DNA Replication Practice Worksheet

Prokaryotic cells lack a nucleus, so DNA replication occurs in the cytoplasm. Prokaryotic DNA is found in the nucleoid region, which is an irregularly shaped area within the cytoplasm. Unlike eukaryotic cells, prokaryotes do not have membrane-bound organelles, so mitochondria and the endoplasmic reticulum are not involved. Bacteria and archaea undergo replication using a single origin of replication in their circular DNA.

In eukaryotic cells, DNA replication occurs in the nucleus, where the chromosomes are enclosed within a nuclear membrane. Unlike prokaryotes, eukaryotic DNA is linear and requires multiple origins of replication to efficiently duplicate its large genome. While mitochondria have their own small DNA that undergoes replication, the main genomic replication occurs in the nucleus during the S phase of the cell cycle. The endoplasmic reticulum is involved in protein and lipid synthesis, not DNA replication.

Base pairing follows Chargaff's rules, where adenine (A) pairs with thymine (T) and guanine (G) pairs with cytosine (C). These complementary base pairings are held together by hydrogen bonds — A and T share two hydrogen bonds, while G and C share three. This strict pairing ensures accurate replication and maintenance of genetic information. Any deviation from this pattern can lead to mutations and errors in genetic material.

DNA replication is essential for cell division, ensuring that each daughter cell receives an identical copy of genetic material. This process occurs during the S phase of the cell cycle and is critical for growth, development, and tissue repair. While proteins are synthesized through transcription and translation, DNA replication does not directly create proteins. Energy production is mainly handled by cellular respiration, not DNA replication.

Errors in DNA replication can lead to mutations, which may cause genetic disorders, cancer, or cell malfunction. Some mutations may be harmless, while others can be detrimental or even lethal to the cell or organism. Cells have repair mechanisms, such as DNA polymerase proofreading and mismatch repair enzymes, to fix replication errors. If the damage is too severe and cannot be repaired, the cell may undergo apoptosis (programmed cell death) to prevent further harm.

DNA replication occurs in three main steps: unwinding, complementary base pairing, and joining of nucleotides. Unwinding: The DNA double helix is separated by helicase, exposing the nitrogenous bases. Complementary Filling: DNA polymerase adds new nucleotides to each strand according to base-pairing rules. Joining: The newly synthesized strands are sealed by ligase, forming two identical DNA molecules. Although mutations can occur, they are not an intended step of replication but rather an error that may be corrected by DNA repair enzymes.