BIO1011 Week 7 - DNA Replication and Cell Division

These flashcards cover key vocabulary and concepts related to DNA replication, cell division, and the cell cycle as outlined in the BIO1011 Week 7 lecture notes.

Chromosome

An organelle that contains genetic material in a eukaryotic cell.

A chromosome is a thread-like structure located within the nucleus of eukaryotic cells, composed of DNA and proteins. Each chromosome contains a single, long molecule of DNA that is tightly coiled and packed to fit within the cell nucleus. In humans, there are 46 chromosomes (23 pairs), and they play a crucial role in heredity, as they carry genes that determine various traits and characteristics.

Homologous chromosomes

Homologous chromosomes are pairs of chromosomes in a diploid organism that have the same structure, size, and genetic content.

Each pair consists of one chromosome inherited from the mother and one from the father. These chromosomes contain genes for the same traits, although the alleles (variant forms of a gene) may differ.

Homologous chromosomes are crucial during meiosis, where they undergo processes such as crossing over and independent assortment, contributing to genetic diversity in offspring.

They ensure that gametes receive an accurate set of chromosomes for fertilization

Sister chromatids

Sister chromatids are identical copies of a single chromosome that are formed during DNA replication.

Each chromosome, after replication, consists of two sister chromatids held together at a region called the centromere. These chromatids are genetically identical and play a crucial role during cell division, particularly in mitosis and meiosis.

During anaphase of mitosis, sister chromatids are separated and pulled to opposite poles of the cell, ensuring that each daughter cell receives an identical set of chromosomes.

This process is essential for maintaining genetic stability in organisms and is a key mechanism in growth, development, and tissue repair.

DNA replication

The process by which a cell makes an exact copy of its DNA before cell division.

This process occurs during the S phase of the cell cycle and involves several key steps:

1. Initiation: The process begins at specific locations on the DNA molecule called 'origins of replication.' Enzymes called helicases unwind the double helix, separating the two strands of DNA to create a 'replication fork.'

2. Primer Binding: Short RNA primers are synthesized by the enzyme primase to provide a starting point for DNA synthesis.

3. Elongation: DNA polymerase enzymes add nucleotide triphosphates to the growing DNA strand, complementary to the template strand. The leading strand is synthesized continuously, while the lagging strand is synthesized in short segments known as Okazaki fragments.

4. Termination: Once the entire DNA molecule has been replicated, the RNA primers are removed, and the gaps are filled in with DNA, followed by the joining of Okazaki fragments by DNA ligase, creating a continuous double strand.

5. Proofreading and Repair: DNA polymerases also have proofreading capabilities to correct any errors that occur during replication, ensuring high fidelity and minimizing mutations.

Leading strand

The strand of DNA that is synthesized continuously during DNA replication.

It is characterized by the following details:

1. Synthesis Direction: The leading strand is synthesized continuously in the same direction as the replication fork unwinds. It grows toward the replication fork and is completed in the 5' to 3' direction, following the template strand.

2. Enzymatic Activity: This strand is synthesized by DNA polymerase III, which adds nucleotides to the growing chain. The addition is guided by complementary base-pairing: adenine pairs with thymine, and cytosine pairs with guanine.

3. No Need for Primers: The leading strand can be synthesized continuously because it only requires a single RNA primer at the origin of replication. Once this primer is laid down, DNA polymerase can proceed without interruption.

4. Relationship with Lagging Strand: Unlike the lagging strand, which is synthesized discontinuously in short segments known as Okazaki fragments, the leading strand remains intact as one continuous piece. This is crucial for the efficient and accurate replication of the entire DNA molecule.

5. Role in Cell Division: The leading strand plays a vital role in the duplication of genetic material prior to cell division, ensuring that each daughter cell receives an exact copy of the DNA.

Overall, the leading strand is essential for efficient DNA replication, acting as a model for synthesizing the strand opposite to it, known as the lagging strand.

Lagging strand

The strand of DNA that is synthesized discontinuously in short sections during DNA replication.

Here are key details about the lagging strand:

1. Synthesis Direction: The lagging strand is synthesized in the direction opposite to the replication fork movement. This means that as the DNA unwinds and the replication fork opens, the lagging strand is built away from the fork rather than toward it.

2. Okazaki Fragments: The lagging strand is formed in short segments called Okazaki fragments, which are typically around 100-200 nucleotides long in eukaryotes. Each fragment is synthesized separately to allow for the overall directionality of DNA synthesis (5' to 3').

3. Role of RNA Primers: Each Okazaki fragment begins with an RNA primer synthesized by the enzyme primase. This primer provides a free 3' hydroxyl (OH) group for DNA polymerase to begin adding nucleotides. New RNA primers must be laid down for each Okazaki fragment, making this process more complex than for the leading strand.

4. Enzymatic Action: DNA polymerase III is responsible for synthesizing the Okazaki fragments. However, it can only add nucleotides to an existing strand; thus, it works in conjunction with RNA primers to create each fragment. Once a fragment is synthesized, DNA polymerase I replaces the RNA primer with DNA nucleotides.

5. Joining of Fragments: After the Okazaki fragments are synthesized and the RNA primers replaced, DNA ligase comes into play. This enzyme seals the gaps between the fragments, creating a continuous DNA molecule.

6. Importance in Replication: The lagging strand is essential for accurately replicating the entire genomic DNA of the cell. Despite its discontinuous nature, it ensures that both strands of DNA are copied efficiently and accurately during cell division, allowing for the proper transmission of genetic information.

7. Comparison with Leading Strand: Unlike the leading strand, which is synthesized continuously in the direction of the replication fork, the lagging strand highlights the complexities of DNA replication and the necessity of coordinated enzyme activity for complete DNA synthesis.

DNA polymerase

An enzyme that synthesizes new DNA strands by adding nucleotides complementary to the template strand.

Replication fork

The area where the double helix of DNA is unwound and replicated.

Mitosis

The process of cell division that results in two identical daughter cells.

Meiosis

A type of cell division that reduces the chromosome number by half, resulting in four non-identical daughter cells.

Cell cycle

The series of events that cells go through as they grow and divide.

Non-disjunction

The failure of homologous chromosomes or sister chromatids to separate properly during cell division, which can lead to aneuploidy.

G1 phase

The phase of the cell cycle where the cell grows and prepares for DNA synthesis.

G2 phase

The phase of the cell cycle where the cell prepares for mitosis.

Cyclin

A protein that regulates the cell cycle and ensures progression through different phases.

Centrosome

An organelle that serves as the main microtubule organizing center and is involved in cell division.

Chiasma

The point where two homologous non-sister chromatids exchange genetic material during meiosis.

Aneuploidy

An abnormal number of chromosomes in a cell, which can be a result of non-disjunction.

Cytokinesis

The final step of cell division where the cytoplasm divides, resulting in two daughter cells.

Karyotype

A visual representation of the chromosomes in a cell, arranged in pairs and organized by size.

Spindle fibers

Structures that separate the chromosomes during cell division.

Complementary strand

A strand of DNA that is synthesized based on the sequence of a template strand.

5’ and 3’ ends

The two ends of a DNA or RNA strand, indicating the direction of synthesis or reading.

Equational division

A process of division that maintains the chromosome number, as seen in mitosis.

Reductional division

A process of division that reduces the chromosome number by half, as seen in meiosis.