Anatomy of dicot roots

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The document discusses the anatomy of dicot roots. It describes a cross section of a dicot root showing the different tissues including the epidermis, cortex, endodermis, pericycle, vascular bundles, and pith. It notes that in dicot roots, the primary xylem is exarch, meaning the protoxylem lies towards the periphery and the metaxylem lies towards the pith.

Science

Enlarged view of cross section of
Dicot root
⚫ Anatomy

Endarch: The protoxylem lies towards the
pith in stem and the metaxylem lies towards
the periphery. This type of primary xylem is
called endarch.
Exarch: The protoxylem lies towards the
periphery in roots and the metaxylem lies
towards the pith. This type of primary xylem
is called exarch.

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Introduction: RNA interference (RNAi) or Post-Transcriptional Gene Silencing (PTGS) is an important biological process for modulating eukaryotic gene expression. It is highly conserved process of posttranscriptional gene silencing by which double stranded RNA (dsRNA) causes sequence-specific degradation of mRNA sequences. dsRNA-induced gene silencing (RNAi) is reported in a wide range of eukaryotes ranging from worms, insects, mammals and plants. This process mediates resistance to both endogenous parasitic and exogenous pathogenic nucleic acids, and regulates the expression of protein-coding genes. What are small ncRNAs? micro RNA (miRNA) short interfering RNA (siRNA) Properties of small non-coding RNA: Involved in silencing mRNA transcripts. Called “small” because they are usually only about 21-24 nucleotides long. Synthesized by first cutting up longer precursor sequences (like the 61nt one that Lee discovered). Silence an mRNA by base pairing with some sequence on the mRNA. Discovery of siRNA? The first small RNA: In 1993 Rosalind Lee (Victor Ambros lab) was studying a non- coding gene in C. elegans, lin-4, that was involved in silencing of another gene, lin-14, at the appropriate time in the development of the worm C. elegans. Two small transcripts of lin-4 (22nt and 61nt) were found to be complementary to a sequence in the 3' UTR of lin-14. Because lin-4 encoded no protein, she deduced that it must be these transcripts that are causing the silencing by RNA-RNA interactions. Types of RNAi ( non coding RNA) MiRNA Length (23-25 nt) Trans acting Binds with target MRNA in mismatch Translation inhibition Si RNA Length 21 nt. Cis acting Bind with target Mrna in perfect complementary sequence Piwi-RNA Length ; 25 to 36 nt. Expressed in Germ Cells Regulates trnasposomes activity MECHANISM OF RNAI: First the double-stranded RNA teams up with a protein complex named Dicer, which cuts the long RNA into short pieces. Then another protein complex called RISC (RNA-induced silencing complex) discards one of the two RNA strands. The RISC-docked, single-stranded RNA then pairs with the homologous mRNA and destroys it. THE RISC COMPLEX: RISC is large(>500kD) RNA multi- protein Binding complex which triggers MRNA degradation in response to MRNA Unwinding of double stranded Si RNA by ATP independent Helicase Active component of RISC is Ago proteins( ENDONUCLEASE) which cleave target MRNA. DICER: endonuclease (RNase Family III) Argonaute: Central Component of the RNA-Induced Silencing Complex (RISC) One strand of the dsRNA produced by Dicer is retained in the RISC complex in association with Argonaute ARGONAUTE PROTEIN : 1.PAZ(PIWI/Argonaute/ Zwille)- Recognition of target MRNA 2.PIWI (p-element induced wimpy Testis)- breaks Phosphodiester bond of mRNA.)RNAse H activity. MiRNA: The Double-stranded RNAs are naturally produced in eukaryotic cells during development, and they have a key role in regulating gene expression .

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