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General idea about Evolution

From biological view, evolution of living things could not have occurred without the ability to pass their characteristics from one generation to the next. Chromosomes has its role in transmitting an organism’s hereditary characteristics to its young by means of  ‘units of heredity’ called genes.

  • Formation of new species

New species can evolve if creatures become isolated by water, mountains, or other barriers, so can no longer breed with their own kind.

  • Natural Selection

Living things are always exposed to risk from disease, predators, and other dangers. If the young are born with differences that make them tougher, or stronger, or better suited to their surroundings, they will be able to live longer and have more babies than their weaker brothers and sisters.

This idea is called survival of the fittest, or natural selection. Nature selects the fittest and strongest for survival by killing off the weaklings. The idea was first introduced by Charles Darwin and Alfred Russel Wallace over a hundred years ago.

  • Extinction…. (멸종)

There are many reasons why some creatures gradually declined in numbers until they became extinct. Climate can change from burning deserts to freezing ice ages, so that, unless living things can migrate to better conditions, or evolve which are in some way better equipped to survive. These take over the living space and food supply of some existing species, depriving them of the necessities of life.

  • A criticism of evolution: Evolution is only a theory!?

A scientific definition of evolution is ‘a change in the genes of a population over time’. This leads, for example, to insects developing resistance of pesticides. Evolution in this sense of the word is an indisputable fact. What critics forget is that this mechanism is all that is needed to produce a variety of creatures from a common ancestor and with time, new species.

People who still insist that evolution is ‘only a theory’ imply that it is no more than a tentative suggestion. In fact this ‘theory’ is based on a mass of evidence from fossils, anatomy, ecology, genetics, animal behaviour, and more..


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End of Protein Synthesis. Then what?

Breakdown of mRNA

  • Each mRNA molecule is read many times over before being eventually breakdown by an RNase enzyme.
  • The cell can therefore only continue to make the same protein if new mRNA molecule continue to pass out from the nucleus.
  • To decrease the rate of protein synthesis, few of the mRNA molecule coding for it are made in a given time.
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Protein Synthesis 5: tRNA

  • About 80 nucleotides in length and are made under the direction of genes in thechromosomes.
  • Different tRNA molecule for each of the different possible anti-codons because of the degeneracy of the genetic code.
  • Anti-codon at the base of each tRNA must make a perfect complementary match with the codon on the mRNA before the amino acid is released.
  • Once released, the amino acid is added to the growing polypeptide chain by enzymes.
  • Since there is different kinds of tRNA for each amino acids there must be at least 2kinds of tRNA.
  • tRNA molecule is folded back on itself giving a complex 3-D shape.
  • Each tRNA molecule must be able to combine with:
  1. One of the 20kinds of amino acid
  2. mRNA codon that specifies the amino acid
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Protein Synthesis 4: Ribosome

Translation: Ribosome (rRNA)

  • Single mRNA molecule is served by many ribosomes at the same time.
  • Made up of a complex of ribosomal RNA and proteins.
  • 2 Separate sub-units until they are attracted to a binding site on the mRNA molecule.
  • Have binding sites that attract tRNA molecules loaded with amino acids.
  • Ability of a ribosome to translate the coded information in an mRNA molecule and move along it one triplet at a time.
  • It allows codon-anticodon pairing to occur.
  • It moves along the mRNA one triplet at a time.
  • Has 3 binding sites for tRNA: E-site, P-site, A-site

tRNA and Ribosome

  • tRNA in the A site is translocated to the P site, taking the mRNA along with it. Meanwhile the tRNA in the P site moves to the E site and is released from the ribosome.
  • P site is occupied by the tRNA attached to the growing polypeptide chain.
  • A site binds the incoming tRNA the next amino acid to be added.

  • Ribosome joins with the 5’end of an mRNA molecule.
  • AUG codon acts as a ‘begin here’ symbol.
  • When the ribosome reaches a ‘stop’ codon, it breaks free and can then begin reading another mRNA molecule to make another protein molecule.
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Protein Synthesis 3: RNA Polymerase

Transcription과 Translation과정을 자세히 알아보자.

1. Transcription에 사용되는 Enzyme: RNA Polymerase

  • Transcription is the process by which the code contained in the DNA molecule is transcribed into a mRNA molecule.
  • Under the control of the cell’s metabolic processes.
  • The gene must be activated before this process can begin.

2. The role of RNA Polymerase

  1. Enzyme that directly controls the process
  2. Moves 3’–>5’ direction
  3. Makes a strand of mRNA using the single strand of DNA (template strand) –Only transcribe a gene length of DNA at a times.
  4. Recognizes start and stop codes at the beginning and end of the gene.
  5. Only enzyme involved in mRNA synthesis.
  6. Unwinds the DNA

3. Terms

  • Template strand: contains the information for the construction of a protein
  • Coding strand: DNA has nucleotide sequence complementary to the template strand.
  • mRNA: Complementary to the template strand. Containing same ‘message’ as the coding strand.

4. DNA처럼 RNA도 5′–>3’방향으로 자란다. (3′–>5′ 읽어나가기 때문에..)

  • RNA grows in 5’–>3’ direction by adding new nucleotides to the 3’ end.
  • Two polynucleotide strands are anti-parallel.
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Protein Synthesis 2: Translation

NCEA LV2에서 배웠기때문에 Protein Synthesis가 생소하지는 않겠지만 Gene expression을 이해하려면 모든 과정을 디테일하게 알아야 헷갈리지 않는다. Animation을 보면서 3D로 이해해보자.

and bit of Mutation

  • •mRNA travels from the nucleus where it is formed to ribosomes in the cytoplasm.
  • tRNA, found in the cytoplasm, carries a particular amino acid that is related to its particular anti-codon.
  • An anti-codon is part of tRNA molecule and consists of 3 bases that pair with the complementary codon on mRNA.
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Protein Synthesis 1: Transcription

The transfer of the genetic code occurs in 2stages:

1. Transcription: The code in DNA is transcribed or copied into mRNA, which travels of the cell nucleus to the ribosomes in the cytoplasm.

2. Translation: At the ribosomes, the code in mRNA is translated into a specific amino acid sequence in a protein.

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Transcription

  • The genetic code in DNA is transcribed into molecule of mRNA.
  • mRNA molecule is a single strand of nucleotides joined together.
  • The order of nucleotides in an mRNA molecule is determined by the order of nucleotides in the DNA molecule from which the mRNA has been transcribed.
  • Four different kinds of nucleotides are found in RNA: (A) Adenine, (U)Uracil, (C) Cytosine, (G) Guanine
  • No thymine found in RNA strand.

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Splicing

• Initially, the RNA copy of a gene includes both introns and exons.

•Introns are removed and the exons are spliced together to make a continuous strand of mRNA.

•After removal of the introns, the RNA leaves the nucleus via the pores in the nucleus envelope and passes to the cytoplasm.

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Nucleic acid

  • Nucleic acid can be made up of a double helix structure or a single stranded structure.
  • DNA and RNA both are Nucleic acids
  • DNA consists of a base made up of: Cytosine, Thymine, Adenine, and Guanine.
  • RNA consists of a base made up of: Cytosine, Adenine, and Guanine. Uracil
  • Nucleic acids are built up by single nucleotides linked together.
  • Nucleotides are made up of a five carbon sugar, a nitrogen base , and a phosphoric acid.
  • Nucleotides are held together by phosphate ester bonds, which occur with the presence of oxygen and carbon atoms bonded together.

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Comparison of DNA and RNA

  • Deoxyribonucleic acid (DNA) is known to have a double helix structure, a deoxyribose sugar, and consists Thymine but not Uracil.
  • Ribonucleic acid (RNA) consists of a ribose sugar and Uracil instead of Thymine.
  • RNA is single stranded, and it translates the genetic material of DNA, into protein structures, carrying out the instructions of DNA.
  • The three subcategories of RNA are Ribosomal RNA (rRNA),messanger RNA (mRNA), and transfer RNA (RNA).
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Gene Expression: intron과 exon

Mitosis 와 DNA Replication을 이해했다면 이제 본격적으로 Gene expression을 공부해야할 시간! ㅎㅎㅎ

Intron과 Exon에대해서 알아보도록 하자. Intron에 대해서는 그다지 알려진 바가 없다. Gene expression의 모든 중요한 정보는 exon이 가지고 있으므로 Protein Synthesis를 공부하기전에 mRNA를 만드는 과정부터 알아보도록 하자.

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Review

• DNA is made of nucleotides joined together to form a double helical structure.

• Each strand contains the genetic code for the particular gene.

• Different DNAs are responsible for the production of different proteins.

How does DNA carry genetic information?

• The DNA molecule carries genetic information in the unique base-pairing and base sequences of the gene.

•The genetic information, which codes for characteristics of an organism, results from the sequence of nucleotides in the DNA, which make up units called genes.

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WHAT IS GENE EXPRESSION?

• The process of transferring the information encoded in a gene to its functional gene product.

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RNA processing in Eukaryotes

• Coding regions of eukaryotic gene are separated by sections of DNA that are not translated into proteins.

DNA gene: Intronic and exonic DNA

• DNA sequences that does not code for proteins are called introns or intronic DNA.

• DNA sequence that gives rise to a translated polypeptide sequence are called exons.

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Eukaryotic genomes

• Eukaryotic genomes contain a large amount of introns.

• Introns were termed as “junk DNA” and assumed to have no function. New evidence suggests that this DNA may encode a vast number of RNA molecules with regulatory functions.

• An increase in complexity is associated with an increase in the proportion of nonprotein-coding DNA.

• Intronic DNA may has a role in regulating genomic function.

인간 DNA 의 경우 염기서열중 5660만개 염기가 단백질을 합성하는 유전자 부분이고, exon은 전체 28억 3천만개의 유전자 중 2%에 해당한다. 엑손(exon)은 DNA 가닥상 단백질의 발현정보를 code 하고 있는 부분이고, 인트론(intron) 은 아직까지는 정확한 해석이 내려지지 않고 있는 부분이다. 일반적으로는 exon 부분을 보호하기 위한 일종의 껍질 같은 역할을 한다는 설과 gene expression의 과정을 조절하거나 제어하는 과정에 필요하다는 설이있다.

DNA의 정보가 mRNA로 전사되면서 단백질을 만드는 모든 정보가 복사된다. 여기엔 불필요한 정보(intron)까지 복사되고 이 불필요한 정보를 스플라이싱(Splicing)과정을 거쳐 필요한 정보만으로 구성된 mRNA로 다시 만들게 된다. Mature mRNA는 핵막을 통과할 수 있다.

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이 그림은 mRNA 의 정밀한 모습이다. 기본적으로 엑손이 모인 중간 부분과 캡을 쓴 머리 부분, 그리고 여러개의 A 로 이어진 꼬리 부분이 있다. 여기서 모자같은 머리부분과 꼬리 부분을 제거하면 DNA의 정보를 그대로 담고 있는 부분이 된다.

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Prokaryotic genomes

• Lack nucleus

• Transcription and translation occur together

• RNA is translated into protein almost as fast as it is transcribed from DNA.

• Lack of introns.

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Large Intestine 큰창자 스토리(2)

The large intestine (or large bowel) is the part of the digestive system where waste products from the food you eat are collected and processed into faeces.

The large intestine is about 1.5 m long and consists of the caecum, appendix, colon and rectum – which are distributed in the abdominal cavity.

What does it do?

  • The large intestine performs the following functions:
  • reabsorbs water and maintains the fluid balance of the body
  • absorbs certain vitamins
  • processes undigested material (fibre)
  • stores waste before it is eliminated.

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위를 지나 작은창자에서 소화된 음식은 잘게 쪼개져 창자 벽을 통해 흡수가 된다. 길고 긴 작은창자를 지나면서 몸에서 필요로 하는 영양소의 대부분이 흡수되고 나면 남은 찌꺼기는 큰창자로 들어간다.

큰 창자는 흔히 물을 흡수한다고 알려져 있으나 그 외에도 여러 기능을 하며, 이러한 기능은 큰창자 내에 존재하는 세균(대장균)과 깊은 관계가 있다. 큰창자에 사는 세균들이 일부의 노폐물이나 독소를 우리 몸에 필요한 물질로 바꿔주면 큰창자가 이를 흡수하는 것이다. 사람의 몸에 전혀 쓸모 없는 물질을 유용한 물질로 바꾸고, 이를 흡수하여 재활용하게 하니 큰창자와 그 속에 사는 세균은 참으로 고마운 존재이기도 하다.

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큰창자는 작은창자(Small intestine)가 끝나는 지점부터 항문(Anus)까지를 가리킨다. 6미터에 이르는 작은창자 길이의 1/4에 불과한 약 1.5미터 정도지만 폭이 약 7.5cm이므로 작은창자보다 두 배 이상 굵다.

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1. 큰 창자의 첫 번째 기능 : 물의 흡수

인체는 필요로 하는 영양소를 얻기 위해 음식물을 소화시킨 다음 몸 안으로 흡수하는데, 이때 가장 큰 역할을 하는 것은 작은창자다. 음식물은 입에서부터 식도와 위를 지나 작은창자에 이르면 이자에서 분비된 소화효소와 작은창자의 기계적인 운동, 간에서 분비된쓸개즙염에 의해 산산조각이 난다. 그리고는 길고도 긴 작은창자의 통로를 지나가면서 사람에게 필요한 것은 모두 몸 속으로 들어가게 된다. 작은창자를 통과하여 큰창자로 들어갈 때쯤에는 미처 흡수되지 못한 영양소가 일부 포함되긴 하지만 영양가 없는 찌꺼기가 주로 남는다.

이들 찌꺼기들이 몸 밖으로 나가기 위해 창자를 통과해 갈 때 그 흐름을 쉽게 하기 위한 물이 포함되어 있을 뿐이다. 그런데 물은 인체를 구성하는 가장 많은 성분이므로 함부로 몸 밖으로 내보낼 수가 없다. 큰창자는 찌꺼기가 배출되는 통로 역할을 하면서 동시에 마지막까지 남아 있는 물을 흡수하는 기능을 한다.

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2. 큰 창자의 두 번째 기능 : 대장균에 의한 비타민 생산과 흡수

흔히 큰창자는 물을 흡수하는 것 외에 별다른 기능이 없는 것으로 알려져 있지만 큰창자가 물만 흡수하는 것은 아니다. 하루에 작은창자에서 큰창자로 들어오는 물질이 약 1,500ml이고, 이중 약 3/4(1,125ml)이 물이며, 대변으로 배출되는 양은 약 200ml이라 했으니 얼른 생각해도 계산에서 175ml가 비어 있음을 알 수 있다. 이 수치는 큰창자에서 흡수되는 비타민의 일부, 쓸개즙염, 빌리루빈(bilirubin) 등의 양을 가리키는 것이다.

인간의 몸이 진화를 통해 작은창자에서 모든 소화를 끝내고 필요한 영양소를 흡수하게 된 것은 자연의 섭리라고도 할 수 있는데 어째서 일부 비타민은 작은창자에서 흡수되지 못하고 큰창자에 이르러서야 몸 안으로 들어가는 것일까?

그것은 큰창자에서 흡수되는 비타민이 섭취한 음식에 포함된 것이 아니라 큰창자 안에 존재하는 세균에 의해 합성된 것이기 때문이다. 언제부터 큰창자에 세균(대장균)이 살게 되었는지는 확실치 않으나 유인원 이전부터 세균이 존재했을 것으로 추정된다. 큰창자에서 살기 시작한 세균이 대사를 하는 과정에서 사람의 몸에 쓸만한 것(비타민)을 만들게 되자 우리 몸은 그 물질을 흡수하는 식으로 진화해 왔을 것이다. 큰창자에서 흡수하는 비타민은 비타민 B5(판토텐산)와 바이오틴, 비타민 K가 전부다. 이 세 가지 비타민은 음식으로 섭취하지 않아도 구할 수 있으므로 결핍되어도 별 문제가 생기지 않는다.

글 예병일 / 연세대학교 원주의과대학 교수