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Skip to 0 minutes and 11 seconds In this chapter, we are going to see heredity and the role of DNA on it. Thus, it is one of the basic concepts of biology to understand where the information for making us as living beings is held. The first thing is to understand the concept of heredity. We all use this word for many concepts, but we all know that when a woman is pregnant she will deliver a living being, a human being, a specific individual that will resemble her ancestors. The idea is that we have heredity as the transmission of the genetic characteristics from parents to offspring.

Skip to 1 minute and 2 seconds And the concept we are going to hold is that it is hereditary, meaning that it comes biologically from one generation to the next. Sometimes we also talk about familiar traits but in this case they may be genetic or not, hereditary or not… We may talk on congenital meaning that we have from birth, but maybe it is not genetic, and in some cases we talk on sporadic traits; sporadic means that it comes without any ancestor having that. The main idea then, is that this concept of heredity is what is shaped by genes, which are inherited. But our traits are also shaped by the environment and lots of complex interactions.

Skip to 1 minute and 54 seconds When we talk about heredity, we have to separate two main groups of traits. One is of simple traits, what we call Mendelian traits, that follow perfectly the rules of inheritance we studied. These are the Mendelian rules that apply to simple, rare diseases and to simple traits; it’s nice to see how blood groups are inherited, for example. But the most interesting part are the concepts of multiple genes that interact towards a single trait with the environment, and this applies to complex traits, and it also applies to the most common diseases. This is the big challenge of biology nowadays. In any case, the idea is that this heredity, how it’s transmitted .

Skip to 2 minutes and 55 seconds And now we know that it is transmitted through the molecule of life. DNA is where the information is held, and it constitutes the genes, that in its set the overall amount of DNA that holds the genetic information in a cell, is called the genome. So, now we have to understand what DNA is like and how information is embedded in this complex molecule.

Skip to 3 minutes and 32 seconds Sydney Brenner wrote a very interesting phrase: “Living organisms may be viewed as the only part of the natural world whose members contain internal descriptions of themselves.” This is why all of biology must be rooted in DNA, and our task is still to discover how these DNA sequences arose in evolution, and how they are interpreted in the building of diversity of the living world, including disease.

Skip to 4 minutes and 7 seconds Thus, the interesting point: we are all living beings, we have all our book of instructions inside ourselves. This is the book of instructions, written in the language of genes, written in the DNA. We have to go and look for that inside of our bodies; we are made of many cells, within the cells we have the nucleus, inside this nucleus we have the chromosomes, and the chromosomes are made, among other constituents of DNA, here in the nucleus of the cell.

Skip to 4 minutes and 50 seconds In these chromosomes, if we look not just with a microscope but at much smaller scale, we are going to see that inside the chromosomes, there are fibres of DNA down to the basic structure, which is the very well-known double helix of the DNA. This double helix is made of a set of blocks, units of information, that may have only four states,

Skip to 5 minutes and 28 seconds the four letters of the language of genes: A, C, D, G. These are the names that we use, but these are names of the chemical compounds that are in a given sequence within a genome. The main idea then is that we have this double helix and one helix and the other will have complementary blocks,

Skip to 5 minutes and 55 seconds meaning that the information is doubled: the information is complementary between the two strands. You may see a double helix – there are not many – but a very interesting one when you visit La Pedrera in Barcelona, in the main façade there is a double helix that is worth having a look. If we put numbers on DNA it’s really astonishing. We have a very long double helix of DNA within each of our cells.

Skip to 6 minutes and 37 seconds Let’s put some numbers: the numbers of genes we humans have is around 20,000 and these are units of information, units many times of function . But if we go to the basic blocks, we have three billion.

Skip to 7 minutes and 1 second This number is huge, and this is the length of the information

Skip to 7 minutes and 9 seconds we have within each cell in two copies: one from the genome of the father and one from the genome of the mother. To have a nice idea of that,

Skip to 7 minutes and 26 seconds of just a single cell it would become a double helix of more than 2 meters. If you look at the number of cells that you have in your body, make the calculations and you’ll see that the distance between the Earth and the Sun is rather small.

Skip to 7 minutes and 48 seconds In 2001, it was published for the first time one of the key achievements of science: the first human genome sequence. This means that the whole 3 billion letters of the genome –of a human genome– were published and were worked out, and for the first time very well-known. After that, it has been improving and improving through time. At the very end, a sequence, a genome, is one of the most boring books you may have because it’s a set of letters, huge amounts of letters, that now we have to make sense of them.

Skip to 8 minutes and 46 seconds To make sense of that means to understand the grammar, to understand how the letters in the information of the genome are being encapsulated, , how we understand how it may work. If we have a look at our genome, we are going to see that most of our genome is made of repetitive sequences, many of which – as we understand today – do not have a function. It’s what we call many times “junk DNA”, and in our genome only a small part is the part that we recognize very well as important in the sense that is going to be used to build our body.

Skip to 9 minutes and 35 seconds In fact, there is only 1.5 % of our genome that is really going to be translated into proteins.

Skip to 9 minutes and 46 seconds And this is the point we are going to see: how these genes turn into proteins or, in other words, how from the genes we get the phenotypes or the genes in action. René Magritte painted this interesting picture in which the painter looks at an egg and paints a bird.

Skip to 10 minutes and 14 seconds The question is: which is the information he is looking at? How does he know that the egg contains the information to make a bird? He is not able to read the information inside the egg, but he knows that in other cases from the egg came a bird, so he guesses that it’s going to become that. What biology intends to do is something similar but looking at the DNA sequence to understand that there is the information for making a bird. This is the interesting point of biology today. And to understand that begins by going from the DNA into the function.

Skip to 11 minutes and 4 seconds And to make the function the DNA first is transcribed into RNA, a similar molecule to DNA, but it’s not double stranded, and from this RNA into proteins, that are the constituents of our body. To understand how DNA makes proteins is the basic understanding of the language of genes,

Skip to 11 minutes and 32 seconds or what we call the genetic code: how the information is manifested, how the genotype gets into the phenotype and how the genome informs the traits. To know that in detail is still a challenge. We are learning a lot, and every day, thousands of labs are working on that and even if we know very well many aspects of that, there are many others that still remain to be understood, so we usually say that this is the big challenge of the 21st century. To finish, just a concept; a concept of mutation, the concept of getting something new.

Skip to 12 minutes and 34 seconds In general, as we have a double stranded double helix of DNA, the information in each of the strands may be used to make a second strand. This is the DNA replication at the base of every cell division. When replicating, DNA may make mistakes, and there are some mistakes. This is what we call mutation. Mutation, thus, is the way of making a new generation of cells. Something new will arise, something new that will be at the basis of information to produce innovations in the future; but also as the changes that are going to be produced in the human being in a very complex system, many times will produce a disease.

Skip to 13 minutes and 27 seconds So, mutation, is at the same time a cause of disease and the base of innovation for the future of life. This mutation may happen in our somatic cells and this will not be transmitted, but it may be in our germ cells and then will pass to the next generation. So, in this chapter we have seen the concept of heredity and how this heredity is made chemically in our bodies, in the DNA, the molecule of life, and how this information gets into making ourselves.

DNA, the information for life

We shall see the concept of heredity; how DNA works; how proteins are made; how genes define phenotypes, and the concept of mutation. What has life added to the physical world?

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Why Biology Matters: Basic Concepts

Pompeu Fabra University Barcelona