Skip to content
Quick Guides

Biology - Gene Expression (Molecular Genetics)

Preview 8 of 24 pages

See the first 8 pages exactly as they appear in the document.

  1. Page 1
    Read page 1 as text

    Study Guide Biology – Gene Expression (Molecular Genetics) 1. DNA Defined 1.1. A New Era in Biology In the 1950s, biology entered an exciting new phase. Scientists made rapid discoveries that helped explain how genes work inside cells. These breakthroughs led to the development of molecular genetics , a field that focuses on how genetic information is used by living organisms. 1.2 What Is DNA and Why Is It Important? At the center of molecular genetics is deoxyribonucleic acid , better known as DNA . DNA is the molecule that stores all genetic information. This information determines how cells function and what traits an organism inherits. DNA does not work alone. Instead, it passes instructions along in a step - by - step process that leads to the production of proteins — the molecules that do most of the work in cells. 1.3 From DNA to Protein: The Information Pathway The flow of genetic information follows a clear sequence: DNA → mRNA → protein Here’s how it works: • DNA holds the original genetic instructions. • These instructions are copied into messenger RNA (mRNA) . • mRNA then provides the code needed to build proteins . This one - directional flow of information — from DNA to RNA to protein — is called the Central Dogma of molecular biology . It explains how genetic information is expressed in cells.

  2. Page 2
    Read page 2 as text

    Study Guide 1.4 Discovering the Structure of DNA A major breakthrough came in 1953 , when scientists James D. Watson and Francis H. C. Crick proposed a model for the structure of DNA. Their discovery was so important that they later received the Nobel Prize in 1962 . Understanding DNA’s structure helped scientists explain: • How genes control cell activities • How DNA copies itself during mitosis • How hereditary traits are passed from one generation to the next 1.5 Why This Discovery Matters The discovery of DNA’s structure opened the door to many new scientific advances. It clarified how proteins are made and laid the foundation for modern fields such as biotechnology and genetic engineering . These areas of science continue to shape medicine, agriculture, and research today. In short, learning about DNA helps us understand how life works at its most basic level — and why genetics is such a powerful part of biology. 2. DNA Replication 2.1 Why DNA Replication Is Important Before a cell can divide by mitosis , it must first make an exact copy of its DNA. This step is crucial because, at the end of cell division, each new (daughter) cell must receive the same genetic information as the original cell. In human cells, this means that 46 chromosomes are copied to make 92 chromosomes . These copies are then evenly divided so each daughter cell ends up with 46 chromosomes again. Step 1: Unzipping the DNA DNA replication begins when special enzymes separate the DNA molecule. Think of the DNA double helix like a zipper being pulled apart. • The two strands of DNA separate.

  3. Page 3
    Read page 3 as text

    Study Guide • This exposes the nitrogen bases along each strand. • These bases are either purines or pyrimidines , which pair in specific ways. Step 2: Matching the Bases Once the strands are separated, each exposed base attracts its complementary base : • Adenine pairs with thymine. • Cytosine pairs with guanine. Inside the nucleus, there are plenty of nucleotides available. Each nucleotide contains: • A nitrogen base • A deoxyribose sugar • A phosphate group The enzyme DNA polymerase links these nucleotides together, forming a new strand alongside the original one. Step 3: Building New DNA Strands Each original DNA strand acts as a template for making a new strand. When the new strand forms, it joins with the original strand to create a new double helix. This method is called semiconservative replication because: • Each new DNA molecule contains one original (old) strand and one newly made strand . 2.2 Leading Strand vs. Lagging Strand DNA polymerase can only build DNA in one direction: 5′ to 3′ . This creates two different situations during replication: • Leading strand : ○ Made smoothly and continuously in the 5′ – 3′ direction. • Lagging strand : ○ Built in short sections because it runs in the opposite direction.

  4. Page 4
    Read page 4 as text

    Study Guide ○ These short pieces are called Okazaki fragments . ○ Another enzyme, DNA ligase , later joins these fragments together to form a complete strand. 2.3 When Does DNA Replication Happen? DNA replication occurs during the S phase (synthesis phase) of the cell cycle. After replication: • The DNA shortens and thickens. • The copied chromosomes become visible as chromatids during prophase of mitosis. 2.4 The Final Result As mitosis continues, the cell divides into two daughter cells . Each daughter cell receives: • The same amount of DNA • The same type of DNA This careful copying process ensures that genetic information is passed on accurately and equally , allowing cells — and organisms — to grow, repair, and function properly. In short, DNA replication is the cell’s way of making sure nothing important is lost when it divides. 3. Quiz DNA Replication 1. Question What is the function of the enzyme DNA polymerase? Answer Choices • gluing together Okazaki fragments • joining together nucleotides during replication • “unzipping” the two strands of DNA

  5. Page 5
    Read page 5 as text

    Study Guide Correct Answer joining together nucleotides during replication Why This Is Correct DNA polymerase is the enzyme responsible for building new DNA strands during DNA replication. It works by adding and joining nucleotides together to form a complete strand that is complementary to the original DNA template. For example: As DNA replicates, DNA polymerase connects each new nucleotide in the correct order, creating an exact copy of the DNA strand. The other options describe the functions of different enzymes: • Okazaki fragments are joined by DNA ligase • DNA strands are “unzipped” by helicase 2. Question Okazaki fragments occur with replicating: Answer Choices • both strands • the lagging strand • the leading strand Correct Answer the lagging strand Why This Is Correct DNA replication happens in opposite directions on the two strands of DNA. The leading strand is made continuously, while the lagging strand is made in short segments called Okazaki fragments . For example:

  6. Page 6
    Read page 6 as text

    Study Guide As the lagging strand is copied, DNA polymerase can only add nucleotides in one direction, so it builds the strand in pieces. These pieces are later joined together to form a complete strand. 3. Question When does DNA replication occur during the cell cycle? Answer Choices • G1 phase • G2 phase • S phase Correct Answer S phase Why This Is Correct DNA replication takes place during the S (Synthesis) phase of the cell cycle. This is the stage when the cell copies its DNA so that each daughter cell will receive an identical set of chromosomes during cell division. For example: Before a cell enters mitosis, it must duplicate its DNA. This duplication happens in the S phase, ensuring that both new cells have complete genetic information. 4. Protein Synthesis 4.1 Why Proteins Matter By the 1950s and 1960s, scientists clearly understood that DNA plays a central role in making proteins . Proteins are essential because they perform many different jobs in the cell. For example, proteins: • Act as enzymes that speed up chemical reactions

  7. Page 7
    Read page 7 as text

    Study Guide • Form structures inside cells • Make up hormones like insulin • Build muscle fibers , hair , skin , and nails In fact, the human body contains hundreds of thousands of different proteins , each with a specific role. 4.2 The Protein “Code” What makes one protein different from another is the order of amino acids that form it. Proteins are built from 20 different amino acids , and the sequence of these amino acids acts like a code. This amino acid sequence is determined by a genetic code in DNA . The code is based on the order of nitrogenous bases in DNA. Understanding how this base sequence is converted into an amino acid sequence is the key to understanding protein synthesis . 4.3 What Is Needed for Protein Synthesis? For protein synthesis to occur, several important components must be present: • A supply of the 20 amino acids • A set of specific enzymes • DNA , which contains the original instructions • RNA , which helps carry out those instructions 4.4 RNA: The Messenger Between DNA and Protein Ribonucleic acid (RNA) carries genetic instructions from DNA in the nucleus to the cytoplasm, where proteins are made. RNA is similar to DNA but has two important differences: 1. RNA contains ribose sugar instead of deoxyribose. 2. RNA uses the base uracil (U) instead of thymine (T).

  8. Page 8
    Read page 8 as text

    Study Guide 4.5 The Three Types of RNA Ribosomal RNA (rRNA) • rRNA combines with proteins to form ribosomes . • Ribosomes are the cell’s protein - building sites , often described as chemical “workbenches.” • They are found in the cytoplasm and attached to the rough endoplasmic reticulum . Transfer RNA (tRNA) • tRNA carries amino acids from the cytoplasm to the ribosomes. • Each tRNA is specific — it binds only to one particular amino acid . Messenger RNA (mRNA) • mRNA is made in the nucleus using DNA as a template. • It carries the genetic instructions from DNA to the ribosomes. • The enzyme RNA polymerase is responsible for making mRNA (as well as tRNA and rRNA). There are also non - coding RNAs (ncRNAs) . These do not directly help build proteins but play roles in controlling genes, which are discussed later in the chapter. 4.6 Transcription: Making mRNA from DNA The first major step in protein synthesis is transcription . Here’s how transcription works: • RNA polymerase binds to a specific region of DNA. • Only one strand of DNA is used as a template. • The enzyme reads the DNA bases one by one. • A complementary mRNA strand is built in the 5′ – 3′ direction . • When a stop signal is reached, mRNA synthesis ends. The DNA strands then rewind into a double helix, while the mRNA leaves the nucleus through a pore and enters the cytoplasm.

16 more pages in the full document

Unlock it once and it stays in your library, ready to chat with or turn into flashcards.

Unlock the full document

More Biology study materials

See all

Study this document with CramX

Once it is in your library, every tool can work from it.