标签归档:Biology

What is a Scaffold?

A scaffold is a portion of the genome sequence reconstructed from end-sequenced whole-genome shotgun clones. Scaffolds are composed of contigs and gaps. A contig is a contiguous length of genomic sequence in which the order of bases is known to a high confidence level. Gaps occur where reads from the two sequenced ends of at least one fragment overlap with other reads in two different contigs (as long as the arrangement is otherwise consistent with the contigs being adjacent). Since the lengths of the fragments are roughly known, the number of bases between contigs can be estimated.

The goal of whole-genome shotgun assembly is to represent each genomic sequence in one scaffold; however, this is not always possible. One chromosome may be represented by many scaffolds (e.g., Chlamydomonas reinhardtii) or just a single scaffold (e.g., Human chromosome 19), depending on how completely the genome can be reconstructed, or assembled, from the available reads. The relative locations of scaffolds in the genome are unknown.

Scaffolds are normally numbered approximately from largest to smallest. Some scaffolds may ultimately be filtered out of the assembly, resulting in skipped scaffold numbers.
In some cases, scaffolds can overlap. For example, in polymorphic genomes, regions with a high density of allelic differences between haplotypes may be split into separate sets of scaffolds, each representing one allele. Thus, a sequence that exists in only one location in the genome may appear on more than one scaffold.
Gaps are shown in the Genome Viewer as red lines or rectangles in the scaffold track (viewed in “full” mode). Contigs are shown in black. In FASTA sequences, gaps are represented by a series of Ns.

miRNA和siRNA的基本介绍及区别(转)

1998年,Andrew Fire和Craig Mello提出了一项新技术:通过dsRNA诱导特异基因的沉默,即所谓RNAi。2000年,Amy Pasquinelli等将lin-4和let-7作小时序RNAs(stRNAs,mall temporal RNAs)。

RNA干涉(RNAi)在实验室中是一种强大的实验工具,利用具有同源性的双链RNA(dsRNA)诱导序列特异的目标基因的沉寂,迅速阻断基因活性。SiRNA在RNA沉寂通道中起中心作用,是对特定信使RNA(mRNA)进行降解的指导要素。siRNA是RNAi途径中的中间产物,是RNAi发挥效应所必需的因子。SiRNA的形成主要由Dicer和Rde-1调控完成。由于RNA 病毒入侵、转座子转录、基因组中反向重复序列转录等原因,细胞中出现了dsRNA,Rde-1(RNAi缺陷基因-1)编码的蛋白质识别外源dsRNA,当dsRNA达到一定量的时候,Rde-1引导dsRNA与Rde-1编码的Dicer(Dicer是一种RNaseIII 活性核酸内切酶,具有四个结构域:Argonaute家族的PAZ结构域,III型RNA酶活性区域,dsRNA结合区域以及DEAH/DEXHRNA解旋酶活性区)结合,形成酶-dsRNA复合体。在Dicer酶的作用下,细胞中的单链靶mRNA(与dsRNA具有同源序列)与dsRNA的正义链互换,原来dsRNA中的正义链被mRNA代替而从酶-dsRNA复合物中释放出来,然后,在ATP的参与下,细胞中存在的一种RNA诱导的沉默复合体RNA-induced silencing complex (RISC,由核酸内切酶、核酸外切酶、解旋酶等构成,作用是对靶mRNA进行识别和切割)利用结合在其上的核酸内切酶的活性来切割dsRNA上处于原来正义链位置的靶mRNA分子中与dsRNA反义链互补的区域,形成21-23nt的dsRNA小片段,这些小片段即为siRNA。RNAi干涉的关键步骤是组装RISC和合成介导特异性反应的siRNA蛋白。SiRNA并入RISC中,然后与靶标基因编码区或UTR区完全配对,降解靶标基因,因此说siRNA只降解与其序列互补配对的mRNA。其调控的机制是通过互补配对而沉默相应靶位基因的表达,所以是一种典型的负调控机制。siRNA识别靶序列是有高度特异性的,因为降解首先在相对于siRNA来说的中央位置发生,所以这些中央的碱基位点就显得极为重要,一旦发生错配就会严重抑制RNAi的效应,相对而言,3′末端的核苷酸序列并不要求与靶mRNA完全匹配。 继续阅读