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Chapter 16: The Molecular Basis of Inheritance - Coggle Diagram
Chapter 16: The Molecular Basis of Inheritance
DNA Replication
Process of DNA Replication
Topoisomerase and Helicase unwind and unzip the parental double helix
Topoisomerase
Relieves overwinding strain ahead of replication forks by breaking, swiveling, and rejoining DNA strands
Helicase
Unzips parental double helix at replication fork
Molecules of single-strand binding protein stabilize the unwound template strand
The leading strand is synthesized continuously in the 5' to 3' direction by DNA Polymearse III
DNA Polymerase
Using parental DNA as a template, synthesizes new DNA strand by adding nucleotides to an RNA primer or a pre-existing DNA Strand
Leading Strand
The strand synthesized continuously in the 5' to 3' direction
Single-Stranded Binding Protein
Binds to and stabilizes single-stranded DNA until it is used as a template
Primase begins synthesis of the RNA primer for the Okazaki Fragments
RNA Primer
A short segment of RNA that is synthesized by the enzyme Primase
Okazaki Fragment
Short sequences of DNA nucleotides synthesized discontinuously on the lagging stran
Primase
A crucial enzyme that synthesizes short RNA primers
DNA Polymerase III completes synthesis on one Okazaki Fragment, DNA Polymerase III will detach and reattach to the next fragment's RNA Primer
Lagging Strand
The strand synthesized discontinuously in short segments known as Okazaki Fragments
DNA Polymerase I will remove RNA primers and replaces with DNA bases
DNA Polymerase I
Removes RNA nucleotides of primer from 5' end and replaces them with DNA nucleotides added to 3' end of adjcent fragment
DNA Ligase joins the 3' end of fragment 2 to the 5' end of fragment 1
DNA Ligase
On lagging strand, joins Okazaki Fragments
On leading strand, joins 3' ends of DNA that replaces primer to rest of leading strand of DNA
Proofreading and Repairing
DNA Polymerase
DNA Polymerase has a proofreading ability to it
DNA polymerases proofread each nucleotide against its template as soon as it is covalently bonded to the growing strand
Errors in the completed DNA molecule amount to only one in 10 billion nucleotides, an error rate that is 10,000 times lower
Types
Mismatch Repair
The cellular process that uses specific enzymes to remove and replace incorrectly paired nucleotides
Results from replication errors
Mismatched nucleotides sometimes evade proofreading by a DNA polymerase
Applications
Hereditary Defect
One defect found to be associated with colon cancer
This defect allows cancer-causing errors to accumulate in the DNA faster than normal
Nucleotide Excision Repair
A repair system that removes and then correctly replaces a damaged segment of DNA using the undamaged strand as a guide
A nucleases cuts out and replaces damaged stretches of DNA
Nuclease
An enzyme that cuts DNA or RNA, either removing one or a few bases or hydrolyzing the DNA or RNA completely into its component nucleotides
Applications
Xeroderma Pigmentosum (XP)
Caused by an inherited defect in a nucleotide excision repair enzyme
Mutations in their skin cells caused by ultraviolet light are left uncorrected, often resulting in skin cancer
DNA Makeup/DNA Molecules
DNA Makeup
Structure
Semiconservative Model of Replication
Predicts that when a double helix replicates, each daughter molecules will have one old strand of DNA
Double Helix Model
DNA is composed of two antiparallel strands forming a double helix
The sugar-phosphate backbones are on the outside, and the nitrogenous bases are on the inside
X-Ray Crystallography
A powerful technique used to determine the three-dimensional structure of molecules, such as proteins and nucleic acids
The X-ray diffraction images showed that DNA is helical
The images indicated the width of the helix and the spacing of the nitrogenous bases
DNA Replication
The double helix structure suggested a mechanism for DNA replication
Each strand serves as a template for a new complementary strand.
Base Pairing
The nitrogenous bases pair specifically: adenine (A) with thymine (T) and guanine (G) with cytosine (C)
This pairing is due to Hydrogen Bonding between the bases
2 Bonds between A and T
3 Bonds between G and C
Virus
Bacteriophages
A virus that infects bacteria; also called a phage
Virus
An infectious particle incapable of replicating outside of a cell, consisting of an RNA or DNA genome surrounded by a protein coat (capsid) and, for some viruses, a membranous envelope
Makeup
Nucleotides
The monomers for DNA
Composed of a Phosphate group, a Sugar, and a Nitrogen Base
Pyrine
Adenosine (A) and Guanine (G)
Pyrimidine
Cytosine (C) and Thymine (T)
Chargaff's Rule
The percentage of adenine (A) equals thymine (T), and the percentage of cytosine (C) equals guanine (G) in a DNA molecule
Base Composition Variation
The ratio of A/T and C/G varies between species due to unique DNA sequences
DNA Molecules
Replicating the Ends of DNA Molecules
Eukaryotic Chromosomal DNA molecules have special nucleotides sequences at their ends called Telomeres
Telomeres
The tandemly repetitive DNA at the end of a eukaryotic chromosome’s DNA molecule
Do not contain genes; instead, the DNA typically consists of multiple repetitions of one short nucleotide sequence
Functions
Protection from DNA Damage
Prevent the staggered ends of DNA from being mistaken for DNA damage, which could otherwise trigger cell cycle arrest or cell death
Buffer against Gene Erosion
Act as buffer zones, delaying the shortening of essential genes during DNA replication, similar to how plastic tips protect shoelaces from fraying
Eukaryotic/Prokaryotic
For linear DNA, such as the DNA of eukaryotic chromosomes, the usual replication machinery cannot complete the 5' ends of daughter DNA strands because there is no 3' end of a preexisting polynucleotide for DNA polymerase to add onto
Most prokaryotes have a circular chromosome, with no ends, so the shortening of DNA does not occur
DNA Molecules Packed Together with Proteins
Prokaryotic
The main component is a double-stranded, circular DNA molecule that is associated with specific proteins
Eukaryotic
DNA is precisely combined with a large amount of protein
Chromatin
A complex of DNA and proteins found in eukaryotic cells
Euchromatin
The less condensed form of eukaryotic chromatin that is available for transcription
Appears as less dense regions in the nucleus
Allows for gene expression
Heterochromatin
Eukaryotic chromatin that remains highly compacted during interphase and is generally not transcribed
Doesn't typically allow for gene expression
Appears as dense clumps, often near centromeres and telomeres
Nucleosomes
The basic units of DNA packing, appearing as "beads on a string" with DNA wrapped around histone cores
Involved in regulating gene expression by controlling the accessibility of DNA to transcription machinery
Histones
Proteins that play a crucial role in the first level of DNA packing in chromatin
They're positively charged, allowing them to bind tightly to the negatively charged DNA
Acetylation
Histone tails generally promotes transcription by loosening chromatin structure, making DNA more accessible
Methylation
Leads to chromatin condensation, reducing transcription