Unit 3 Review

​Unit 3 Review! :)

By Shay Paddie

DNA Structure

Middle School

Key Vocabulary

​

DNA

A molecule carrying the genetic instructions for the growth, development, and reproduction of all living organisms.

​

Nucleotide

The basic building block of DNA, made of a sugar, a phosphate group, and a nitrogenous base.

​

Double Helix

The shape of a DNA molecule, which looks like a twisted ladder with two intertwined strands.

​

Base Pairing Rule

The principle that adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C).

​

Antiparallel Strands

A key feature of DNA where the two strands of the double helix run in opposite directions.

​

​

DNA inside the Cells

DNA in Eukaryotic Cells

• DNA holds genetic instructions for complex organisms like plants and animals.

• It is safely stored within a membrane-bound structure called the nucleus.

• This organized storage helps manage the complex functions of eukaryotic organisms.

DNA in Prokaryotic Cells

• DNA carries the essential genetic code for simpler organisms like bacteria.

• It is located in the cytoplasm in a region known as the nucleoid.

• This simpler arrangement allows for rapid growth and reproduction in prokaryotic cells.

Multiple Choice

What is the primary role of DNA and where is it located within a eukaryotic cell?

• To create energy; found in the mitochondria

• To store genetic information; found in the nucleus

• To transport materials; found in the cytoplasm

• To build proteins; found in the ribosomes

​

​

The Structure of DNA

• DNA is shaped like a twisted ladder, a structure called a double helix.

• The building blocks that make up the DNA chain are called nucleotides.

• A nucleotide contains a sugar (deoxyribose), a phosphate group, and a nitrogenous base.

​

Multiple Choice

What are the three components that make up a single nucleotide?

• Deoxyribose sugar, phosphate group, and nitrogenous base

• Ribose sugar, amino acid, and phosphate group

• Deoxyribose sugar, lipid, and nitrogenous base

• Phosphate group, fatty acid, and amino acid

​

​

Assembling the DNA Ladder

The Backbone

• ​The sides of the ladder are made of a sugar-phosphate backbone.

• ​​The sugar and phosphate groups are held together by strong covalent bonds.

• ​These strong bonds make the DNA molecule stable and durable.

The Rungs

• ​The rungs of the ladder are made of pairs of nitrogenous bases.

• ​​These base pairs are connected to each other by weak hydrogen bonds.

• ​The weak bonds allow the DNA strands to separate for important processes.

Multiple Choice

What forms the 'backbone' of the DNA ladder, and what type of bond holds it together?

• Nitrogenous bases, held by hydrogen bonds

• Sugar and phosphate groups, held by covalent bonds

• Deoxyribose sugars only, held by ionic bonds

• Phosphate groups only, held by hydrogen bonds

​

​

Base Pairing Rules in DNA

Adenine & Thymine

• In DNA, the base Adenine (A) consistently pairs up with the base Thymine (T).

• This base pair is held together by two chemical connections called hydrogen bonds.

• This complementary relationship is a fundamental component of Chargaff's rules of base pairing.

Guanine & Cytosine

• The base Guanine (G) consistently connects with the base Cytosine (C) within the DNA molecule.

• Three hydrogen bonds are formed between Guanine and Cytosine, holding them securely together.

• The presence of three bonds makes this pair slightly stronger than the Adenine-Thymine pair.

Multiple Choice

According to the rules of base pairing, which base pairs with Guanine (G) and how many hydrogen bonds are formed between them?

• It pairs with Thymine (T) via 2 hydrogen bonds.

• It pairs with Adenine (A) via 3 hydrogen bonds.

• It pairs with Cytosine (C) via 3 hydrogen bonds.

• It pairs with Guanine (G) via 2 hydrogen bonds.

​

​

Antiparallel Strands

• The two strands of a DNA double helix are antiparallel.

• This means the two DNA strands run in opposite directions.

• This structure lets the nitrogenous bases meet in the center.

• It allows complementary bases (A-T and G-C) to pair correctly.

​

Multiple Choice

What is the significance of the DNA strands being 'antiparallel'?

• It prevents the DNA from replicating.

• It allows the base pairs to properly connect in the center of the molecule.

• It makes the sugar-phosphate backbone stronger.

• It ensures that both strands have the exact same sequence.

​

Common Misconceptions

Misconception	Correction
The two strands of DNA are identical.	The strands are complementary, not identical.
All bonds in DNA are equally strong.	Covalent bonds are strong, while hydrogen bonds are weak.
The order of bases does not matter.	The sequence of bases contains genetic instructions.

Multiple Choice

Why is it structurally important for the hydrogen bonds between bases to be weak while the covalent bonds of the backbone are strong?

• The weak backbone and strong base connections allow the DNA to be flexible.

• The strong backbone provides stability, while the weak bonds between bases allow the strands to separate for replication.

• Both types of bonds are actually equal in strength.

• The weak covalent bonds allow the DNA to be easily broken down for energy.

Multiple Choice

If a scientist determines that a DNA sample contains 30% Adenine, what percentage of the sample should be Thyamine?

• 30%

• 60%

• 40%

• 20%

Poll

On a scale of 1-4, how confident are you about the structure of DNA after this review?

• 1

• 2

• 3

• 4

Genotype - this is the genetic makeup of the organism

• heterozygous - different alleles

• homozygous - same alleles

Multiple Choice

Heterozygous or homozygous?

BB

• heterozygous

• homozygous

Multiple Choice

Heterozygous or homozygous?

Bb

• heterozygous

• homozygous

Phenotype - the physical appearance of the organism

• Think "ph" in phenotype and "ph" in physical

Multiple Choice

Genotype or phenotype?

• genotype

• phenotype

Multiple Choice

Genotype or phenotype?

Bb

• genotype

• phenotype

How to draw a Punnett Square...

• Find the genotype for each parent.

• Draw a 2 x 2 square

• The male parent goes at the top.

• The female parent goes along the left side.

• Bring the male letters down.

• Bring the female letters across.

Remember for a recessive trait to be expressed, both genes must be recessive.

All recessive traits would come from homozygous alleles.

Remember for a dominant trait to be expressed, the alleles can be heterozygous or homozygous.

You only need one dominant allele for a dominant trait to show up.

Multiple Choice

How chance the offspring born from this cross will have a homozygous dominant genotype?

• 25%

• 50%

• 75%

• 100%

Multiple Choice

Fill in the Yellow Box

• Bb

• BB

• bb

Open Ended

What is Meiosis? (Use complete sentences)

Open Ended

Why is Meiosis Important? (Use complete sentences)

Multiple Choice

Which of the following events occur during prophase 1 of meiosis 1?

• a) Chromosomes align at the equator of the cell
  

• b) Homologous chromosomes exchange genetic material
  

• c) Sister chromatids separate and move to opposite poles
  

• d) The nuclear envelope forms back

Multiple Choice

Homologous pairs line up in the middle of the cell in what phase of Meiosis?

• Metaphase I

• Metaphase II

• Mitosis

• Prophase II

Multiple Choice

What part of the cell disappears during Prophase and reappears in Telophase?

• nuclear membrane

• cell membrane

• chromosomes

Multiple Select

What happens in Anaphase 1?

• Chromosomes are lined up in the middle of the cell

• Two new cells are formed

• Chromosome pairs separate

• This phase ensures that each cell has HALf the number of chromosomes

Multiple Select

What happens during Telophase 1?

• The cytoplasm splits and two haploid daughter cells are formed

• Two new nuclear membranes are formed

• The cytoplasm splits and four haploid daughter cells are formed

• Chromosome pairs separate

Multiple Choice

What happens in Prophase 2

• The two haploid cells separate to make 4 new cells

• The chromosomes condense and nuclear membrane disappears

• The chromosomes line up in the middle of the cell

• The nuclear membrane reappears and diploid cells are formed

Open Ended

What happens in Metaphase 2? (Use complete Sentences)

Multiple Choice

What happens in Anaphase 2?

• The sister chromatids separate and move to opposite ends of the cell

• The sister chromatids separate and move to the same side of the cell

• The sister chromatids line up in the center of the cell

• The nuclear membrane reappears and four new cells are formed.

Multiple Select

What happens during Telophase 2?

• Four new nuclear membranes form around the chromosomes.

• The cytoplasm splits and four new haploid cells are formed

• The cytoplasm splits and two new haploid cells are formed

• The cytoplasm splits and four new diploid cells are formed

Open Ended

What are some advantages for meiosis? What are some examples of this advantage that you can see in real life? (Use complete sentences)

Multiple Choice

What phase of Meiosis is this?

• Telophase I

• Anaphase I

• Anaphase II

• Telophase II

Multiple Choice

What phase is the picture showing?

• Metaphase I

• Metaphase II

• Prophase II

• Anaphase I

Multiple Choice

How many chromosomes are in an egg or sperm cell at the end of Meiosis?

• 23

• 46

• 64

• 32

Multiple Choice

In what phase is the DNA of the cell replicated or copied?

• Interphase

• Prophase I

• Telophase I

• Metaphase I

Multiple Choice

What part of the cell disappears during Prophase and reappears in Telophase?

• nuclear membrane

• cell membrane

• chromosomes

Multiple Choice

Homologous pairs line up in the middle of the cell in what phase of Meiosis?

• Metaphase I

• Metaphase II

• Mitosis

• Prophase II