DNA Structure: Key Concepts and Discoveries

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Questions and Answers

Where does the chromosome lie in prokaryotes?

In the endoplasmic reticulum

In membrane-bound organelles

In the cytoplasm in an area called the nucleoid (correct)

In the nucleus

What is the structure formed when DNA wraps around histones?

Chromatin

Chromatid

Centromere

Nucleosome (correct)

What packing strategy do eukaryotes use to fit their DNA inside the nucleus?

Random arrangement

Beads on a string and further coiling (correct)

Supercoiling

Linear alignment

What are the proteins called that DNA wraps around in eukaryotic cells?

Histones Signup and view all the answers

At what stage of mitosis are chromosomes most compacted?

Metaphase Signup and view all the answers

What do the darkly staining regions of eukaryotic chromosomes usually contain?

Inactive genes and heterochromatin Signup and view all the answers

What is the approximate width of chromosomes at their most condensed during metaphase?

700 nm Signup and view all the answers

What does the short strand of DNA between nucleosomes lack?

Histones Signup and view all the answers

What is the primary purpose of DNA replication?

To ensure each daughter cell receives an identical copy of DNA Signup and view all the answers

During which phase of the cell cycle does DNA replication occur?

S phase Signup and view all the answers

Which model describes the mechanism of DNA replication using the existing strands as templates?

Semiconservative model Signup and view all the answers

Which statement about the complementary nature of DNA strands is true?

Cytosine pairs with guanine Signup and view all the answers

What is a consequence of the complementarity of DNA strands during replication?

One strand can recreate a complementary strand Signup and view all the answers

How does the semiconservative replication model benefit DNA replication?

It allows for accurate copying of genetic information Signup and view all the answers

What happens to the original DNA strands during the process of replication?

They remain intact and serve as templates Signup and view all the answers

Why is telomerase important in the context of DNA replication?

It extends the telomeres during DNA replication Signup and view all the answers

What is the primary reason for the extensive study of DNA replication in prokaryotes?

The small size of prokaryotic genomes and availability of variants Signup and view all the answers

How long does it take for Escherichia coli to replicate its entire genome?

42 minutes Signup and view all the answers

What is the rate of DNA replication in prokaryotes?

1000 nucleotides/s Signup and view all the answers

Which of the following statements about eukaryotic DNA replication is correct?

Eukaryotes possess telomerase. Signup and view all the answers

What mechanism is employed when mismatched bases are found during DNA replication?

Mismatch repair Signup and view all the answers

Which type of DNA repair is particularly important for correcting thymine dimers?

Nucleotide excision repair Signup and view all the answers

What happens to incorrectly incorporated bases during DNA replication?

They are edited through a proofreading mechanism Signup and view all the answers

Which feature distinguishes prokaryotic chromosomes from eukaryotic chromosomes?

Prokaryotic chromosomes are circular Signup and view all the answers

What corresponds to one amino acid in the protein sequence?

Three mRNA nucleotides Signup and view all the answers

Where does transcription occur in eukaryotic cells?

Nucleus Signup and view all the answers

What is the role of a promoter in transcription?

It determines the rate of gene transcription Signup and view all the answers

What is the function of RNA polymerase during transcription?

To synthesize RNA from the DNA template Signup and view all the answers

Which strand of DNA is the template for mRNA synthesis?

The template strand Signup and view all the answers

Which component is present in RNA but not in DNA?

Uracil Signup and view all the answers

What must occur for transcription to be initiated?

The RNA polymerase must bind to a promoter Signup and view all the answers

In prokaryotes, where does transcription take place?

In the cytoplasm Signup and view all the answers

What is the term for the relationship between a nucleotide codon and its corresponding amino acid?

Genetic code Signup and view all the answers

How many possible combinations of nucleotide triplets are there in the genetic code?

64 Signup and view all the answers

What does the AUG codon specify in addition to being the start codon?

Methionine Signup and view all the answers

What is the role of stop codons in protein synthesis?

They signal the end of protein synthesis. Signup and view all the answers

Which component is NOT part of the initiation complex in E. coli protein synthesis?

Enzymatic proteins Signup and view all the answers

Which site in the large ribosomal subunit of E. coli is responsible for binding incoming charged tRNAs?

A site Signup and view all the answers

What evidence suggests that all life on Earth shares a common origin according to the genetic code?

The universal use of the genetic code Signup and view all the answers

Which phase is NOT part of the protein synthesis process?

Replication Signup and view all the answers

What role does the repressor protein play in the lac operon when lactose is not present?

It prevents the binding of RNA polymerase to the promoter. Signup and view all the answers

What happens to the repressor protein when lactose is present in the environment?

It undergoes a conformational change and can no longer bind to the operator. Signup and view all the answers

At which level can gene expression regulation occur in eukaryotic cells?

At multiple stages including transcription and translation. Signup and view all the answers

Where does transcription of eukaryotic DNA occur?

Within the nucleus. Signup and view all the answers

What is the function of the promoter sequence in the lac operon?

To initiate transcription when RNA polymerase binds to it. Signup and view all the answers

How does the intranuclear environment affect gene expression in eukaryotes?

It separates transcription and translation processes. Signup and view all the answers

What allows a bacterium like E.coli to metabolize lactose?

The binding of lactose to the repressor protein. Signup and view all the answers

In which cellular component does the translation of mRNA into protein occur?

Ribosomes in the cytoplasm. Signup and view all the answers

Study Notes

DNA Structure

DNA was discovered in 1953
Scientists Francis Crick and James Watson determined the structure of DNA
Linus Pauling and Maurice Wilkins were also involved in research
Rosalind Franklin used X-ray crystallography to understand DNA structure
Watson and Crick used Franklin's data and Chargaff's rules to determine the structure.
Chargaff's rules showed that in DNA, the amounts of adenine and thymine were equal, and guanine and cytosine were equal.
DNA is a double helix composed of two strands twisted around each other.
Bases pair in a complementary manner: adenine with thymine, and guanine with cytosine.
DNA is made up of four kinds of monomers called nucleotides
Each nucleotide contains a deoxyribose sugar, a phosphate group, and a nitrogenous base
Two types of nitrogenous bases are double-ringed purines (adenine and guanine)
Two types of nitrogenous bases are single-ringed pyrimidines (cytosine and thymine).
The phosphate group of one nucleotide joins covalently with the sugar molecule of the next.
DNA is a long polymer of nucleotides with sugar-phosphate backbones, and nucleotide bases that stick out.
The sugar-phosphate groups form a "backbone" for each strand.
Carbon atoms of the five-carbon sugar are numbered clockwise.
The phosphate group attaches to the 5' carbon of one nucleotide and 3' carbon of the next.
Complementary strands are held together by hydrogen bonds between the bases.

DNA Arrangement in Cells

Prokaryotic DNA is simpler than eukaryotic DNA
Most prokaryotes have one circular chromosome in an area called the nucleoid.
Eukaryotic DNA is bound to proteins known as histones that form nucleosomes.
Nucleosomes stack compactly onto each other to form a 30-nm-wide fiber.
This fiber is further coiled into a more compact structure during the metaphase stage of mitosis.
Eukaryotic chromosomes have two distinct regions
Darkly staining regions usually contain inactive genes
Lightly staining regions usually contain active genes.

DNA Replication

DNA is copied during DNA replication in the synthesis phase (S phase) of cell cycle before cell division.
The double helix separates into two strands.
Each strand serves as a template from which a new complementary strand.
Semiconservative Replication: Each new double strand is composed of one parental strand and one new daughter strand.

DNA Replication in Eukaryotes

Eukaryotic DNA is very complex and involves several enzymes and proteins for replication.
Replication starts at specific nucleotide sequences called origins of replication and proceeds in both directions.
Replication forks (Y-shapes structures) are formed at the origins of replication as the DNA double helix opens up.
DNA polymerase adds DNA nucleotides to the 3' end of the template.
Leading strand is synthesized continuously toward the replication fork, and this allows the DNA polymerase to add DNA nucleotides continuously.
Lagging strand is synthesized in short fragments called Okazaki fragments, this means that DNA is not synthesized continuously in the lagging strand.
RNA primers are required to start the synthesis of Okazaki fragments.
RNA primers are removed from the lagging strand by DNA polymerase and replaced by DNA nucleotides.
The gaps between fragments are sealed by DNA ligase.

DNA Replication in Prokaryotes

Easier DNA replication process due to the small genome size relative to eukaryotic DNA.
Replication starts at a single origin and proceeds in both directions around the circular chromosome
There is rapid replication rate of roughly 1000 nucleotides/second.

DNA Repair

DNA polymerase proofreads incoming bases after adding to prevent errors.
Mismatch repair mechanisms detect and remove incorrect bases later in the process.
Nucleotide excision repair removes damaged or incorrect bases and nucleotides.
Errors lead to mutations (permanent DNA sequence changes), some of which can result in diseases like cancer.

Transcription

DNA is transcribed into mRNA via transcription and the processes lead to protein creation within the cell.
DNA serves to provide the genetic information required for the formation of proteins necessary for the functions of a cell.
Transcription involves the synthesis of an mRNA molecule that is complementary to the DNA template strand.
Proceeds through initiation, elongation, and termination stages in Eukaryotes and Prokaryotes
Transcription takes place in the nucleus in eukaryotic cells and the cytoplasm in prokaryotic cells.
Multiple RNA polymerase enzymes are used in transcribing different genes .
DNA is partially unwound to produce a transcription bubble during initiation.
Promoter is the region where transcription factors and RNA polymerase bind to initiate transcription.
RNA polymerase proceeds along the template strand, synthesizing the mRNA.
RNA polymerase continues the process while unwinding and rewinding the DNA.
Terminator signals tell the RNA polymerase to stop and release the newly formed mRNA in prokaryotic cell termination.

RNA Processing in Eukaryotes

Eukaryotic mRNAs undergo processing steps before leaving the nucleus.
Modifications protect mRNA from degradation and aid in protein synthesis.
A 5' cap and poly-A tail are added to the 5' and 3' ends, respectively.
Introns (non-coding regions) are removed by splicing
Exons (coding regions) are joined together.
The mRNA is transported to cytoplasm after processing

Translation

Translation involves using the mRNA code to synthesize proteins.
It uses ribosomes, tRNA molecules, and amino acids.
mRNA specifies the sequence of amino acids that should be linked to form a protein.
Ribosomes are composed of rRNA and multiple proteins and function to help facilitate translation of process.
tRNAs bring amino acids to the ribosome based on the mRNA codons.
Several steps: initiation, elongation, and termination
Initiation: Brings together the small subunit of the ribosome, mRNA molecule, tRNA molecule and initiation factors
Elongation: Amino acids are added to the growing polypeptide chain.
Termination: The stop codon is recognized and the translation machinery leaves the template and disassembles.
The process generates a polypeptide chain which then folds into a functional protein.

Gene Regulation

Gene expression is controlled in prokaryotes mainly at the transcriptional level (how often a gene is transcribed).
Prokaryotes use operons (e.g., lac operon). The operons can be controlled by repressors that bind to DNA and block transcription or activators that do the opposite.
Gene expression in eukaryotes is regulated at multiple levels, including epigenetic, transcriptional, post-transcriptional, translational, and post-translational levels.
Eukaryotic cells use a complex system of proteins to control gene expression. These proteins influence where and when genes or certain parts of genes are expressed in response to signals.

Alternative RNA Splicing

Eukaryotic genes have multiple different sites that can be used during splicing
Splicing mechanism can sometimes fail to identify intron boundaries which can affect the proteins produced from genes.
Alternative splicing generates different protein isoforms from the same gene, influencing cell specialization

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Description

Explore the essential concepts surrounding the structure of DNA, including its discovery by scientists like Crick, Watson, Franklin, and others. Understand the significance of Chargaff's rules, the composition of nucleotides, and the double helix formation. This quiz provides a comprehensive overview of the molecular biology of DNA.