesson 6:
DNA Structure and
Function
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
• 1869
• 1951
• 1953
– Gregor Mendel –
– Johann Fredrich Miechner - Rosalind Franklin & Maurice Wilkins–
– 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
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
• 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
• 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.
Unit 2 Lesson 6 DNA Structure and Function
• Which type of mutation is shown in each row? (The
first row is the original sequence.)
Copyright © Houghton Mifflin Harcourt Publishing Company
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”