Biology Chapter: Nucleotides and DNA Formation

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Questions and Answers

What distinguishes ribose from deoxyribose in the nucleotide structure?

Presence of a phosphate group

Difference in the nitrogen base

Presence of a double-ring structure

Presence of an OH group on the 2nd carbon (correct)

How are the two strands of DNA oriented in relation to each other?

Both strands run in the same direction

Base pairs connect the two strands instead of phosphodiester bonds

The strands run in an anti-parallel direction (correct)

One strand has a 3’ end aligned with the other strand’s 3’ end

Which of the following nitrogen bases is unique to RNA?

Thymine

Cytosine

Adenine

Uracil (correct)

What type of bond forms the sugar-phosphate backbone of a DNA strand?

Phosphodiester bonds Signup and view all the answers

Which statement correctly describes the size and structure of purines and pyrimidines?

Purines are large, double-ringed bases Signup and view all the answers

What role do hydrogen bonds play in the structure of DNA?

They connect the nitrogen bases of opposite strands Signup and view all the answers

Which nitrogen bases are classified as pyrimidines in DNA?

Cytosine and Thymine Signup and view all the answers

In the context of DNA, what does the term 'anti-parallel' refer to?

The orientation of the nucleotide sugar-carbon backbones Signup and view all the answers

What is the primary function of the enzyme primase in DNA replication?

To synthesize RNA primers complementary to template strands Signup and view all the answers

Why can DNA polymerase only add nucleotides in a 5’ to 3’ direction?

It requires a 3' hydroxyl group to form a bond Signup and view all the answers

What enzyme is responsible for joining Okazaki fragments on the lagging strand?

DNA ligase Signup and view all the answers

What occurs during the DNA mismatch repair process?

Mistakes are identified, removed, and fixed by DNA polymerase and ligase Signup and view all the answers

Which type of DNA damage is caused by the removal of NH3 from a nitrogen base?

Deamination Signup and view all the answers

What is the role of nuclease enzymes in DNA replication?

To remove RNA primers Signup and view all the answers

What is the consequence of thymine dimer formation?

It stalls DNA replication Signup and view all the answers

What type of mutation can result from depurination?

Deletion mutation Signup and view all the answers

What type of bond forms between adenine and thymine in DNA?

Hydrogen bond Signup and view all the answers

In human chromosomes, how many pairs are categorized as somatic chromosomes?

22 Signup and view all the answers

Which method can be used to distinguish different chromosome pairs?

Fluorescent dyes Signup and view all the answers

What are introns primarily characterized as in gene structure?

Non-coding regions Signup and view all the answers

During which phase are chromosomes highly condensed compared to interphase?

Prophase Signup and view all the answers

What is the principal function of telomeres in chromosomes?

To ensure complete DNA replication Signup and view all the answers

Which type of chromatin is characterized by being highly condensed and inaccessible for transcription?

Heterochromatin Signup and view all the answers

Which component is necessary to form a nucleosome?

Histone octamer Signup and view all the answers

What role does miRNA play in gene expression?

Inhibits translation of mRNA Signup and view all the answers

Study Notes

Nucleotides

Nucleotides are the building blocks of nucleic acids (DNA and RNA).
Each nucleotide has a pentose sugar (ribose in RNA, deoxyribose in DNA), a nitrogenous base, and a phosphate group.
The only difference between ribose and deoxyribose is the presence of an OH group on the 2' carbon in ribose, absent in deoxyribose.
Nitrogenous bases are either purines (double-ringed: Adenine (A), Guanine (G)) or pyrimidines (single-ringed: Cytosine (C), Thymine (T) in DNA; Uracil (U) in RNA).

Formation of DNA

DNA is a double helix formed by two antiparallel strands of nucleotides.
Phosphodiester bonds link nucleotides within each strand (sugar-phosphate backbone).
The strands run in opposite 5' to 3' directions.
Hydrogen bonds form between complementary base pairs: A=T (2 hydrogen bonds), C≡G (3 hydrogen bonds).
Complementary base pairing maintains a constant distance between the two DNA strands.

Chromosomes

Chromosomes are highly condensed DNA molecules.
Humans have 23 pairs of chromosomes; 22 pairs are somatic (homologous), and one pair are sex chromosomes (XX in females, XY in males).
Different chromosome pairs can be distinguished by size, fluorescent dye binding, and staining patterns (e.g., Giemsa stain).
Genes are specific DNA segments coding for RNA molecules, which may be translated into proteins or remain as non-protein coding RNA.
Chromosomes also contain non-protein coding regions: introns, promotor and regulatory regions.
Repeated sequences (satellites) include centromeres (attachment sites for spindle fibers), telomeres (protective caps), and replication origins (DNA replication initiation sites).
Repeated sequences can cause mutations.

Structure of Interphase Chromosomes

Interphase chromosomes are less condensed than mitotic chromosomes. They're longer and finer, and located within the nucleus.
Specific regions of the nucleus are occupied by different chromosomes. Some chromosomes are tethered to the nuclear membrane or nuclear lamina.
The nucleolus is formed by the convergence of certain chromosomes (containing genes for rRNA). This is where rRNA is transcribed and ribosomes assemble.
Chromatin is the complex of DNA and histone proteins.
Nucleosomes are formed when DNA wraps around histone cores ('beads on a string').
DNA is further packaged into chromatin fibers, and then into interphase chromosomes.
The final condensation step results in the creation of mitotic chromosomes.

Euchromatin vs Heterochromatin

Chromatin exists as euchromatin and heterochromatin during interphase.
Euchromatin is less condensed which allows transcription. Found in gene-rich regions.
Heterochromatin is highly condensed, making transcription impossible. It's located in gene-poor areas, including centromeres.
Heterochromatin can silence genes; for example, one X chromosome is inactivated in females.

DNA Replication

DNA replication is semi-conservative, where each strand serves as a template.
Replication begins at replication origins, typically rich in A=T base pairs.
Initiator proteins separate DNA strands.
DNA helicase and replication forks open up the DNA helix.
Primers are short RNA sequences synthesized by primase, providing a starting point for DNA polymerase.
DNA polymerase adds free nucleotides to the 3' end of the primer, following complementary base-pairing rules.
Proofreading by DNA polymerase ensures accuracy. Continuous synthesis of the leading strand; lagging strand synthesis leads to Okazaki fragments
RNA primers are removed and replaced by DNA by DNA polymerase.
DNA ligase seals the gaps between Okazaki fragments, completing the new strands.
Mismatch repair systems scan and correct errors.

Depurination – spontaneous loss of purine bases.
Deamination – spontaneous conversion of a base (e.g. C to U).
Dimerization – formation of base dimers (e.g., thymine dimers) due to UV radiation.
These damages are repaired by nucleases, DNA repair polymerase and DNA ligase.

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Description

This quiz covers essential concepts related to nucleotides, the building blocks of DNA and RNA, and the formation of DNA structure. It explores the composition of nucleotides, the formation of the DNA double helix, and the role of chromosomes in genetic information storage. Test your understanding of these fundamental biological concepts!