The centromere is the specialized DNA sequence of a chromosome that links a pair of sister chromatids. During mitosis, spindle fibers attach to the centromere via the kinetochore. Centromeres were first thought to be genetic loci; the physical role of the centromere is to act as the site of assembly of the kinetochores – a complex multiprotein structure, responsible for the actual events of chromosome segregation – i.e. binding microtubules and signalling to the cell cycle machinery when all chromosomes have adopted correct attachments to the spindle, so that it is safe for cell division to proceed to completion and for cells to enter anaphase. There are, broadly speaking, two types of centromeres. "Point centromeres" bind to specific proteins that recognize particular DNA sequences with high efficiency. Any piece of DNA with the point centromere DNA sequence on it will form a centromere if present in the appropriate species; the best characterised point centromeres are those of the budding yeast, Saccharomyces cerevisiae.
"Regional centromeres" is the term coined to describe most centromeres, which form on regions of preferred DNA sequence, but which can form on other DNA sequences as well. The signal for formation of a regional centromere appears to be epigenetic. Most organisms, ranging from the fission yeast Schizosaccharomyces pombe to humans, have regional centromeres. Regarding mitotic chromosome structure, centromeres represent a constricted region of the chromosome where two identical sister chromatids are most in contact; when cells enter mitosis, the sister chromatids are linked along their length by the action of the cohesin complex. It is now believed that this complex is released from chromosome arms during prophase, so that by the time the chromosomes line up at the mid-plane of the mitotic spindle, the last place where they are linked with one another is in the chromatin in and around the centromere; each chromosome has two arms, labeled q. Many remember that the short arm'p' is named for the French word "petit" meaning'small', although this explanation was shown to be apocryphal.
They can be connected in either metacentric, acrocentric or telocentric manner. These are X-shaped chromosomes, with the centromere in the middle so that the two arms of the chromosomes are equal. A chromosome is metacentric if its two arms are equal in length. In a normal human karyotype, five chromosomes are considered metacentric: chromosomes 1, 3, 16, 19, 20. In some cases, a metacentric chromosome is formed by balanced translocation: the fusion of two acrocentric chromosomes to form one metacentric chromosome. If the arms' lengths are unequal, the chromosome is said to be submetacentric, they are L-shaped. If the p arm is so short that it is hard to observe, but still present the chromosome is acrocentric; the human genome includes five acrocentric chromosomes: 13, 14, 15, 21, 22. The Y chromosome is acrocentric. In an acrocentric chromosome the p arm contains genetic material including repeated sequences such as nucleolar organizing regions, can be translocated without significant harm, as in a balanced Robertsonian translocation.
The domestic horse genome includes one metacentric chromosome, homologous to two acrocentric chromosomes in the conspecific but undomesticated Przewalski's horse. This may reflect either fixation of a balanced Robertsonian translocation in domestic horses or, fixation of the fission of one metacentric chromosome into two acrocentric chromosomes in Przewalski's horses. A similar situation exists between the human and great ape genomes, with a reduction of two acrocentric chromosomes in the great apes to one metacentric chromosome in humans. Strikingly, harmful translocations in disease context unbalanced translocations in blood cancers, more involve acrocentric chromosomes than non-acrocentric chromosomes. Although the cause is not known, this relates to the physical location of acrocentric chromosomes within the nucleus. Acrocentric chromosomes are located in and around the nucleolus, so in the center of the nucleus, where chromosomes tend to be less densely packed than chromosomes in the nuclear periphery.
Chromosomal regions that are less densely packed are more prone to chromosomal translocations in cancers. A telocentric chromosome's centromere is located at the terminal end of the chromosome. A telocentric chromosome has therefore only one arm. Telomeres may extend from both ends of the chromosome, their shape is similar to letter "i" during anaphase. For example, the standard house mouse karyotype has only telocentric chromosomes. Humans do not possess telocentric chromosomes. If the chromosome's centromere is located closer to its end than to its center, it may be described as subtelocentric. If a chromosome lacks a centromere, it is said acentric; the macronucleus of ciliates for example contains hundreds of acentric chromosomes. Chromosome-breaking events can generate acentric chromosomes or acentric fragments. A dicentric chromosome is an abnormal chromosome with two centromeres, it is formed through the fusion of two chromosome segments, each with a centromere, resulting in the loss of acentric fragments and the formation of dicentric fragments.
The formation of dicentric chromosomes has been attributed to genetic processes, such as Robertsonian translocation and pa
The.dbf file extension represents the dBase database file. The file type was introduced in 1983 with the introduction of dBASE II; the file structure has evolved over the years to include many more features and capabilities and has introduced various other files to help support data storage and manipulation. The current.dbf file level is called Level 7. The.dbf format is supported by a number of database products. The original dBASE database was known as Project Vulcan. At the time the file that held the data was a simple table that could have data added, modified and printed using ASCII characters set; as the product became more popular, the underlying file type.dbf was expanded and additional files were added to increase the capabilities of the database system. Keep in mind that dBASE is an IDE, a database system, a compiler, a database application builder. However, underneath all, the.dbf file, the actual data storage mechanism. If you want to understand the structure at a much lower level, review the Level 5 DOS headers section of this article.
Project Vulcan There are no public records on the exact layout of the file the best information at this time is that it was a simple table that allows for adding, deleting and printing out ASCII information. It was designed to run on 8-bit machine running CP/M. dBASE II – MS-DOS was the first major release of Ashton-Tate and offered many advancements above and beyond the simple table structure of the original tables found in Project Vulcan. Still written for 8-bit computing Increased the number of fields from 16 to 32 Introduced a SORT routine 16-bit version released in April 1983 version 2.4dBASE III – MSDOS was now focused on 16-bit operations and was introduced in 1.0 was released in June 1984. The underlying dBASE database was still based on an intermediate version of the dBASE II file format; the dBASE III file format is not compatible with the dBASE III+ format. DBASE III+ – MS-DOS this starts the modern era of dBASE.dbf files. The dBASE III + was introduced in December 1985. 16-bit version released in April 1983 version 2.4Structure layout of the file type:dBASE IV – MS-DOS' 1.0 x322 Oct 1988 1.0 x55 Mar 1993 – dBASE Compiler 2.0 x12 Oct 1993 Includes fixes in dBASE IV v2.0 x16 above.
It's a new version that contain new features not in the V1.0 product: 32-bit generation Auto compiling and linking Smaller. EXE size. DBO output Linker can produce a. MAP file Compiler supports alternate date formats Support for wildcard character in file names used with command-line switches.dBASE V – MS-DOS 1.0 x46 Jun 1994BDE – Borland Database Engine 2.52 This is the last update to the 16-bit version of the Borland Database Engine. Download and unzip to a temporary folder, run the SETUP program.dBASE V – MS-Windows 5.5 b673 Jul 1995 While dBASE has been around for many years, the files themselves seem to be shrouded in mystery. Below is the structure of a database file for dBASE V for MS-DOS. A database file is composed of a header, data records, deletion flags, an end-of-file marker; the header contains information about the file structure, the records contain the actual data. One byte of each record is reserved for the deletion flag. Database header structure The header structure, detailed in Table D.l and Table D.2, provides information dBASE for DOS uses to maintain the database file.
Database records The records follow the header in the database file. Data records are preceded by one byte: a space if the record is not deleted, or an asterisk if the record is deleted. Fields are packed into records without field separators or record terminators; the end of the file is marked by a single byte 0x1A. You can input ASCII data. A memo file consists of blocks numbered sequentially. SET BLOCKSIZE determines the size of each block; the first block in the memo file, block 0, is the memo file header. Each memo field of each record in the. DBF file contains the number of the block. If the memo field contains no data, the. DBF file contains blanks rather than a number; when data is changed in a memo field, the block numbers may change, the number in the. DBF may be changed to reflect the new location. Unlike dBASE III PLUS, if you delete text in a memo field, dBASE for DOS may reuse the space from the deleted text when you input new text. DBASE IQ PLUS always appended new text to the end of the.dbt file.
In dBASE III PLUS, the. DBT file size grew whenever new text was added if other text in the file was deleted. DBASE 7 – MS-Windows 7.0 b1345 Dec 1997 Full 32 bit version for Win 95/NT Level 7 brought many improvements. The field names can have up to 31 characters; some new fields types have appeared. If your tables have to be used by other software, you might have to sacrifice these advantages for the sake of compatibility, as few applications can use a level 7 table; the Level 7 structure is the latest supported by dBASE and BDE. BDE version 5.1.0 Significant improvements over the prior releases. There are some limitations with regards to what the BDE can handle. There are many file types or files that have extensions that can be used by dBASE; the following list is presented in the order of use. This is a combination list of files that are related to dBASE
The 1927 VFL Grand Final was an Australian rules football game contested between the Collingwood Football Club and Richmond Football Club, held at the Melbourne Cricket Ground in Melbourne on 1 October 1927. It was the season's grand final of the Victorian Football League, staged to determine the premiers for the 1927 VFL season. Before the 1927 season, Collingwood had last won a premiership in 1919, had suffered grand finals losses in 1920, 1922, 1925 & 1926. Great dissatisfaction brewed amongst the supporters, at a meeting at the Collingwood Town Hall in March 1927 the committee was put under enormous pressure to end the seven-year premiership drought. In a remarkable response, the committee sacked the in-form Charlie Tyson as captain and player, gave the captaincy to Syd Coventry. Coventry went on to win the inaugural Copeland Trophy. In the 1927 home-and-away season Collingwood had won 15 of its 18 matches to finish top of the table, with Richmond next, a game behind. Geelong and Carlton made up the four.
In the semi-finals, Richmond had narrowly beaten Carlton, while Collingwood comfortably defeated Geelong by 66 points, went into the match as favourites. This season was played under the amended Argus system. If Richmond had won this match, Collingwood would have had the right to challenge Richmond to a rematch for the premiership on the following weekend, because Collingwood was the minor premier; the winner of that match would have won the premiership. Torrential rain and freezing winds before and during the match made for atrocious conditions on the day. While Richmond tried to pick up the ball, Collingwood followed the instructions of coach Jock McHale by "marking on the chest, not out in front" and "kicking the ball off the ground wherever possible". With Syd Coventry impassable in defence, Gordon Coventry booted two goals in the 2nd quarter to take his season tally to 97 and have Collingwood lead by 14 points at half time. Both goals came from mistakes by the Richmond backmen. In the 3rd quarter, Richmond responded with a fierce attack on the football and the man, with Collingwood's Syd Coventry and Rumney all knocked out in the opening minutes, but the Tigers could not convert the effort into a score, the Magpies maintained their 14-point margin at 3-quarter time.
Richmond's Fincher scored a goal early in the last quarter but the Collingwood defence held, with the Magpies winning by 12 points at the final bell. The game's aggregate score of 3.20 was the lowest-scoring game in the VFL since Round 6 1900, is the lowest in any VFL/AFL grand final, the equal 11th lowest-scoring game in VFL/AFL history, the lowest-scoring game in the 20th century or the 21st century. Collingwood's 2.13 is the lowest winning score in a grand final, Richmond's 1.7 is the lowest score in a grand final. The 1927 flag was Collingwood's sixth premiership victory, they went on to win the next three grand finals. In winning four premierships in a row from 1927–1930 they set a record that has not been equaled to date; the teams from this era became known as "The Machine" for the teamwork and effectiveness with which they played. Umpire – Jack McMurray Attendance – 34,551 Gate – ₤1,779 1927 VFL season Atkinson, Graeme: The Complete Book of AFL Finals, 1996. ISBN 1-875971-47-5 McFarlane and Roberts, Michael: The Machine – The Inside Story of Football's Greatest Team, 2005.