Anatomy books

Tuesday, January 5, 2021

Short discussion about autosome

 

Autosome

An autosome is any of the numbered chromosomes, as opposed to the sex chromosomes. Humans have 22 pairs of autosomes and one pair of sex chromosomes (the X and Y). Autosomes are numbered roughly in relation to their sizes. That is, Chromosome 1 has approximately 2,800 genes, while chromosome 22 has approximately 750 genes.

An autosome is any chromosome that is not a sex chromosome (an allosome).

 The members of an autosome pair in a diploid cell have the same morphology, unlike those in allosome pairs which may have different structures. The DNA in autosomes is collectively known as atDNA or auDNA.

For example, humans have a diploid genome that usually contains 22 pairs of autosomes and one allosome pair (46 chromosomes total). The autosome pairs are labeled with numbers (1–22 in humans) roughly in order of their sizes in base pairs, while allosomes are labelled with their letters. By contrast, the allosome pair consists of two X chromosomes in females or one X and one Y chromosome in males. Unusual combinations of XYY, XXY, XXX, XXXX, XXXXX or XXYY, among other allosome combinations, are known to occur and usually cause developmental abnormalities.

Autosomes still contain sexual determination genes even though they are not sex chromosomes. For example, the SRY gene on the Y chromosome encodes the transcription factor TDF and is vital for male sex determination during development. TDF functions by activating the SOX9 gene on chromosome 17, so mutations of the SOX9 gene can cause humans with an ordinary Y chromosome to develop as females.

All human autosomes have been identified and mapped by extracting the chromosomes from a cell arrested in metaphase or prometaphase and then staining them with a type of dye (most commonly, Giemsa). These chromosomes are typically viewed as karyograms for easy comparison. Clinical geneticists can compare the karyogram of an individual to a reference karyogram to discover the cytogenetic basis of certain phenotypes. For example, the karyogram of someone with Patau Syndrome would show that they possess three copies of chromosome 13. Karyograms and staining techniques can only detect large-scale disruptions to chromosomes—chromosomal aberrations

Autosomal genetic disorders

Autosomal genetic disorders can arise due to a number of causes, some of the most common being nondisjunction in parental germ cells or Mendelian inheritance of deleterious alleles from parents. Autosomal genetic disorders which exhibit Mendelian inheritance can be inherited either in an autosomal dominant or recessive fashion

 These disorders manifest in and are passed on by either sex with equal frequency

 Autosomal dominant disorders are often present in both parent and child, as the child needs to inherit only one copy of the deleterious allele to manifest the disease. Autosomal recessive diseases, however, require two copies of the deleterious allele for the disease to manifest. Because it is possible to possess one copy of a deleterious allele without presenting a disease phenotype, two phenotypically normal parents can have a child with the disease if both parents are carriers (also known as heterozygotes) for the condition.

Autosomal aneuploidy can also result in disease conditions. Aneuploidy of autosomes is not well tolerated and usually results in miscarriage of the developing fetus. Fetuses with aneuploidy of gene-rich chromosomes—such as chromosome 1—never survive to term and fetuses with aneuploidy of gene-poor chromosomes—such as chromosome 21— are still miscarried over 23% of the time Possessing a single copy of an autosome (known as a monosomy) is nearly always incompatible with life, though very rarely some monosomies can survive past birth. Having three copies of an autosome (known as a trisomy) is far more compatible with life, however. A common example is Down syndrome, which is caused by possessing three copies of chromosome 21 instead of the usual two

Partial aneuploidy can also occur as a result of unbalanced translocations during meiosis Deletions of part of a chromosome cause partial monosomies, while duplications can cause partial trisomies. If the duplication or deletion is large enough, it can be discovered by analyzing a karyogram of the individual. Autosomal translocations can be responsible for a number of diseases, ranging from cancer to schizophrenia

 Unlike single gene disorders, diseases caused by aneuploidy are the result of improper gene dosage, not nonfunctional gene product smaller than a few million base pairs generally cannot be seen on a karyogram

An autosome is one of the 22 numbered pairs of chromosomes that most of us carry in almost all of the cells of our body. We actually have a total of 23 pairs of chromosomes in these cells, for a total of 46 chromosomes, but two of those are referred to by letter rather than by number and are called sex chromosomes rather than autosomes, since they--that is the X and Y chromosome--help determine what sex, or gender, we are. The 22 pairs of autosomes are referred to by number basically in inverse correlation with their size. That is, Chromosome 1, with the smallest number, is actually the largest chromosome. It has almost 3,000 genes on it. And we go down to the smallest chromosomes, the ones with the largest numbers. You think that would be Chromosome 22, since we have Chromosomes 1 through 22, which only has about 750 genes, but in fact Chromosome number 22 is not the smallest of the autosomes. We thought it was when it was first described, so that's how it got named 22. It turns out that Chromosome 21 is actually a little bit smaller than Chromosome 22.

 

Monday, January 4, 2021

Transcription

What is transcription?


Transcription is the synthesis of any type of complimentary RNA from a DNA template: note, several types of RNA can be encoded by a DNA strand [see DNA vs. RNA list]. Here, we focus specifically on transcription that leads to pre-mRNA, mRNA and eventually proteins. 

In the process of gene expression, transcription involves the production of messenger RNA (mRNA) from a DNA template. It takes place in the nucleus of a cell and is catalyzed by the enzyme RNA polymerase II.

RNA polymerase  All eukaryotes have three different types of RNA polymerase

RNA polymerase I transcribes rRNA genes

RNA polymerase II transcribes mRNA, miRNA, snRNA, and snRNA genes

RNA polymerase III transcribes an array of RNA genes, including but not limited to tRNA and 5S rRNA gene


 

 

 

 

 

The steps of transcription


The process of transcription entails several steps: 


1. Initiation


The first step of transcription to form mRNA involves RNA polymerase II binding to a promoter region  just upstream of the gene that is to be transcribed. Promoters are often classified as strong or weak based on their effects on transcription rates and thus gene expression. Transcription factors are proteins that help to position RNA polymerase II and assist in the breaking of the hydrogen bonds in the DNA helix. 3

2. Elongation


RNA polymerase II breaks the hydrogen bonds connecting two strands of DNA in the double helix. The enzyme then uses the single DNA strand as a template to build an RNA strand in the 5' to 3' direction, adding each complementary nucleotide to the 3' end of the strand. In RNA, the nucleotide thymine is replaced by the nucleotide uracil.


What do we mean by 5' and 3'? This refers to the carbon numbers in DNA and RNA's backbone. The 5' carbon ribose ring frequently has a phosphate group attached, and the 3' carbon end has a hydroxyl (-OH) group attached. The asymmetry gives the DNA and RNA strands a "direction

The DNA strand moves through the RNA polymerase II enzyme. In the region behind where the nucleotides are being added to form the pre-mRNA strand, the DNA helix re-forms. This means that the pre-mRNA produced is eventually released from the DNA template a single strand. 

3. Termination


Termination marks the end of RNA polymerase II adding nucleotides to the pre-mRNA strand and the release of the pre-mRNA. Despite extensive research, there is still ambiguity surrounding the precise physiological cause of termination - several mechanisms are outlined in this review paper .

From pre-mRNA to mRNA


Eukaryotic pre-mRNAs must go through several additional processing steps before translation can occur. Firstly, they have a 5' cap added and a 3' poly-A-tail added to protect against transcript degradation.

Many eukaryotic pre-mRNAs are subject to splicing. Here, the non-coding sections of the pre-mRNA (introns) are cut out, and the coding sections (the exons) are effectively glued back together.

Schematic showing pre-mRNA undergoing splicing to form mature mRNA.  

Alternative splicing may also take place, whereby exons or noncoding regions within the pre-Mrna transcript are joined or skipped, resulting in multiple mRNAs being encoded by a single gene.

After these modifications have taken place, the resulting strand is known as mature mRNA. This mature mRNA is then able to leave the nucleus and enter the cell cytoplasm where translation takes place.