Genetics and Evolution

Molecular Basis of Inheritance

Formation of Chromatin Fibre

Introduction to Chromatin Fibre
Structure and Components of Chromatin Fibre
Nucleosomes and Histones
Histone Modifications
Heterochromatin and Euchromatin

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Introduction to Chromatin Fibre

Chromatin fibre is a complex of DNA, proteins, and RNA found in the nucleus of eukaryotic cells.
It plays a critical role in packaging DNA to fit inside the nucleus.
Chromatin fibre undergoes structural changes during various cellular processes.
The organization of chromatin fibre is tightly regulated to control gene expression.

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Structure and Components of Chromatin Fibre

Chromatin fibre is made up of repeating subunits called nucleosomes.
Nucleosomes consist of DNA wrapped around a protein core called histones.
Histones are positively charged proteins that help in DNA packaging.
Linker DNA connects adjacent nucleosomes.
Other proteins, such as histone variants and non-histone proteins, also contribute to chromatin fibre structure.

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Nucleosomes and Histones

Nucleosomes are composed of eight histone proteins: two copies each of H2A, H2B, H3, and H4.
The histone proteins form an octamer core around which DNA is wrapped.
The DNA wraps around the histone octamer in 1.65 turns, forming a left-handed superhelix.
This wrapping of DNA results in condensed chromatin structure and helps in DNA compaction.

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Histone Modifications

Histone proteins undergo various chemical modifications, including acetylation, methylation, phosphorylation, etc.
These modifications can affect chromatin structure and gene expression.
For example, acetylation of histones is associated with gene activation, while methylation can be associated with gene repression.
Histone modifications can be added or removed by specific enzymes, called histone modifying enzymes.

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Heterochromatin and Euchromatin

Chromatin fibre can exist in two main forms: heterochromatin and euchromatin.
Heterochromatin is a highly condensed form of chromatin, usually transcriptionally inactive.
Euchromatin is a less condensed form of chromatin, usually transcriptionally active.
The balance between heterochromatin and euchromatin is crucial for proper gene regulation and cellular function.

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Conclusion

Chromatin fibre is a dynamic structure that undergoes changes to control gene expression.
Nucleosomes and histones play a crucial role in the packaging and organization of DNA.
Histone modifications can affect chromatin accessibility and gene expression.
The balance between heterochromatin and euchromatin is essential for proper gene regulation.

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Slide 11

X-Inactivation and Barr Body Formation

X-inactivation is a process that occurs in female mammals to compensate for the presence of two X chromosomes.
One of the X chromosomes in each cell is randomly inactivated and forms a heterochromatic structure called a Barr body.
The Barr body ensures that both males and females have the same dosage of X-linked genes.
The inactivation of an X chromosome is permanent and is passed on to all daughter cells.

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Slide 12

Chromatin Remodeling

Chromatin remodeling refers to the changes in chromatin structure that allow regulatory factors to access DNA.
Remodeling complexes use ATP to alter nucleosome positions or to evict nucleosomes, making DNA more accessible.
Chromatin remodeling is essential for gene activation, DNA repair, and other cellular processes.
Examples of chromatin remodeling complexes include SWI/SNF and ISWI complexes.

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Slide 13

Epigenetics and Chromatin Modifications

Epigenetics refers to heritable changes in gene expression that do not involve changes to the DNA sequence.
Chromatin modifications, such as histone modifications and DNA methylation, play a crucial role in epigenetic regulation.
These modifications can be maintained through cell divisions and can affect gene expression patterns.
Epigenetic changes can be influenced by environmental factors and can be reversible.

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Slide 14

Chromatin Immunoprecipitation (ChIP)

ChIP is a technique used to study protein-DNA interactions and chromatin modifications.
In ChIP, specific proteins or chromatin modifications are “pulled down” using antibodies.
The DNA that is associated with the protein or modification of interest can then be analyzed.
ChIP allows us to determine which regions of the genome are bound by specific proteins and study their functional role.

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Slide 15

Chromosome Conformation Capture (3C)

3C is a molecular biology technique used to study the three-dimensional organization of the genome.
In 3C, cross-linking is performed to freeze interactions between DNA segments that are spatially close in the nucleus.
The cross-linked DNA is then digested, ligated, and analyzed using PCR or sequencing.
3C allows us to determine the physical proximity of specific genomic regions and study their interactions.

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Slide 16

Long Non-Coding RNAs (lncRNAs)

lncRNAs are a class of non-coding RNAs that are longer than 200 nucleotides.
They play diverse regulatory roles in gene expression, chromatin organization, and cellular processes.
lncRNAs can interact with chromatin, DNA, and proteins to modulate gene expression and chromatin structure.
Examples of lncRNAs include Xist, HOTAIR, and MALAT1.

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Slide 17

Small Interfering RNAs (siRNAs)

siRNAs are small RNA molecules that regulate gene expression through RNA interference (RNAi).
They can bind to target messenger RNAs (mRNAs), leading to their degradation or translational inhibition.
siRNAs are involved in many biological processes, including defense against viral infections and regulation of gene expression.
Researchers have used siRNAs as tools for gene silencing and studying gene function.

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Slide 18

RNA Interference (RNAi)

RNAi is a biological process in which RNA molecules silence gene expression.
It can be triggered by endogenous small RNAs or introduced siRNAs.
RNAi regulates gene expression at the post-transcriptional level, impacting mRNA stability or translation.
RNAi has applications in gene therapy, crop improvement, and functional genomics research.

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Slide 19

Chromatin Disorders and Diseases

Abnormalities in chromatin structure and function can lead to various genetic disorders and diseases.
Examples include Rett syndrome, Kabuki syndrome, and immunodeficiency-centromeric instability-facial anomalies (ICF) syndrome.
Dysregulation of chromatin remodeling, histone modifications, and DNA methylation are often observed in cancer.
Understanding chromatin dynamics and its role in disease can lead to new therapeutic strategies.

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Slide 20

Summary

Chromatin fibre is a complex structure formed by DNA and associated proteins.
Nucleosomes and histones play a critical role in DNA packaging and compaction.
Chromatin undergoes various modifications and remodeling to regulate gene expression.
Epigenetic modifications can influence gene expression patterns.
Techniques like ChIP and 3C help in studying chromatin and genomic interactions.
Long non-coding RNAs and small interfering RNAs are involved in gene regulation.
Chromatin disorders and abnormalities contribute to human diseases.
Understanding chromatin dynamics is crucial for unraveling the molecular basis of inheritance.

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Slide 21

DNA Replication

DNA replication is the process by which DNA is synthesized. It occurs before cell division.
The original DNA molecule serves as a template for the synthesis of a new complementary strand.
The replication process is highly accurate due to the base-pairing rules (A-T, G-C) and proofreading mechanisms.
DNA replication is carried out by a group of enzymes known as DNA polymerases.
Each DNA strand is replicated in a semi-conservative manner, resulting in two identical daughter DNA molecules.

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Slide 22

Transcription

Transcription is the process by which an RNA molecule is synthesized using a DNA template.
It is the first step in gene expression and occurs in the nucleus of eukaryotic cells.
RNA polymerase enzyme catalyzes the formation of a complementary RNA strand using DNA as a template.
The newly synthesized RNA molecule undergoes post-transcriptional modifications, such as capping, splicing, and polyadenylation.
Transcription plays a crucial role in regulating gene expression and producing different types of RNA molecules.

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Slide 23

Translation

Translation is the process by which proteins are synthesized using the information encoded in RNA molecules.
It occurs on ribosomes in the cytoplasm.
During translation, the sequence of nucleotides in mRNA is decoded into a sequence of amino acids.
Transfer RNA (tRNA) molecules bring the appropriate amino acids to the ribosome.
The process of translation involves initiation, elongation, and termination.

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Slide 24

Genetic Code

The genetic code is a set of rules that determines how nucleotide sequences are translated into amino acid sequences.
It is a triplet code, meaning that three nucleotides (codon) encode one amino acid.
There are a total of 64 possible codons, including start and stop codons.
The genetic code is almost universal across organisms, with a few exceptions.
Examples: AUG codes for methionine, UUU codes for phenylalanine, UGA is a stop codon.

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Slide 25

Gene Regulation

Gene regulation refers to the mechanisms that control the expression of genes.
It allows cells to respond to various signals and environmental changes.
Gene regulation can occur at multiple levels, including transcriptional, post-transcriptional, translational, and post-translational.
Transcriptional regulation is the most common and important mechanism.
It involves the binding of regulatory proteins (transcription factors) to specific DNA sequences.

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Slide 26

Operons

Operons are functional units in prokaryotes that consist of a cluster of genes and their regulatory elements.
They allow for the coordinate regulation of gene expression.
The operon consists of a promoter, operator, and genes coding for proteins involved in the same metabolic pathway.
The lac operon is a well-known example of an inducible operon.
The trp operon is an example of a repressible operon.

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Slide 27

Transcription Factors

Transcription factors are proteins that bind to specific DNA sequences and regulate gene expression.
They can activate or repress the transcription of target genes.
Transcription factors can interact with other proteins and modify chromatin structure to control gene expression.
Different combinations of transcription factors determine cell-specific gene expression patterns.
Mutations in transcription factor-encoding genes can lead to developmental disorders and diseases.

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Slide 28

Mutation

Mutations are changes in the DNA sequence that can affect gene expression and protein structure.
Mutations can occur spontaneously or as a result of environmental factors, replication errors, or exposure to mutagens.
Types of mutations include point mutations, insertions, deletions, and chromosomal rearrangements.
Some mutations can be beneficial, neutral, or deleterious, depending on their effects.
Mutations are a driving force in evolution and can lead to the development of new traits and species.

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Slide 29

Genetic Disorders

Genetic disorders are diseases that result from abnormalities in DNA sequence or chromosomal structure.
They can be inherited or occur de novo (new mutations).
Examples of genetic disorders include cystic fibrosis, Down syndrome, sickle cell anemia, and Duchenne muscular dystrophy.
Genetic disorders can be diagnosed through various techniques, including karyotyping, DNA sequencing, and genetic testing.
Some genetic disorders can be treated or managed through early diagnosis and medical interventions.

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Slide 30

Gene Therapy

Gene therapy is a promising approach for treating genetic disorders by introducing functional genes into cells.
It involves delivering therapeutic genes using viral vectors, liposomes, or other delivery systems.
Gene therapy can be used to replace defective genes, introduce new genes, or modify gene expression.
Challenges in gene therapy include the efficiency of gene delivery, immune responses, and long-term safety concerns.
Despite the challenges, gene therapy offers great potential for the treatment of genetic disorders.

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