Telomeres

Telomeres are the terminal portions of the chromosomes, made up of long repetitive sequences of TTAGGG associated with proteins and act as a protection cap of the chromosomes. They serve as a physical barrier against fusion, recombination and degradation of chromosomes. When a somatic cell reaches a time when it loses its capacity of replicating, it has reached the replication senescence - event that happens when a cell has lost enough telomere that it can't replicate anymore, this happens because the cell can only replicate so much telomere each cell cycle ^[1]. This point is also called 'Hayflick limit', discovered in the 1960s, which is the limit of replications that an in vitro diploid cell can undergo ^[2].

            Ageing is then, related to cell divisions and when the cell reaches that point, it goes through biochemical and morphological changes that lead to senescence. Something that is under scientific interest is ageing reversal, in other words, putting a halt in replicative senescence. This is where telomerase comes in - an enzymatic ribonucleoprotein complex that regulates telomeres. When it is active, it performs telomere maintenance, so that when the cell replicates, it doesn't lose telomere parts, leading to a state where the cell doesn't reach replicative senescence anymore ^{[1, 3]}.

                                Image 1. Telomere (Source: http://upload.wikimedia.org/)

References:

[1] Wai LK. (2004). Telomeres, Telomerase, and Tumorigenesis -- A Review. MedGenMed 6(3): 19.

[2] Shay JW, Wright WE. (2000). Hayflick, his limit, and cellular ageing. Mol Cell Biol;1:72-76.

[3] Reddel RR. (2000). The role of senescence and immortalization in carcinogenesis. Carcinogenesis 121(3): 477-84

Licenciatura em Biologia Aplicada, 2º ano

PL3 - Grupo 10

Daniel Figueiredo

Domingos Gomes

Leandro Lopes

Sofia Beatriz Silva

Interphase

Interphase is one of the phases of the cell cycle.  During this phase, the cell is preparing itself for division. The great majority of eukaryotic cells spend most of their lifetime in this stage (for example, most mature brain cells remain in interphase throughout their lives). Even though the cell is in the preparation for its division, the interphase is not a part of the mitosis/meiosis processes.

                This phase is constituted into three different stages. Each phase will only end after a "cellular checkpoint" checks for certain mistakes that can occur before letting the cell advance to the next stage, thus avoiding mutations and other malfunctions that could endanger the organism.

                G1 is the first stage. The cell grows to about the double of its former size. It synthesises a high amount of protein and organelles, which result in an increase of its cytoplasm's volume. If the cell's mechanisms "decide" that it should not divide itself, it enters G0 phase, remaining dormant in terms of division. Even though they do not divide themselves, they still continue to perform their main life functions.

                The second stage is called S (synthesis). Via semiconservative replication, the cell duplicates its DNA. When this stage is completed, all of the cell's chromosomes have two chromatids each, which means that they have two molecules of DNA per chromosome.

                After S, the cell goes into G2. In this phase, the cell continues to grow, preparing for division, which starts with prophase, implying that mitosis and cytokinesis are separate processes from interphase.

                                           1. Illustration of interphase and mitosis   

Alunos de Biologia Aplicada:

G11
Alice Loureiro

Cátia Mendes

Miguel Pacheco

Gustavo Pereira

Cell cycle

Cell cycle is a series of events that leads to the division and duplication of cells.

In prokaryotic cells the cycle is called binary fission. This type of division starts with the replication of one DNA molecule. Then, the DNA is pulled to the poles of the cell and increases its size before splitting. The growth of a new cell wall begins to separate the cell and when fully developed leads to the split of the cell.

In cells with a nucleus (eukaryotic) the cycle includes 4 phases: G₁, S, G₂ and M. There is also a resting phase where the cell has left the cycle and stopped dividing (G₀). In G₁ the cell increase in size and function normally. At some point (the first checkpoint), the cell becomes “committed” to the cycle no longer requiring extracellular stimuli (this is called restriction point). Next comes the DNA replication (synthesis or S phase).  During G₂­ phase the cell continue to grow. When the second checkpoint is reached and if all the requirements are met then the cell is ready to enter the last phase – mitosis (M). Here the cell stops growing and directs its energy to the division into the daughter cells. The last checkpoint is the metaphase, which ensures that the cell is ready to complete the cycle.

 

                                                                                                    Nguyen Quynh Anh, Renata Oliveira

                                                                                                                                    Applied Biology

Interphasic Chromosome

          Interphasic chromosome is a structure in the nucleus which contains DNA that is in a period of cell cycle when the cell is not dividing. This structure, capable of transmitting genetic information, contains RNA and histones associated with DNA and during this phase it is under its least condensed state, looking diffused throughout the nucleus and not completely visible under the optical microscope.

Figure 1: Simplified scheme of a cell in interphase. An inter phasic chromosome is visible in the nucleus.

André Gomes, Cátia C Silva, Margarida Borges, Sílvia Miranda

BA

Ionic Bond

Chemical elements with low ionization energies, such as metals, tend to loose electrons and form cations (positive ions), while elements with high ionization energies, such as halogens and oxygen, tend to capture electrons, thus forming anions (negative ions). The resulting ions are attracted to ions with its opposite charge and form an ionic bond. This bond is energetically favourable since bonded ions have lower energy than free ions. Usually, substances with ionic bonds are soluble in water, have a high melting point and show electric conductivity when in aqueous solution, but not when in solid state.

Degree in Biochemistry

Group 15

Ana Patrícia Carvalho
Filipe Lima

Humberto Pereira

Vítor Martins

Transcription

Transcription is the first step of genetic expression. Transcription reaction is catalyzed by RNA polymerases that, along with other protein factors, copy genetic information from genome to messenger RNA (mRNA), in three main steps: initiation, elongation, and termination. This is a mechanism which uses 3’-5’ DNA strand as a template in mRNA synthesis. This process in eukaryotes occurs in the nucleus and in prokaryotes occurs in the nucleoid.

Licenciatura em Biologia Aplicada, 2º ano

João Cardoso
Paulo Silva
Samuel Gonçalves

Grupo 6
  

The Cell Cycle

The cell cycle, or cell-division cycle, is a regulation process that takes place in a cell leading to its division and duplication. The division cycle of most cells consists of four coordinated stages: cell growth, DNA replication, distribution of the duplicated chromosomes to daughter cells, and cell division.

The cell cycle is an ordered sequence of events in which a cell duplicates its DNA and divides into two. Progression between each coordinated stages of the cell cycle is controlled by a conserved regulatory apparatus, which not only coordinates the different events of the cell cycle but also links the cell cycle with extracellular signals that control cell proliferation.

Licenciatura de Biologia Aplicada, 2ºAno

Diogo Esteves

Fernando Cruz

Hugo Magalhães

Chromatin

Chromatin is a complex combination of nucleic acids (DNA and RNA) and basic proteins, such as histones and non-histone chromossomal proteins, in the eukaryotic cell nucleus. However, prokaryotic cells have a very different organization of their DNA, which is a chromosome without chromatin, which is found in the nucleoid region. The chromatin has the function to package DNA into a smaller volume that fits in the nucleus, it also prevents DNA damage, controls gene expression and it will strengthen the DNA that allow mitosis and meiosis.

Licenciatura em Biologia Aplicada, 2º ano

PL2 (Grupo 7)

Casimira Lima

Catarina Lopes

Pedro Silva

Sofia Granja 

Genetic code

The genetic code is a set of rules defining how the four-letter code of DNA is translated into the 20-letter code of amino acids, which are the building blocks of proteins. The genetic code consists of sequences of three nucleotides that are called “codons” and the correspondence to a given amino acid. Genes are the functional segments of DNA, which allow the production of a functional RNA molecule upon transcription. With four possible bases, we have 4³=64 different codons to encode the 20 different amino acids. Although each codon is specific for only one amino acid (or one translation stop signal), the genetic code is described as redundant because there are examples in which more than one codon can codify a single amino acid. An important thing to note is that the genetic code does not overlap, this means that a single nucleotide cannot be part of two adjacent codons, each nucleotide is part of only one codon. Furthermore, with only rare variations reported, the genetic code is universal. Remarkable exceptions can be found in mitochondria, which have a genetic code with slight variations.

G6 TP4
Lic. Bioquímica

Chargaff's rules

Erwin Chargaff was an Austrian biochemist that found out some quantitative relationships in the 1940’s, with his collaborators, that helped to the discovery of the double helix structure of DNA. 

Through research he concluded that the base composition of DNA varies from one species to another and that all tissues of individuals of the same species have the same composition of bases, which allowed him to establish the “Chargaff’s rules”.

The first rule tells us that in every cellular DNA, regardless of the species, the amount of residues of adenine is equal to thymine (A=T) and the amount of guanine residues is equal to that of cytosine (G=C). On the other hand, the second rule says that A+G=C+T, i.e. the sum of the residues of pyrimidines is equal to the sum of the purine residues.

Licenciatura em Biologia Aplicada, 2º ano

Estela Macedo
Luciana Rodrigues
Tânia Pinheiro

Grupo 3

Polynucleotide

Is an organic polymer derived from successive nucleotide junctions. This is formed when two nucleotides make covalent bonds in a linear chain. The phosphate group of the nucleotide 5’ carbon binds to the 3’ carbon of the sugar of the subsequent nucleotide. This phosphodiester linkage generates phosphate bridges.

Currently, the living cells have polynucleotides (DNA and RNA known as information-storage molecules). Such molecules are viewed as essential constituents to the cells.

Bioquímica 2º ano

Grupo 16:

Bárbara Costa

Rosana Monteiro

Centromere

The Centromere is the ever-condensed region of a chromosome, which allows the connection of the sister chromatids. It also harbors a protein complex, the kinetocore, responsible for the adhesion of microtubules of mitotic fuse to the centromere during cell division.

The location of the centromere is a term of classification of the chromosome, which may be, for instance, metacentric if the centromere is located in the middle of it.

The centromere is a very important piece during the cell division because a malfunctioning centromere can lead to incorrect alignment and separation of the sister chromatids, resulting in wrong number of chromosomes in daughter cells, which lead to many genetic diseases like Klinefelter syndrome.

Biologia Aplicada 2°ano

Pl4 grupo 13 

André Santos 

Jorge Ribeiro

Mario Silva

Rui Oliveira

Chromosome G-banding

When treated with stains, chromosomes often display banded patterns, alternating between light and dark, which happens especially during metaphase where the more condensed segments of DNA tend to stain darkly. Therefore, chromosomes can be treated with different methods including G-banding, Q-banding, R-banding, C-banding, NOR-banding or T-banding.

The most commonly used in human cytogenetic analysis is G-banding, allowing between 400 and 600 bands to appear along the length of the whole chromosome in metaphase. This technique uses chromosomes pre-treated with salt or a proteolytic enzyme like trypsin and then stained with Giemsa, which highlights preferentially regions rich in adenine and thymine. Also, the darker stained regions usually are heterochromatic (non functional genes) and late-replicating and the lighter regions are euchromatic (actively transcribed) and early-replicating.

The importance of banding techniques is to diagnose chromosomal anomalies such as chromosome breakage, loss, duplication or inverted segments by identifying unique banding patterns.

Bioquímica 2º ano

Grupo 13:

Adriana Gomes

Andreia Campos

Bárbara Nogueira

Patrícia Machado

Introns

Introns are segments of a gene situated between exons that are not translated. The term intron refers not only to the DNA sequence within a gene but also to the corresponding sequence in RNA transcripts. Introns are present in the genes of most eukaryotic organisms and many virus and their number, length, location and composition greatly varies among genes. They are removed from the pre-mRNA sequence during RNA splicing, resulting in a mRNA sequence ready to be translated. Alternative splicing of introns introduces greater variability of protein sequences translated from a single gene, allowing the translation of far more proteins than the actual amount of genes present in the genome.

Bioquímica (Grupo 9)

Alexandra Gonçalves       67330

Manuela Proença             67340

Sara Ribeiro                     67336

Telma Afonso                   67377

Nucleosome

Nucleosome is the basic unit of DNA packaging in eukaryotes. It consists of a segment of DNA wound twice around eight histone protein cores. The DNA is folded through a series of successively higher order structures into a chromosome, which allow the large eukaryotic genomes to be packed in the nucleus while still ensuring appropriate access to it. The degree of chromatin packaging determines whether or not genes within that segment are expressed, being very important to control gene expression.

The basic structure of a nucleosome consists of approximately 147 base pairs of DNA, wrapped in 1.67 left-handed superhelical turns around a histone octamer, which consists of a central tetramer with 2 dimers each of the core histones H2A, H2B, H3, and H4. Adjacent nucleosomes are connected by stretches of "linker DNA", about 80 bp long. This structure is often called “beads-on-a-string”. Higher levels of organization of the chromatin involves the linker histones, H1 providing more chromatin compaction, such as the 30 nm fiber. The N-terminal tails of the histone octamer interact with each other allowing condensation. Their acetylation reduces the affinity for DNA and the interaction between individual nucleosomes, being potentially relevant for the higher-order structure of nucleosomes and for DNA expression. In heterochromatin the histones are generally unacetylated while those in functional domains are acetylated, indicating that this type of modification is linked to DNA packaging. 

Biologia Aplicada grupo 5

Cecília Cristelo a66766;

Diana Silva a68847; 

Filipa Martins a68872