Unit 2:
1. What are genes composed of and where are they located?
• Composition of Genes
• Deoxyribonucleic Acid (DNA):
• Genes are composed of DNA, which is a double-stranded helical molecule.
• DNA contains nucleotides, each consisting of:
• A sugar (deoxyribose).
• A phosphate group.
• A nitrogenous base (adenine [A], thymine [T], guanine [G], cytosine [C]).
• The specific sequence of these bases encodes genetic information.
• Structure of Genes
• Genes include coding regions (exons) and non-coding regions (introns).
• The coding regions are transcribed into messenger RNA (mRNA) to produce proteins.
• Regulatory sequences control gene expression (e.g., promoters, enhancers, silencers).
• Location of Genes
• Chromosomes:
• Genes are located on chromosomes, which are thread-like structures made of DNA and proteins.
• In humans, there are 23 pairs of chromosomes (46 total), including:
• 22 pairs of autosomes.
• 1 pair of sex chromosomes (XX in females, XY in males).
• Cellular Location:
• Chromosomes are found in the nucleus of eukaryotic cells.
• Mitochondria also contain a small number of genes (mitochondrial DNA).
• Additional Notes
• The human genome contains approximately 20,000-25,000 genes.
• Each gene provides instructions for making specific proteins or functional RNA molecules.
2. What are the four types of nitrogenous bases that constitute DNA?
DNA is composed of four nitrogenous bases, categorized into purines and pyrimidines:
, • Purines (Double-Ring Structure):
• Adenine (A)
• Guanine (G)
• Pyrimidines (Single-Ring Structure):
• Cytosine (C)
• Thymine (T)
Base-Pairing Rules:
• Adenine (A) pairs with Thymine (T) via two hydrogen bonds.
• Cytosine (C) pairs with Guanine (G) via three hydrogen bonds.
Function:
• The specific sequence of these nitrogenous bases encodes genetic information critical for protein synthesis and cellular
function.
3. How are new strands of DNA formed?
New DNA strands are formed through a process called DNA replication. This is a semi-conservative process, meaning each
new DNA molecule contains one original strand and one newly synthesized strand.
Steps in DNA Replication:
• Initiation:
• Unwinding: DNA helicase unwinds the double helix, breaking hydrogen bonds between the base pairs,
creating a replication fork.
• Stabilization: Single-stranded binding proteins (SSBs) keep the strands separated.
• Primer Addition:
• Primase synthesizes short RNA primers, which serve as starting points for DNA synthesis.
• Elongation:
• Leading Strand: DNA polymerase synthesizes the new strand continuously in the 5' to 3' direction.
• Lagging Strand: DNA polymerase synthesizes the new strand discontinuously, forming Okazaki
fragments, which are later joined by DNA ligase.
• Proofreading and Error Correction:
• DNA polymerase checks for errors and corrects mismatched base pairs during synthesis.
• Termination:
• When replication is complete, the DNA strands rewind into a double helix, forming two identical DNA
, molecules.
Key Enzymes Involved:
• Helicase: Unwinds the DNA helix.
• DNA Polymerase: Adds nucleotides to the growing strand.
• Primase: Synthesizes RNA primers.
• Ligase: Joins Okazaki fragments on the lagging strand.
4. How is the process of transcription regulated?
1. Chromatin Structure Regulation
• Euchromatin vs. Heterochromatin:
• Euchromatin: Loosely packed DNA, accessible for transcription.
• Heterochromatin: Densely packed DNA, transcriptionally inactive.
• Histone Modifications:
• Acetylation (by HATs): Loosens chromatin, increasing transcription.
• Deacetylation (by HDACs): Compacts chromatin, decreasing transcription.
• Methylation: Can activate or repress transcription depending on the location.
2. Epigenetic Regulation
• DNA Methylation:
• Addition of methyl groups to cytosine bases (CpG islands) represses transcription by preventing transcription
factors from binding.
3. Transcription Factor Binding
• General Transcription Factors:
• Bind to the promoter region (e.g., TATA box) to recruit RNA polymerase.
• Specific Transcription Factors:
• Bind to enhancers or silencers to either upregulate or downregulate transcription.
• Examples: Activators and repressors.
4. Regulatory DNA Elements
• Promoters:
• Regions upstream of the gene where RNA polymerase and transcription factors bind to initiate transcription.
• Enhancers:
, • Distal DNA elements that enhance transcription when bound by activators.
• Silencers:
• Distal DNA elements that repress transcription when bound by repressors.
5. Post-Transcriptional Regulation
• Alternative Splicing:
• Generates multiple mRNA variants from a single gene, influencing protein expression.
• RNA Interference (RNAi):
• Small RNAs (e.g., miRNA, siRNA) can degrade mRNA or block translation.
6. Environmental and Cellular Signals
• Hormones:
• Steroid hormones can bind intracellular receptors and act as transcription factors.
• Stress or Nutritional Signals:
• Can activate signaling pathways (e.g., MAPK, cAMP), influencing transcription factor activity.
5. How many pairs of chromosomes do humans have?
• Number of Chromosomes: Humans have 23 pairs of chromosomes, totaling 46 chromosomes.
• Composition:
• 22 pairs are autosomes, which are non-sex chromosomes.
• 1 pair is the sex chromosomes:
• XX in females.
• XY in males.
• Location: Chromosomes are found in the nucleus of nearly all human cells and carry the genetic material (DNA)
organized into genes.
6. What are some of the most common types of chromosome abnormalities?
• Numerical Abnormalities
• Definition: An abnormal number of chromosomes due to errors in cell division.
• Examples:
• Trisomy: Presence of an extra chromosome (47 total).
1. What are genes composed of and where are they located?
• Composition of Genes
• Deoxyribonucleic Acid (DNA):
• Genes are composed of DNA, which is a double-stranded helical molecule.
• DNA contains nucleotides, each consisting of:
• A sugar (deoxyribose).
• A phosphate group.
• A nitrogenous base (adenine [A], thymine [T], guanine [G], cytosine [C]).
• The specific sequence of these bases encodes genetic information.
• Structure of Genes
• Genes include coding regions (exons) and non-coding regions (introns).
• The coding regions are transcribed into messenger RNA (mRNA) to produce proteins.
• Regulatory sequences control gene expression (e.g., promoters, enhancers, silencers).
• Location of Genes
• Chromosomes:
• Genes are located on chromosomes, which are thread-like structures made of DNA and proteins.
• In humans, there are 23 pairs of chromosomes (46 total), including:
• 22 pairs of autosomes.
• 1 pair of sex chromosomes (XX in females, XY in males).
• Cellular Location:
• Chromosomes are found in the nucleus of eukaryotic cells.
• Mitochondria also contain a small number of genes (mitochondrial DNA).
• Additional Notes
• The human genome contains approximately 20,000-25,000 genes.
• Each gene provides instructions for making specific proteins or functional RNA molecules.
2. What are the four types of nitrogenous bases that constitute DNA?
DNA is composed of four nitrogenous bases, categorized into purines and pyrimidines:
, • Purines (Double-Ring Structure):
• Adenine (A)
• Guanine (G)
• Pyrimidines (Single-Ring Structure):
• Cytosine (C)
• Thymine (T)
Base-Pairing Rules:
• Adenine (A) pairs with Thymine (T) via two hydrogen bonds.
• Cytosine (C) pairs with Guanine (G) via three hydrogen bonds.
Function:
• The specific sequence of these nitrogenous bases encodes genetic information critical for protein synthesis and cellular
function.
3. How are new strands of DNA formed?
New DNA strands are formed through a process called DNA replication. This is a semi-conservative process, meaning each
new DNA molecule contains one original strand and one newly synthesized strand.
Steps in DNA Replication:
• Initiation:
• Unwinding: DNA helicase unwinds the double helix, breaking hydrogen bonds between the base pairs,
creating a replication fork.
• Stabilization: Single-stranded binding proteins (SSBs) keep the strands separated.
• Primer Addition:
• Primase synthesizes short RNA primers, which serve as starting points for DNA synthesis.
• Elongation:
• Leading Strand: DNA polymerase synthesizes the new strand continuously in the 5' to 3' direction.
• Lagging Strand: DNA polymerase synthesizes the new strand discontinuously, forming Okazaki
fragments, which are later joined by DNA ligase.
• Proofreading and Error Correction:
• DNA polymerase checks for errors and corrects mismatched base pairs during synthesis.
• Termination:
• When replication is complete, the DNA strands rewind into a double helix, forming two identical DNA
, molecules.
Key Enzymes Involved:
• Helicase: Unwinds the DNA helix.
• DNA Polymerase: Adds nucleotides to the growing strand.
• Primase: Synthesizes RNA primers.
• Ligase: Joins Okazaki fragments on the lagging strand.
4. How is the process of transcription regulated?
1. Chromatin Structure Regulation
• Euchromatin vs. Heterochromatin:
• Euchromatin: Loosely packed DNA, accessible for transcription.
• Heterochromatin: Densely packed DNA, transcriptionally inactive.
• Histone Modifications:
• Acetylation (by HATs): Loosens chromatin, increasing transcription.
• Deacetylation (by HDACs): Compacts chromatin, decreasing transcription.
• Methylation: Can activate or repress transcription depending on the location.
2. Epigenetic Regulation
• DNA Methylation:
• Addition of methyl groups to cytosine bases (CpG islands) represses transcription by preventing transcription
factors from binding.
3. Transcription Factor Binding
• General Transcription Factors:
• Bind to the promoter region (e.g., TATA box) to recruit RNA polymerase.
• Specific Transcription Factors:
• Bind to enhancers or silencers to either upregulate or downregulate transcription.
• Examples: Activators and repressors.
4. Regulatory DNA Elements
• Promoters:
• Regions upstream of the gene where RNA polymerase and transcription factors bind to initiate transcription.
• Enhancers:
, • Distal DNA elements that enhance transcription when bound by activators.
• Silencers:
• Distal DNA elements that repress transcription when bound by repressors.
5. Post-Transcriptional Regulation
• Alternative Splicing:
• Generates multiple mRNA variants from a single gene, influencing protein expression.
• RNA Interference (RNAi):
• Small RNAs (e.g., miRNA, siRNA) can degrade mRNA or block translation.
6. Environmental and Cellular Signals
• Hormones:
• Steroid hormones can bind intracellular receptors and act as transcription factors.
• Stress or Nutritional Signals:
• Can activate signaling pathways (e.g., MAPK, cAMP), influencing transcription factor activity.
5. How many pairs of chromosomes do humans have?
• Number of Chromosomes: Humans have 23 pairs of chromosomes, totaling 46 chromosomes.
• Composition:
• 22 pairs are autosomes, which are non-sex chromosomes.
• 1 pair is the sex chromosomes:
• XX in females.
• XY in males.
• Location: Chromosomes are found in the nucleus of nearly all human cells and carry the genetic material (DNA)
organized into genes.
6. What are some of the most common types of chromosome abnormalities?
• Numerical Abnormalities
• Definition: An abnormal number of chromosomes due to errors in cell division.
• Examples:
• Trisomy: Presence of an extra chromosome (47 total).
