Following splicing, the segment of mRNA containing the code that directs synthesis of a protein product is called the open reading frame (ORF). The inclusion of a particular amino acid in the protein is specified by a codon composed of three contiguous bases. There are 64 different codons with some redundancy in the system: 61 codons encode one of the 20 amino acids, and the remaining three codons – UAA, UAG and UGA (known as stop codons) – cause termination of the growing polypeptide chain. ORFs in humans most commonly start with the amino acid methionine. All mRNA molecules have domains before and after the ORF called the 5 untranslated region (UTR) and 3 UTR, respectively. The start of the 5 UTR contains a cap structure that protects mRNA from enzymatic degradation, and other elements within the 5 UTR are required for efficient translation. The 3 UTR also contains elements that regulate efficiency of translation and mRNA stability, including a stretch of adenine bases known as a polyA tail (see Fig. 1).

Fig1. RNA synthesis and its translation into protein. Gene transcription involves binding of RNA polymerase I to the promoter of genes being transcribed with other proteins (transcription factors) that regulate the transcription rate. The primary RNA transcript is a copy of the whole gene and includes both introns and exons, but the introns are removed within the nucleus by splicing and the exons are joined to form the messenger RNA (mRNA). Prior to export from the nucleus, a methylated guanosine nucleotide is added to the 5 end of the RNA (‘cap’) and a string of adenine nucleotides is added to the 3 (‘polyA tail’). This protects the RNA from degradation and facilitates transport into the cytoplasm. In the cytoplasm, the mRNA binds to ribosomes and forms a template for protein production. (tRNA = transfer RNA; UTR = untranslated region)

The mRNAs then leave the nucleus via nuclear pores and associate with ribosomes, the sites of protein production (see Fig. ). Each ribosome consists of two subunits (40S and 60S), which comprise non-coding rRNA molecules (see Fig. 2) complexed with proteins. During translation, a different RNA molecule known as transfer RNA (tRNA) binds to the ribosome. The tRNAs deliver amino acids to the ribosome so that the newly synthesised protein can be assembled in a stepwise fashion. Individual tRNA molecules bind a specific amino acid and ‘read’ the mRNA ORF via an ‘anticodon’ of three nucleotides that is complementary to the codon in mRNA (see Fig. 1). A proportion of ribosomes is bound to the membrane of the endoplasmic reticulum (ER), a complex tubular structure that surrounds the nucleus.

Fig2. Mitochondria. [A] Mitochondrial structure. There is a smooth outer membrane surrounding a convoluted inner membrane, which has inward projections called cristae. The membranes create two compartments: the inter-membrane compartment, which plays a crucial role in the electron transport chain, and the inner compartment (or matrix), which contains mitochondrial DNA and the enzymes responsible for the citric acid (Krebs) cycle and the fatty acid $eta$-oxidation cycle. [B] Mitochondrial DNA. The mitochondrion contains several copies of a circular double-stranded DNA molecule, which has a non-coding region, and a coding region that encodes the genes responsible for energy production, mitochondrial transfer RNA (tRNA) molecules and mitochondrial ribosomal RNA (rRNA) molecules. [C] Mitochondrial energy production. Fatty acids enter the mitochondrion conjugated to carnitine by carnitine-palmityl transferase type 1 (CPT I) and, once inside the matrix, are unconjugated by CPT II to release free fatty acids (FFA). These are broken down by the $eta$-oxidation cycle to produce acetyl-co-enzyme A (acetyl-CoA). Pyruvate can enter the mitochondrion directly and is metabolised by pyruvate dehydrogenase (PDH) to produce acetyl-CoA. The acetyl-CoA enters the Krebs cycle, leading to the production of NADH and flavine adenine dinucleotide (reduced form) ($ ext{FADH}_2$), which are used by proteins in the electron transport chain to generate a hydrogen ion gradient across the inter-membrane compartment. Reduction of NADH and $ ext{FADH}_2$ by proteins I and II, respectively, releases electrons (e), and the energy released is used to pump protons into the inter-membrane compartment. Coenzyme $ ext{Q}_{10}$/ubiquinone (Q) is an intensely hydrophobic electron carrier that is mobile within the inner membrane. As electrons are exchanged between proteins in the chain, more protons are pumped across the membrane, until the electrons reach complex IV (cytochrome oxidase), which uses the energy to reduce oxygen to water. The hydrogen ion gradient is used to produce ATP by the enzyme ATP synthase, which consists of a proton channel and catalytic sites for the synthesis of ATP from ADP. When the channel opens, hydrogen ions enter the matrix down the concentration gradient, and energy is released that is used to make ATP. (ATP = adenosine triphosphate; NADH = the reduced form of nicotinamide adenine dinucleotide)

Proteins synthesised on these ribosomes are translocated into the lumen of the ER, where they undergo folding and processing. From here, the protein may be transferred to the Golgi apparatus, where it under goes post-translational modifcations, such as glycosylation (covalent attachment of sugar moieties), to form the mature protein that can be exported into the cytoplasm or packaged into vesicles for secretion. The clinical importance of post-translational modifcation of proteins is shown by the severe developmental, neurological, haemostatic and soft tissue abnormalities that are associated with the many different congenital dis orders of glycosylation. Post-translational modifcations can also be disrupted by the synthesis of proteins with abnormal amino acid sequences. For example, the most common CFTR gene variant that causes cystic fbrosis (ΔF508) results in an abnormal protein that cannot be exported from the ER and Golgi.

اخر الاخبار

اخبار العتبة العباسية المقدسة

وفد العتبة العباسية المقدسة: توزيع المساعدات إلى الشعب الإيراني أُجري بالتنسيق مع مكتب المرجعية العليا في مدينة قم

وفد العتبة العباسية المقدسة يعلن إيصال المساعدات الإنسانية لتوزيعها على المتضررين في إيران

في مقابلة خاصة.. رئيس وفد قافلة العتبة العباسية المقدسة يكشف عن تفاصيل رحلة إيصال المساعدات للشعب الإيراني

ضمن حملتها الإنسانية.. وفد العتبة العباسية المقدسة يزور الجرحى وعوائل الشهداء في إيران

المزيد

الآخبار الصحية

الشاشات ليست مجرد ترفيه.. تأثيرات طويلة المدى على دماغ طفلك

الأسباب الرئيسية لنقص الطاقة في الجسم

الكاكاو وتأثيره على العين.. نتائج واعدة من تجارب مخبرية

طبيب يحذر: أطعمة شائعة الاستهلاك أخطر على صحتنا من التدخين

المزيد

الآخبار التكنلوجية

باحثون في "نيويورك أبوظبي" يطورون تقنيات لرصد وعلاج الأورام

جيتكس إفريقيا.. المغرب يجمع "رواد التكنولوجيا" من 103 دول

طاقم أرتميس 2 يحطم رقما قياسيا ويستعرض الجانب البعيد للقمر

"أرتيميس 2" تنطلق حول القمر وتحطم الرقم القياسي

المزيد

مواضيع ذات صلة

Genetic Material can be Altered & Rearranged

Epigenetic Regulation of Transcription

Chromatin-Remodeling Complexes Help Activate or Repress Transcription

Activators Can Direct Histone Acetylation at Specific Genes

Repressors Can Direct Histone Deacetylation at Specific Genes

Formation of Heterochromatin Silences Gene Expression at Telomeres, near Centromeres, and in Other Regions

Multiprotein Complexes Form on Enhancers

Transcription Factor Interactions Increase Gene-Control Options

Structurally Diverse Activation and Repression Domains Regulate Transcription

Electrophoresis of Nucleic Acids

Regulation of Transcription Factor Activity

Activators Are Composed of Distinct Functional Domains