Molecular Genetics: DNA Replication and Mutagenesis

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Questions and Answers

What is the primary function of the S phase in the cell cycle?

Cell entry into quiescent phase

Chromosome segregation

DNA synthesis (correct)

Cell preparation for mitosis

What is the approximate percentage of the human genome that encodes proteins?

51%

20%

1% (correct)

What is the name of the short interspersed elements that are 160bp repeats?

Satellite DNA

SINES (correct)

LINES

Transposons

What is the term for the problem of chromosome ends?

Chromosome end-problem Signup and view all the answers

What is the primary function of the G1 phase in the cell cycle?

Cell growth and preparation for DNA replication Signup and view all the answers

What is the term for the repeated sequences found in the human genome?

Satellite DNA Signup and view all the answers

What is the number of pairs of chromosomes found in human cells?

23 Signup and view all the answers

What is the term for the mobile genetic elements that are derived from retroviruses?

LINES Signup and view all the answers

What is the role of topoisomerase in DNA replication?

To break a phosphodiester bond in one parental strand ahead of the replication fork Signup and view all the answers

What is the purpose of the RNA primer in DNA replication?

To provide a free 3'OH for DNA synthesis Signup and view all the answers

What is the direction of DNA synthesis in the leading strand?

5' to 3' Signup and view all the answers

What is the role of helicase in DNA replication?

To unwind the double helix Signup and view all the answers

What is the result of the loss of telomeres?

The cell stops dividing Signup and view all the answers

What is the primary function of DNA polymerase?

To synthesize new DNA strands using the parental strands as templates Signup and view all the answers

What is the result of a nonsense mutation?

A change from encoding an amino acid to a stop codon Signup and view all the answers

What is the purpose of DNA ligase in DNA replication?

To join adjacent Okazaki fragments Signup and view all the answers

What is the role of the Shelterin complex in telomere function?

To prevent the loss of functional parts of the genome following DNA replication Signup and view all the answers

What is the result of the Philadelphia Chromosome translocation?

Formation of a hybrid gene that is active Signup and view all the answers

Study Notes

DNA Replication and Mutagenesis

Overview of DNA Replication

DNA replication is part of the cell cycle in eukaryotic cells
It occurs during the S phase of the cell cycle
The process involves the unwinding of DNA, synthesis of new strands, and the formation of replication forks

Structure of the Human Genome

The human genome consists of 23 pairs of chromosomes
The total number of nucleotides is approximately 3.2 x 10^10
Only 1% of the genome encodes proteins (20,000 genes)
Regulatory regions of genes make up about 4% of the genome
Other unique sequences account for about 44% of the genome
Repeated sequences, including LINES, SINES, and satellite DNA, make up about 51% of the genome

Initiation of Replication

Replication begins at the origin of replication
The double helix is unwound, and the bases are exposed
DNA polymerase synthesizes new strands using the parental strands as templates
The process is semi-conservative, resulting in two new DNA molecules, each containing one parental strand and one daughter strand

The Replication Fork

The replication fork is the region where the DNA is unwound and replication occurs
Helicase unwinds the DNA, and single-strand binding proteins stabilize the single-stranded DNA
Topoisomerase breaks a phosphodiester bond in one parental strand to prevent supercoiling
The leading strand is synthesized continuously, while the lagging strand is synthesized in short, discontinuous segments called Okazaki fragments

Role of Enzymes in DNA Replication

Helicase: unwinds DNA strands
Topoisomerase: releases supercoils in DNA
Single-strand binding protein: stabilizes single-stranded DNA
Primase: makes RNA primer for DNA synthesis
DNA polymerase: synthesizes DNA
Exonuclease: removes RNA primer
DNA ligase: joins adjacent Okazaki fragments

Fidelity of DNA Replication

DNA polymerase has a proof-reading mechanism to correct errors during DNA synthesis
The enzyme also has a 3'-5' exonuclease activity to remove incorrect nucleotides

Origins of Replication

There are multiple origins of replication per eukaryotic chromosome
Bacterial chromosomes are circular and have a single origin of replication

Chromosome End Problem

The end of the chromosome is lost during DNA replication due to the inability of DNA polymerase to synthesize DNA at the very end of the chromosome
Telomeres, specialized structures at the end of chromosomes, consist of hundreds of repeats of the sequence 5' TTAGGG 3'
Telomeres shorten with every round of DNA replication, leading to cellular senescence

Types of Mutation

Point mutation: a single base is changed
Silent mutation: a change that does not affect the encoded amino acid
Missense mutation: a change that affects the encoded amino acid
Nonsense mutation: a change that results in a stop codon
Small scale insertions or deletions: indels
Large scale changes to chromosomes: inversions, deletions, duplications, translocations

Cause of Mutagenesis

Spontaneous errors in DNA replication
Replication slippage associated with repetitive regions
DNA damage caused by spontaneous deamination, UV radiation, ionizing radiation, and alkylating agents
Most DNA damage is repaired through direct repair, nucleotide excision repair, or homologous recombination

Chemotherapy Drugs

Methotrexate: inhibits the synthesis of thymine
Doxorubicin: binds to DNA and prevents the action of topoisomerase
5-fluorouracil: inhibits the synthesis of thymine

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Description

This quiz covers the fundamentals of molecular and cellular genetics, including DNA replication, mutagenesis, and the chromosome end-problem. Understand the chemistry and structure of DNA/RNA and appreciate the clinical importance of DNA mutations.